labeled diagram for photosynthesis

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Photosynthetic Cells

Cells get nutrients from their environment, but where do those nutrients come from? Virtually all organic material on Earth has been produced by cells that convert energy from the Sun into energy-containing macromolecules. This process, called photosynthesis, is essential to the global carbon cycle and organisms that conduct photosynthesis represent the lowest level in most food chains (Figure 1).

What Is Photosynthesis? Why Is it Important?

Most living things depend on photosynthetic cells to manufacture the complex organic molecules they require as a source of energy. Photosynthetic cells are quite diverse and include cells found in green plants, phytoplankton, and cyanobacteria. During the process of photosynthesis, cells use carbon dioxide and energy from the Sun to make sugar molecules and oxygen. These sugar molecules are the basis for more complex molecules made by the photosynthetic cell, such as glucose. Then, via respiration processes, cells use oxygen and glucose to synthesize energy-rich carrier molecules, such as ATP, and carbon dioxide is produced as a waste product. Therefore, the synthesis of glucose and its breakdown by cells are opposing processes.

However, photosynthesis doesn't just drive the carbon cycle — it also creates the oxygen necessary for respiring organisms. Interestingly, although green plants contribute much of the oxygen in the air we breathe, phytoplankton and cyanobacteria in the world's oceans are thought to produce between one-third and one-half of atmospheric oxygen on Earth.

What Cells and Organelles Are Involved in Photosynthesis?

Chlorophyll A is the major pigment used in photosynthesis, but there are several types of chlorophyll and numerous other pigments that respond to light, including red, brown, and blue pigments. These other pigments may help channel light energy to chlorophyll A or protect the cell from photo-damage. For example, the photosynthetic protists called dinoflagellates, which are responsible for the "red tides" that often prompt warnings against eating shellfish, contain a variety of light-sensitive pigments, including both chlorophyll and the red pigments responsible for their dramatic coloration.

What Are the Steps of Photosynthesis?

Photosynthesis consists of both light-dependent reactions and light-independent reactions . In plants, the so-called "light" reactions occur within the chloroplast thylakoids, where the aforementioned chlorophyll pigments reside. When light energy reaches the pigment molecules, it energizes the electrons within them, and these electrons are shunted to an electron transport chain in the thylakoid membrane. Every step in the electron transport chain then brings each electron to a lower energy state and harnesses its energy by producing ATP and NADPH. Meanwhile, each chlorophyll molecule replaces its lost electron with an electron from water; this process essentially splits water molecules to produce oxygen (Figure 5).

Once the light reactions have occurred, the light-independent or "dark" reactions take place in the chloroplast stroma. During this process, also known as carbon fixation, energy from the ATP and NADPH molecules generated by the light reactions drives a chemical pathway that uses the carbon in carbon dioxide (from the atmosphere) to build a three-carbon sugar called glyceraldehyde-3-phosphate (G3P). Cells then use G3P to build a wide variety of other sugars (such as glucose) and organic molecules. Many of these interconversions occur outside the chloroplast, following the transport of G3P from the stroma. The products of these reactions are then transported to other parts of the cell, including the mitochondria, where they are broken down to make more energy carrier molecules to satisfy the metabolic demands of the cell. In plants, some sugar molecules are stored as sucrose or starch.

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This multipart animation series explores the process of photosynthesis and the structures that carry it out.

Photosynthesis converts light energy from the sun into chemical energy stored in organic molecules, which are used to build the cells of many producers and ultimately fuel ecosystems. After providing an overview of photosynthesis, these animations zoom inside the cells of a leaf and into a chloroplast to see where and how the reactions of photosynthesis happen. The animations detail both the light reactions and the Calvin cycle, focusing on the flow of energy and the cycling of matter.

This animation series contains seven parts, which can be watched individually or in sequence. The first three parts are appropriate for middle school through college-level students. The remaining parts are appropriate for high school through college-level students; Parts 5 and 6 are recommended for more advanced students. Depending on students’ background, it may be helpful to pause the animations at various points to discuss different steps or structures.

The accompanying “Student Worksheet” incorporates concepts and information from the animations. The animations are also available in a YouTube playlist or as a full-length YouTube video .

The “Resource Google Folder” link directs to a Google Drive folder of resource documents in the Google Docs format. Not all downloadable documents for the resource may be available in this format. The Google Drive folder is set as “View Only”; to save a copy of a document in this folder to your Google Drive, open that document, then select File → “Make a copy.” These documents can be copied, modified, and distributed online following the Terms of Use listed in the “Details” section below, including crediting BioInteractive.  

Student Learning Targets

• Summarize the overall purpose of photosynthesis, as well as its inputs and outputs.
• Describe the structures used to perform photosynthesis in plants. 
• Describe the main components of the light reactions and Calvin cycle, and how they contribute to photosynthesis.  

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ATP, Calvin cycle, chlorophyll, chloroplast, electron transport chain, G3P, light reactions, NADPH, photosystem, rubisco

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8.2.1 Draw and label a diagram showing the structure of a chloroplast as seen in electron micrographs.

Figure 8.2.1 - Chloroplast

8.2.2 State that photosynthesis consists of light-dependent and light- independent reactions.

Photosynthesis consists of light-dependent and light-independent reactions.

8.2.3 Explain the light-dependent reactions.

Photosynthesis occurs inside chloroplasts. Chloroplasts contain chlorophyll, a green pigment found inside the thylakoid membranes. These chlorophyll molecules are arranged in groups called photosystems. There are two types of photosystems, Photosystem II and Photosystem I. When a chlorophyll molecule absorbs light, the energy from this light raises an electron within the chlorophyll molecule to a higher energy state. The chlorophyll molecule is then said to be photoactivated. Excited electron anywhere within the photosystem are then passed on from one chlorophyll molecule to the next until they reach a special chlorophyll molecule at the reaction centre of the photosystem. This special chlorophyll molecule then passes on the excited electron to a chain of electron carriers. 

The light-dependent reactions starts within Photosystem II. When the excited electron reaches the special chlorophyll molecule at the reaction centre of Photosystem II it is passed on to the chain of electron carriers. This chain of electron carriers is found within the thylakoid membrane. As this excited electron passes from one carrier to the next it releases energy. This energy is used to pump protons (hydrogen ions) across the thylakoid membrane and into the space within the thylakoids. This forms a proton gradient. The protons can travel back across the membrane, down the concentration gradient, however to do so they must pass through ATP synthase. ATP synthase is located in the thylakoid membrane and it uses the energy released from the movement of protons down their concentration gradient to synthesise ATP from ADP and inorganic phosphate. The synthesis of ATP in this manner is called non-cyclic photophosphorylation (uses the energy of excited electrons from photosystem II) .

The electrons from the chain of electron carriers are then accepted by Photosystem I. These electrons replace electrons previously lost from Photosystem I. Photosystem I then absorbs light and becomes photoactivated. The electrons become excited again as they are raised to a higher energy state. These excited electrons then pass along a short chain of electron carriers and are eventually used to reduce NADP +  in the stroma.  NADP +  accepts two excited electrons from the chain of carriers and one H +  ion from the stroma to form NADPH. 

If the light intensity is not a limiting factor, there will usually be a shortage of NADP +  as NADPH accumulates within the stroma (see light independent reaction). NADP +  is needed for the normal flow of electrons in the thylakoid membranes as it is the final electron acceptor. If NADP +  is not available then the normal flow of electrons is inhibited. However, there is an alternative pathway for ATP production in this case and it is called cyclic photophosphorylation. It begins with Photosystem I absorbing light and becoming photoactivated. The excited electrons from Photosystem I are then passed on to a chain of electron carriers between Photosystem I and II. These electrons travel along the chain of carriers back to Photosystem I and as they do so they cause the pumping of protons across the thylakoid membrane and therefore create a proton gradient. As explained previously, the protons move back across the thylakoid membrane through ATP synthase and as they do so, ATP is produced. Therefore, ATP can be produced even when there is a shortage of NADP + . 

In addition to producing NADPH, the light dependent reactions also produce oxygen as a waste product. When the special chlorophyll molecule at the reaction centre passes on the electrons to the chain of electron carriers, it becomes positively charged. With the aid of an enzyme at the reaction centre, water molecules within the thylakoid space are split. Oxygen and H +  ions are formed as a result and the electrons from the splitting of these water molecules are given to chlorophyll. The oxygen is then excreted as a waste product. This splitting of water molecules is called photolysis as it only occurs in the presence of light.

8.2.4 Explain photophosphorylation in terms of chemiosmosis.

Photophosphorylation is the production of ATP using the energy of sunlight. Photophosphorylation is made possible as a result of chemiosmosis. Chemiosmosis is the movement of ions across a selectively permeable membrane, down their concentration gradient. During photosynthesis, light is absorbed by chlorophyll molecules. Electrons within these molecules are then raised to a higher energy state. These electrons then travel through Photosystem II, a chain of electron carriers and Photosystem I. As the electrons travel through the chain of electron carriers, they release energy. This energy is used to pump hydrogen ions across the thylakoid membrane and into the space within the thylakoid. A concentration gradient of hydrogen ions forms within this space. These then move back across the thylakoid membrane, down their concentration gradient through ATP synthase. ATP synthase uses the energy released from the movement of hydrogen ions down their concentration gradient to synthesise ATP from ADP and inorganic phosphate.

8.2.5 Explain the light-independent reactions.

The light-independant reactions of photosynthesis occur in the stroma of the chloroplast and involve the conversion of carbon dioxide and other compounds into glucose. The light-independent reactions can be split into three stages, these are carbon fixation, the reduction reactions and finally the regeneration of ribulose bisphosphate. Collectively these stages are known as the Calvin Cycle. 

During carbon fixation, carbon dioxide in the stroma (which enters the chloroplast by diffusion) reacts with a five-carbon sugar called ribulose bisphosphate (RuBP) to form a six-carbon compound. This reaction is catalysed by an enzyme called ribulose bisphosphate carboxylase (large amounts present within the stroma), otherwise known as rubisco. As soon as the six-carbon compound is formed, it splits to form two molecules of glycerate 3-phosphate. Glycerate 3-phosphate is then used in the reduction reactions.

Glycerate 3-phosphate is reduced during the reduction reactions to a three-carbon sugar called triose phosphate. Energy and hydrogen is needed for the reduction and these are supplied by ATP and NADPH + H +  (both produced during light-dependent reactions) respectively. Two triose phosphate molecules can then react together to form glucose phosphate. The condensation of many molecules of glucose phosphate forms starch which is the form of carbohydrate stored in plants. However, out of six triose phosphates produced during the reduction reactions, only one will be used to synthesise glucose phosphate. The five remaining triose phosphates will be used to regenerate RuBP. 

The regeneration of RuBP is essential for carbon fixation to continue. Five triose phosphate molecules will undergo a series of reactions requiring energy from ATP, to form three molecules of RuBP. RuBP is therefore consumed and produced during the light-independent reactions and therefore these reactions form a cycle which is named the Calvin cycle.

8.2.6 Explain the relationship between the structure of the chloroplast and its function.

The stroma - Contains many enzymes, including rubisco, which are important for the reactions of the Calvin cycle.

The thylakoids - Have a large surface area for light absorption and the space within them allows rapid accumulation of protons.

8.2.7 Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants.

The action spectrum of photosynthesis is a graph showing the rate of photosynthesis for each wavelength of light. The rate of photosynthesis will not be the same for every wavelength of light. The rate of photosynthesis is the least with green-yellow light (525 nm-625 nm). Red-orange light (625nm-700nm) shows a good rate of photosynthesis however the best rate of photosynthesis is seen with violet-blue light (400nm-525nm). 

An absorption spectrum is a graph showing the percentage of light absorbed by pigments within the chloroplast, for each wavelength of light.  An example is the absorption spectrum of chlorophyll a and b. The best absorption is seen with violet-blue light. There is also good absorption with red-orange light. However most of the green-yellow light is reflected and therefore not absorbed. This wavelength of light shows the least absorption. 

As we can see, there is a close relationship between the action spectrum and absorption spectrum of photosynthesis. There are many different types of photosynthetic pigments which will absorb light best at different wavelengths. However the most abundant photosynthetic pigment in plants is chlorophyll and therefore the rate of photosynthesis will be the greatest at wavelengths of light best absorbed by chlorophyll (400nm-525nm corresponding to violet-blue light). Very little light is absorbed by chlorophyll at wavelengths of light between 525nm and 625 (green-yellow light) so the rate of photosynthesis will be the least within this range. However, there are other pigments that are able to absorb green-yellow light such as carotene. Even though these are present in small amounts they allow a low rate of photosynthesis to occur at wavelengths of light that chlorophyll cannot absorb.

8.2.8 Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide.

A limiting factor is a factor that controls a process. Light intensity, temperature and carbon dioxide concentration are all factors which can control the rate of photosynthesis. Usually, only one of these factors will be the limiting factor in a plant at a certain time. This is the factor which is the furthest from its optimum level at a particular point in time. If we change the limiting factor the rate of photosynthesis will change but changes to the other factors will have no effect on the rate. If the levels of the limiting factor increase so that this factor is no longer the furthest from its optimum level, the limiting factor will change to the factor which is at that point in time, the furthest from its optimum level. For example, at night the limiting factor is likely to be the light intensity as this will be the furthest from its optimum level. During the day, the limiting factor is likely to switch to the temperature or the carbon dioxide concentration as the light intensity increases. 

So how can these factors have an effect on the rate of photosynthesis? Lets start off with the light intensity. When the light intensity is poor, there is a shortage of ATP and NADPH, as these are products from the light dependent reactions. Without these products the light independent reactions can't occur as glycerate 3-phosphate cannot be reduced. Therefore a shortage of these products will limit the rate of photosynthesis. When the carbon dioxide concentration is low, the amount of glycerate 3-phosphate produced is limited as carbon dioxide is needed for its production and therefore the rate of photosynthesis is affected. Finally, many enzymes are involved during the process of photosynthesis. At low temperatures these enzymes work slower. At high temperatures the enzymes no longer work effectively. This affects the rate of the reactions in the Calvin cycle and therefore the rate of photosynthesis will be affected.

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Chloroplast Function, Definition, and Diagram

Chloroplasts are cellular organelles that are responsible for the process of photosynthesis . They are the reason Earth is a flourishing, green planet that supports diverse life forms.

Chloroplast Definition

A chloroplast is a type of organelle known as a plastid, predominantly found in plant cells and algae. It is the site of photosynthesis, a process where light energy is converted into chemical energy , fueling the organism’s activities.

Discovery and Word Origin

The discovery of chloroplasts dates back to the 19th century, with early microscopic observations made by botanists such as Julius von Sachs. The term “chloroplast” comes from the Greek words “chloros,” meaning green, and “plastes,” meaning formed or molded, highlighting their characteristic green color and intricate structure.

Organisms with Chloroplasts

Chloroplasts are primarily found in plants and algae. They are absent in animals and fungi .

Some types of lichens contain chloroplasts. Chloroplasts are present when the photosynthetic partner is green algae, but not when cyanobacteria are the partner.

A few types of protozoa and animals contain chloroplasts, but they acquired them through a process known as secondary endosymbiosis. This process involves a eukaryotic host cell engulfing a photosynthetic eukaryotic cell. The primary examples of such organisms are:

• Euglenoids: Euglena, a well-known genus in this group, contains chloroplasts derived from green algae. These single-celled organisms live in freshwater and have both plant-like (photosynthetic) and animal-like (motile and heterotrophic) characteristics.
• Dinoflagellates: Dinoflagellates are a group of mainly marine plankton. Some species have chloroplasts originating from green algae and diatoms.
• Apicomplexans: This is a bit of an unusual case. Most apicomplexans, like the malaria parasite Plasmodium, are not photosynthetic. However, they retain a vestigial organelle called the apicoplast, which comes from chloroplasts. The apicoplast does not perform photosynthesis but it is essential for other vital cellular functions.
• Certain Marine Animals: There are rare instances where marine animals incorporate chloroplasts into their cells in a process called kleptoplasty. For example, sea slugs like Elysia chlorotica consume algae and then retain the chloroplasts within their own cells. While the chloroplasts perform photosynthesis for some time, the integration is not permanent, so the sea slugs must regularly consume algae to maintain their photosynthetic capability.

Location and Number in a Cell

Chloroplasts are a cell’s cytoplasm. The number varies greatly, ranging from one large chloroplast in some algae to hundreds in a single leaf cell of a higher plant, depending on the species and environmental conditions.

Structure of a Chloroplast

Chloroplasts have a lens shape in plants, although they have different shapes in algae, like a cup, a net, or a spiral. A typical chloroplast size is 3-10 μm in diameter and 1–3 μm thick. Each chloroplast contains at least three membrane systems: the outer membrane, inner membrane, and thylakoid system. A chloroplast’s structure is complex, comprising several distinct components:

• Outer Membrane: The outer membrane is a semi-permeable barrier that encases the organelle.
• Inner Membrane: The inner membrane is located just inside the outer membrane. It regulates material entry and exit.
• Stroma: The stroma is a fluid-filled space inside the outer and inner membranes containing enzymes, ribosomes, and DNA . The thylakoid system floats within the stroma. The Calvin cycle occurs in the stroma.
• Thylakoid Membrane (Thylakoids): The thylakoid membrane consists of a system of interconnected membranes where the light-dependent reactions of photosynthesis occur. There are two types of thylakoids. Granal thylakoids are the pancake-like stacks, while stromal thylakoids are helical sheets that wrap around the grana.
• Grana: Grana (singular: granum) are stacks of disc-like structures formed by thylakoid membranes. Each granum has between two to a hundred thylakoids, although stacks of 10-20 thylakoids are common.
• Thylakoid Space: The thylakoid space or lumen is the interior of the granum. It contains proteins that drive the electron transport chain in photosynthesis.
• Lamellae: Lamellae are membrane bridges connecting the grana.
• DNA: Chloroplast DNA is packaged into nucleoids in the stroma. Each organelle may contain many nucleoids.
• Ribosomes: The ribosomes in the stroma of a chloroplast are smaller than those in the cell’s cytoplasm. They synthesize some of the chloroplast’s proteins.
• Starch Granules: Most chloroplasts contain starch granules. Starch granules account for up to 15% of a chloroplast’s volume. They accumulate in the stroma and grow in size during the daytime. Some hornworts and algae contain pyrenoids, which are structures that serve as the site of starch accumulation.
• Plastoglobuli: Plastoglobuli (singular: plastoglobulus) are spheres of proteins and lipids. While they occur in all chloroplasts, they are more common in older organelles or ones under oxidative stress.

Functions of Chloroplasts

The primary function of chloroplasts is photosynthesis, comprising two stages: the light-dependent reactions occurring in the thylakoids, and the light-independent Calvin Cycle happening in the stroma. They also play roles in fatty acid synthesis, amino acid synthesis, and the immune response. Chloroplasts also act as sensors for gravity and defense functions.

Pigments in Chloroplasts

Chlorophyll is the pigment responsible for the green color of plants and algae and the key molecule involved in photosynthesis. However, chloroplasts contain several pigments besides chlorophyll, which play crucial roles in photosynthesis and in protecting the cell from damage caused by sunlight. These pigments include:

• Carotenoids: These are yellow, orange, and red pigments that serve multiple functions. They absorb light energy for use in photosynthesis. They also provide photoprotection by dissipating excess light energy that could otherwise damage chlorophyll or interact with oxygen to produce harmful reactive oxygen species. Carotenoids include compounds like beta-carotene and xanthophylls.
• Phycobilins: Found in the chloroplasts of red algae and cyanobacteria, phycobilins are water-soluble pigments that are present in phycobiliproteins. These pigments, which include phycocyanin and phycoerythrin, absorb different wavelengths of light than chlorophyll. They extend the range of light that can be used for photosynthesis.
• Accessory Pigments: These are additional pigments that help in capturing light energy. They transfer the energy to chlorophyll for the photosynthetic process. While not directly involved in the conversion of light energy into chemical energy, they are essential for efficient photosynthesis, especially under low-light conditions or in water, where light penetration is different from that on land.

Comparison with Other Plastids

Chloroplasts are a type of plastid that are distinct from others like chromoplasts (responsible for pigment synthesis and storage) and leucoplasts (involved in storage and biosynthesis of various molecules). Unlike these other plastids, chloroplasts contain the pigment chlorophyll, essential for photosynthesis.

Comparison with Mitochondria

Both mitochondria and chloroplasts are organelles that have their own DNA and likely originated from endosymbiotic events, but they serve different functions. While chloroplasts are the centers of photosynthesis, mitochondria are the powerhouses of the cell, responsible for cellular respiration. Both organelles occur in plant cells.

Theories of Chloroplast Evolution

The prevailing theory of chloroplast evolution is the endosymbiotic theory. It suggests that chloroplasts originated from photosynthetic bacteria living symbiotically inside eukaryotic cells. This theory is supported by the presence of their own DNA, double membrane, and similarities to cyanobacteria.

Interesting Chloroplast Facts

Here are some interesting and useful facts about chloroplasts:

• Dynamic Movement within Cells: Chloroplasts are not static within cells. They move in response to light intensity in a phenomenon known as chloroplast photorelocation movement. In low light, they spread out to maximize light absorption, while in intense light, they align along cell walls to minimize damage from excessive light.
• Role in Gravity Perception: In some plant cells, especially in the root cap, chloroplasts perform gravity sensing. They settle at the bottom of cells, which helps the plant determine its growth direction.
• Environmental Stress Response: Chloroplasts play a critical role in the plant’s response to environmental stresses. They signal to the nucleus to change the expression of certain genes in response to factors like drought, temperature changes, and light stress.
• Chloroplast DNA: The DNA within chloroplasts is circular, similar to bacterial DNA. It encodes some of the essential proteins and enzymes needed for photosynthesis. This DNA is inherited maternally in most plants, meaning it’s passed down from the mother plant to its offspring.
• Secondary Metabolite Synthesis: Chloroplasts play a role in the synthesis of secondary metabolites, which are important for plant defense, such as flavonoids and terpenoids.
• Ancient Origins: Fossil evidence suggests that early plants contained chloroplasts over a billion years ago.
• Size and Shape Variability: The size and shape of chloroplasts varies between different species and even within different tissues of the same plant. Also, not all plant cells contain chloroplasts.
• Chloroplast Genome Reduction: Over evolutionary time, many chloroplast genes have transferred to the nucleus of the host cell, significantly reducing the size of the chloroplast genome.
• Non-Photosynthetic Chloroplasts: Some plant cells contain chloroplasts that do not perform photosynthesis, especially in roots and non-green tissues. These chloroplasts participate in other biochemical pathways, such as amino acid and fatty acid synthesis.
• Alberts, B. (2002). Molecular Biology of the Cell (4th ed.). New York: Garland. ISBN 0-8153-4072-9.
• Burgess, J. (1989). An Introduction to Plant Cell Development . Cambridge: Cambridge University Press. ISBN 0-521-31611-1.
• Campbell, N.A.; Reece, J.B.; et al. (2009). Biology (8th ed.). Benjamin Cummings (Pearson). ISBN 978-0-8053-6844-4.
• Hoober, J.K. (1984). Chloroplasts . New York: Plenum. ISBN 9781461327677.
• Nakayama, T.; Archibald, J.M. (2012). “Evolving a photosynthetic organelle”. BMC Biology . 10 (1): 35. doi: 10.1186/1741-7007-10-35

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The Process of Photosynthesis in Plants (With Diagram)

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The Process of Photosynthesis in Plants!

Introduction:

Life on earth ultimately depends on energy derived from sun. Photosynthesis is the only process of biological importance that can harvest this energy.

Literally photosynthesis means ‘synthesis using light’. Photosynthetic organisms use solar energy to synthesize carbon compound that cannot be formed without the input of the energy.

Photosynthesis (Photon = Light, Synthesis = Putting together) is an anabolic, endergonic process by which green plant synthesize carbohydrates (initially glucose) requiring carbon dioxide, water, pigments and sunlight. In other words, we can say that photosynthesis is transformation of solar energy/radiant energy/light energy (ultimate source of energy for all living organisms) into chemical energy.

Simple general equation of photo synthesis is as follows:

According to Van Neil and Robert Hill, oxygen liberated during photosynthesis comes from water and not from carbon dioxide.

Thus, the overall correct biochemical reaction for photosynthesis can be written as:

Some photosynthetic bacteria use hydrogen donor other than water. Therefore, photosynthesis is also defined as the anabolic process of manufacture of organic compounds inside the chlorophyll containing cells from carbon dioxide and hydrogen donor with the help of radiant energy.

Significance of Photosynthesis:

1. Photosynthesis is the most important natural process which sustains life on earth.

2. The process of photosynthesis is unique to green and other autotrophic plants. It synthesizes organic food from inorganic raw materials.

3. All animals and heterotrophic plants depend upon the green plants for their organic food, and therefore, the green plants are called producers, while all other organisms are known as consumers.

4. Photosynthesis converts radiant or solar energy into chemical energy. The same gets stored in the organic food as bonds between different atoms. Photosynthetic products provide energy to all organisms to carry out their life activities (all life is bottled sunshine).

5. Coal, petroleum and natural gas are fossil fuels which have been produced by the application of heat and compression on the past plant and animal parts (all formed by photosynthesis) in the deeper layers of the earth. These are extremely important source of energy.

6. All useful plant products are derived from the process of photosynthesis, e.g., timber, rubber, resins, drugs, oils, fibers, etc.

7. It is the only known method by which oxygen is added to the atmosphere to compensate for oxygen being used in the respiration of organisms and burning of organic fuels. Oxygen is important in (a) efficient utilization and complete breakdown of respiratory substrate and (b) formation of ozone in stratosphere that filters out and stops harmful UV radiations in reaching earth.

8. Photosynthesis decreases the concentration of carbon dioxide which is being added to the atmosphere by the respiration of organisms and burning of organic fuels. Higher concentration of carbon dioxide is poisonous to living beings.

9. Productivity of agricultural crops depends upon the rate of photosynthesis. Therefore, scientists are busy in genetically manipulating the crops.

Magnitude of Photosynthesis:

Only 0.2% of light energy falling on earth is utilized by photosynthetic organisms. The total carbon dioxide available to plants for photosynthesis is about 11.2 x 10 14 tonnes. Out of this only 2.2 x 10 13 tonnes are present in the atmosphere @ 0.03%. Oceans contain 11 x 10 14 (110,000 billion) tonnes of carbon dioxide.

About 70 to 80 billion tonnes of carbon dioxide are fixed annually by terrestrial and aquatic autotrophs and it produces near about 1700 million tonnes of dry organic matter. Out of these 10% (170 million tonnes) of dry matter is produced by land plants and rest by ocean (about 90%). This is an estimate by Robinowitch (1951),According to more recent figures given by Ryther and Woodwell (1970) only 1/3 of total global photosynthesis can be attributed to marine plants.

Historical Background:

Functional Relationship between Light and Dark Reactions :

During photosynthesis water is oxidized and carbon dioxide is reduced, but where in the over­all process light energy intervenes to drive the reaction. However, it is possible to show that photo­synthesis consists of a combination of light-requiring reactions (the “light reactions”) and non-light requiring reactions (the “dark reactions”).

It is now clear that tall the reactions for the incorporation of CO 2 into organic materials (i.e., carbohydrate) can occur in the dark (the “dark reactions”). The reactions dependent on light (the “light reactions”) are those in which radiant energy is converted into chemical energy.

According to Arnon, the functional relationship between the “light” and “dark” reactions can be established by examining the requirements of the dark reactions. The “dark reactions” comprise a complex cycle of enzyme-mediated reactions (the Calvin Cycle) which catalyzes the reduction of car­bon dioxide to sugar. This cycle requires reducing power in the form of reduced nicotinamide adenine dinucleotide phosphate (NADPH) and chemical energy in the form of adenosine triphosphate (ATP).

The reduced NADP (NADPH) and ATP are produced by the “light reactions”. It is thus possible to divide a description of photosynthesis into those reactions associated with the Calvin cycle and the fixation of carbon dioxide, and those reactions (i.e., capture of light by pigments, electron transport, photophosphorylation) which are directly driven by light.

Site of Photosynthesis :

Chloroplast (Fig. 6.2) in green plants constitute the photosynthetic apparatus and act as site of photosynthesis. Chloroplasts of higher plants are discoid or ellipsoidal in shape measuring 4 —6 μ in length and 1—2 μ in thickness. It is a double membranous cytoplasmic organelle of eukaryotic green plant cells. The thickness of the two membranes including periplastidial space is approximately 300Å.

Ground substance of chloroplast is filled with a hydrophilic matrix known as stroma. It contains cp-DNA (0.5%), RNA (2—3%), Plastoribosome (70S), enzymes for carbon dioxide assimilation, proteins (50—60%), starch grains and osmophilic droplets, vitamin E and K, Mg, Fe, Mn, P, etc. in traces. In stroma are embedded a number of flattened membranous sacs known as thylakoids. Photosynthetic pigments occur in thylakoid membranes.

Aggregation of thylakoids to form stacks of coin like struc­tures known as granna. A grannum consists near about 20 — 30 thylakoids. Each thylakoid encloses a space known asloculus. The end of disc shape thylakoid is called as margin and the area where the thylakoids membranes are appressed together is called partition.

Some of the granna lamella are connected with thylakoids of other granna by stroma lamella or fret membranes. Thylakoid mem­brane and stroma lamella both are composed of lipid and proteins. In photosynthetic prokaryotes (blue-green algae and Bacteria) chloroplast is absent. Chromatophore is present in photosynthetic bacteria and photosynthetic lamellae in blue-green algae.

Mechanism of Photosynthesis :

Photosynthesis is an oxidation reduction process in which water is oxidized and carbon dioxide is reduced to carbohydrate.

Blackmann (1905) pointed out that the process of photosynthesis consists of two phases:

(1) Light reaction or Light phase or Light-dependent phase or Photochemical phase

(2) Dark reaction or Dark phase or Light independent phase or Biochemical phase.

During light reaction, oxygen is evolved and assimilatory power (ATP and NADPH 2 ) are formed. During dark reaction assimilatory power is utilized to synthesize glucose.

(i) Oxygenic photosynthesis (with evolution of O 2 ) takes place in green eukaryotes and cyanobacteria (blue-green algae).

(ii) An oxygenic photosynthesis (without the evolution of O 2 ) takes place in photosynthetic bacteria.

Photosynthetic Pigments:

Photosynthetic pigments are substances that absorb sunlight and initiate the process of photo­synthesis.

Photosynthetic pigments are grouped into 3 categories:

(i) Chlorophyl l:

These are green coloured most abundant photosynthetic pigments that play a major role during photosynthesis. Major types of chlorophylls are known to exist in plants and photosynthetic bacteria viz., Chlorophyll a, b, c, d and e, Bacteriochlorophyll a, b and g, and Chlorobium chlorophyll (Bacterio viridin).

The structure of chlorophyll was first studied by Wilstatter, Stoll and Fischer in 1912. Chemically a chlorophyll molecule consists of a porphyrin head (15 x 15Å) and phytol tail (20Å). Porphyrin consists of tetrapyrrole rings and central core of Mg. Phytol tail is side chain of hydrocarbon. It is attach to one of the pyrrole ring. This chain helps the chlorophyll molecules to attach with thylakoid membrane.

Out of various types of chlorophyll, chlorophyll a and chlorophyll b are the most important for photosynthetic process. Chlorophyll a is found in all photosynthetic plants except photosynthetic bacteria. For this reason it is designated as Universal Photosynthetic Pigment or Primary Photosynthetic Pigment.

(ii) Carotenoids :

These are yellow, red or orange colour pigments embedded in thylakoid membrane in association with chlorophylls but their amount is less. These are insoluble in water and precursor of Vitamin A. These are of two of types viz., Carotene and Xanthophyll (Carotenol/Xanthol).

Carotenes are pure hydrocarbons, red or orange in colour and their chemical formula is – C 40 H 56 Some of the common carotenes are -α, β, γ and δ carotenes, Phytotene, Neurosporene, Lycopene (Red pigment found in ripe tomato). β—carotene on hydrolysis gives Vitamin A.

Xanthophylls are yellow coloured oxygen containing carotenoids and are most abundant in nature. The ratio of xanthophyll to carotene in nature is 2:1 in young leaves. The most common xanthophyll in green plant is Lutein (C 40 H 56 O 2 ) and it is responsible for yellow colour in autumn foliage. Both carotene and xanthophylls are soluble in organic solvents like chloroform, ethyl ether, carbondisulphide etc.

(iii) Phycobilins (Biliproteins) :

These are water soluble pigments and are abundantly present in algae, and also found in higher plants. There are two important types of phycobilins-Phycoerythrin (Red) and Phycocyanin (Blue). Like chlorophyll, these pigments are open tetrapyrrole but do not contain Mg and Phytol chain.

Nature of Light (Fig. 6.3 ):

The source of light for photosynthesis is sunlight. Sun Light is a form of energy (solar energy) that travels as a stream of tiny particles. Discrete particles present in light are called photons. They carry energy and the energy contained in a photon is termed as quantum. The energy content of a quantum is related to its wave length.

Shorter the wave length, the greater is the energy present in its quantum. Depending upon the wave length electro magnetic spectrum comprises cosmic rays, gamma rays, X-rays,-UV rays, visible spectrum, infra red rays, electric rays and radio waves.

The visible spectrum ranges from 390 nm to 760 nm (3900 – 7600A), however, the plant life is affected by wave length ranging from 300 – 780 nm. Visible spectrum can be resolved into light of different colours i.e., violet (390-430 nm), blue or indigo (430-470 nm), blue green (470-500 nm), green (500 – 580 nm), yellow (580 – 600 nm), orange (600 – 650 nm), orange red (650 – 660 nm) and red (660 – 760 nm). Red light above 700 nm is called far red. Radiation shorter than violet are UV rays (100 – 390 nm). Radiation longer than those of red are called infra red (760 – 10,000 nm).

A ray of light falling upon a leaf behaves in 3 different ways. Part of it is reflected, a part transmitted and a part absorbed. The leaves absorb near about 83% of light, transmit 5% and reflect 12%. From the total absorption, 4% light is absorbed by the chlorophyll. Engelmann (1882) performed an experiment with the freshwater, multicellular filamentous green alga spirogyra.

In a drop of water having numerous aerobic bacteria, the alga was exposed to a narrow beam of light passing through a prism. The bacte­ria after few minutes aggregated more in that re­gions which were exposed to blue and red wave length. It confirms that maximum oxygen evolu­tion takes place in these regions due to high photosynthetic activities.

Absorption Spectrum :

All photosynthetic organisms contain one or more organic pigments capable of absorbing visible radiation which will initiate the photochemical reactions of photosynthesis. When the amount of light absorbed by a pigment is plotted as a function of wave length, we obtain absorption spectrum (Fig. 6.4).

It varies from pigment to pigment. By passing light of specific wave length through a solution of a substance and measuring the fraction absorbed, we obtain the absorption spectrum of that substance. Each type of molecules have a characteristic absorption spectrum, and measuring the absorption spectrum can be useful in identifying some unknown substance isolated from a plant or animal cell.

Action Spectrum :

It represents the extent of response to different wave lengths of light in photosynthesis. It can also be defined as a measure of the process of photosynthesis when a light of different wave lengths is supplied but the intensity is the same. For photochemical reactions involving single pigment, the action spectrum has same general shape as the absorption spectrum of that pigment, otherwise both are quite distinct (Fig. 6.5).

Quantum Requirement and Quantum Yield:

The solar light comes to earth in the form of small packets of energy known as photons. The energy associated with each photon is called Quantum. Thus, requirement of solar light by a plant is measured in terms of number of photons or quanta.

The number of photons or quanta required by a plant or leaf to release one molecule of oxygen during photosynthesis is called quantum requirement. It has been observed that in most of the cases the quantum requirement is 8.

It means that 8 photons or quantum’s are required to release one molecule of oxygen. The number of oxygen molecules released per photon of light during photosynthesis is called Quantum yield. If the quantum requirement is 8 then quantum yield will be 0.125 (1/8).

Photosynthetic Unit or Quantasome:

It is defined as the smallest group of collaborating pigment molecules necessary to affect a photochemical act i.e., absorption and migration of a light quantum to trapping centre where it promotes the release of an electron.

Emmerson and Arnold (1932) on the basis of certain experiments assumed that about 250 chlorophyll molecules are required to fix one molecule of carbon dioxide in photosynthesis. This number of chlorophyll molecules was called the chlorophyll unit but the name was subsequently changed to photosynthetic unit and later it was designated as Quantasome by Park and Biggins (1964).

The size of a quantasome is about 18 x 16 x l0nm and found in the membrane of thylakoids. Each quantasome consists of 200 – 240 chlorophyll (160 Chlorophyll a and 70 – 80 Chlorophyll b), 48 carotenoids, 46 quinone, 116 phospholipids, 144 diagalactosyl diglyceride, 346 monogalactosyl diglyceride, 48 sulpholipids, some sterols and special chlorophyll molecules (P 680 and P 700 ).

‘P’ is pigment, 680 and 700 denotes the wave length of light these molecule absorb. Peso and P 700 constitute the reaction centre or photo centre. Other accessory pigments and chlorophyll molecules are light gatherers or antenna molecules. It capture solar energy and transfer it to the reaction centre by resonance transfer or inductive resonance.

Photoluminescence :

It is the phenomenon of re-radiation of absorbed energy. It is of two types:

(1) Fluorescence and

(2) Phosphorescence.

The normal state of the molecule is called as ground state or singlet state. When an electron of a molecule absorbs a quantum of light it is raised to a higher level of energy a state called Excited Second Singlet State. From first singlet state excited electron may return to the ground state either losing its extra energy in the form of heat or by losing energy in the form of radiant energy. The later process is called fluorescence. The substance which can emit back the absorbed radiations is called fluorescent substance. All photosynthetic pigments have the property of fluorescence.

The excited molecule also losses its electronic excitation energy by internal conversion and comes to another excited state called triplet state. From this triplet state excited molecule may return to ground state in three ways-by losing its extra energy in the form of heat, by losing extra energy in the form of radiant energy is called phosphorescence. The electron carrying extra energy may be expelled from the molecule and is consumed in some other chemical reactions and a fresh normal electron returns to the molecule. This mechanism happens in chlorophyll a (Universal Photosynthetic Pigment).

Emerson Red Drop Effect and Enhancement Effect :

R. Emerson and Lewis (1943) while determining the quantum yield of photosynthesis in Chlorella by using monochromatic light of different wave lengths noticed a sharp decrease in quantum yield at wave length greater than 680 mμ.This decrease in quantum yield took place in the far red part of the spectrum i.e., the curve shows quantum yield drops dramatically in the region above 680 nm (Red region). This decline in photosynthesis is called Red drop effect (Emerson’s first experiment).

Emerson and his co-workers (1957) found that the inefficient far red light in Chlorella beyond 680nm could be made fully efficient if supplemented with light of short wave length. The quantum yield from the two combined beams was found to be greater than the effect of both beams when used separately. This enhancement of photosynthesis is called Emerson Enhancement Effect (Emerson’s second experiment) (Fig. 6.6).

Rate of oxygen evolution in combined beam – Rate of oxygen evolution in red beam/Rate of oxygen evolution in far red beam

E = Emerson effect.

Light Trapping Centres (PSI & PSII) :

The discovery of red drop effect and the Emerson’s enhancement effect concluded in a new concept about the role played bychlorophyll-a and accessary pigments in photosynthesis that photo­synthesis involves two distinct photochemical processes. These processes are associated with two groups of photosynthetic pigments called as Pigment system I (Photoact I or Photosystem I) and Pigment system II (Photoact II or Photosystem II).

Each pigment system consists of a central core complex and light harvesting complex (LHC). LHC comprises antenna pigments associated with proteins (viz.., antenna complex). Their main function is to harvest light energy and transfer it to their respective reaction centre. The core complex consists of reaction centre associated with proteins and also electon donors and acceptors.

Wave length of light shorter than 680 nm affect both the pigment systems while wave length longer than 680 nm affect only pigment system I. PSI is found in thylakoid membrane and stroma lamella. It contains pigments chlorophyll a 660, chlorophyll a 670, chlorophyll a 680, chlorophyll a 690, chlorophyll a 700. Chlorophyll a 700 or P 700 is the reaction centre of PS I. PS II is found in thylakoid membrane and it contains pigments as chlorophyll b 650, chlorophyll a 660, chlorophyll a 670, chlorophyll a 678, chlorophyll a 680 – 690 and phycobillins.

P 680-690 is the reaction centre of PS II. Chlorophyll a content is more in PS I than PS II. Carotenoids are present both in PS II and PS I. PS I is associated with both cyclic and non-cyclic photophosphorylation, but PS II is associated with only non-cyclic photophosphorylation.

Both the pigment systems are believed to be inter-connected by a third integral protein complex called cytochrome b – f complex. The other intermediate components of electron transport chain viz., PQ (plasto quinone) and PC (plastocyanin) act as mobile electron carriers between two pigment systems. PS I is active in both red and far red light and PS II is inactive in far red light (Fig. 6.7).

Evidence in Support of Two Phases of Photosynthesis:

1. Physical Separation of Chloroplast into Granna and Stroma Fraction:

It is now possible to separate granna and stroma fraction of chloroplast. If light is given to granna fraction in the presence of suitable hydrogen acceptor and in complete absence of carbon dioxide then assimilatory power, ATP and NADPH 2 , are produced. If these assimilatory powers are given to stroma fraction in the presence of carbon dioxide and absence of light then carbohydrate is synthesized.

2. Temperature Coefficient (Q 10 ):

Q 10 is the ratio of the rate of reaction at a given temperature and a temperature 10°C lower. Q 10 value of photosynthesis is found to be two or three (for dark reaction) when photosynthesis is fast, but Q 10 is one (for light reaction) when photosynthesis is slow.

3. Evidence from Intermittent Light:

Warburg observed that when intermittent light (flashes of light) of about 1/16 seconds were given to green algae (Chlorella vulgaris and Scenedesmus obliquus), the photosynthetic yield per second was higher as compared to the continuous supply of same intensity of light. This confirms that one phase of photosynthesis is independent of light.

4. Evidence from Carbon dioxide in Dark:

It comes from tracer technique by the use of heavy carbon in carbon dioxide (C 14 O 2 ). The leaves which were first exposed to light have been found to reduce carbon dioxide in the dark It indicates that carbon dioxide is reduced to carbohydrate in dark and it is purely a biochemical phase.

I. Light Reaction (Photochemical Phase):

Light Reaction:

Light reaction or photochemical reaction takes place in thylakoid membrane or granum and it is completely dependent upon the light. The raw materials for this reactions are pigments, water and sunlight.

It can be discussed in the following three steps:

1. Excitation of chlorophyll

2. Photolysis of water

3. Photophosphorylation

1. Excitation of Chlorophyll:

It is the first step of light reaction. When P 680 or P 700 (special type of chlorophyll a) of two pigment systems receives quantum of light then it becomes excited and releases electrons.

2. Photolysis of Water and Oxygen Evolution (Hill Reaction):

Before 1930 it was thought that the oxygen released during photosynthesis comes from carbon dioxide. But for the first time Van Neil discovered that the source of oxygen evolution is not carbon dioxide but H 2 O. In his experiment Neil used green sulphur bacteria which do not release oxygen during photosynthesis. They release sulphur. These bacteria require H 2 S in place of H 2 O.

The idea of Van Neil was supported by R. Hill. Hill observed that the chloroplasts extracted from leaves of Stellaria media and Lamium album when suspended in a test tube containing suitable electron acceptors (Potassium feroxalate or Potassium fericyanide), Oxygen evolution took place due to photochemical splitting of water.

The splitting of water during photosynthesis is called Photolysis of water. Mn, Ca, and CI ions play prominent role in the photolysis of water. This reaction is also known as Hill reaction. To release one molecule of oxygen, two molecules of water are required.

The evolution of oxygen from water was also confirmed by Ruben, Randall, Hassid and Kamen (1941) using heavy isotope (O 18 ) in green alga Chlorella. When the photosynthesis is allowed to proceed with H 2 O 18 and normal CO 2 , the evolved oxygen contains heavy isotope. If photosynthesis is allowed to proceed in presence of CO 2 18 and normal water then heavy oxygen is not evolved.

Thus the fate of different molecules can be summarized as follows:

3. Photophosphorylation:

Synthesis of ATP from ADP and inorganic phosphate (pi) in presence of light in chloroplast is known as photophosphorylation. It was discovered by Arnon et al (1954).

Photophosphorylation is of two types.

(a) Cyclic photophosphorylation

(b) Non-cyclic photophosphorylation.

(a) Cyclic Photophosphorylation (Fig. 6.8) :

It is a process of photophosphorylation in which an electron expelled by the excited photo Centre (PSI) is returned to it after passing through a series of electron carriers. It occurs under conditions of low light intensity, wavelength longer than 680 nm and when CO 2 fixation is inhibited. Absence of CO 2 fixation results in non requirement of electrons as NADPH 2 is not being oxidized to NADP + . Cyclic photophosphorylation is performed by photosystem I only. Its photo Centre P 700 extrudes an electron with a gain of 23 kcal/mole of energy after absorbing a photon of light (hv).

After losing the electron the photo Centre becomes oxidized. The expelled electron passes through a series of carriers including X (a special chlorophyll molecule), FeS, ferredoxin, plastoquinone, cytochrome b- f complex and plastocyanin before returning to photo Centre. While passing between ferredoxin and plastoquinone and/or over the cytochrome complex, the electron loses sufficient energy to form ATP from ADP and inorganic phosphate.

Halobacteria or halophile bacteria also perform photophosphorylation but ATP thus produced is not used in synthesis of food. These bacteria possess purple pigment bacteriorhodopsin attached to plasma membrane. As light falls on the pigment, it creates a proton pump which is used in ATP synthesis.

(b) Noncyclic Photophosphorylation (Z-Scheme) (Fig. 6.9) :

It is the normal process of photophosphorylation in which the electron expelled by the excited photo Centre (reaction centre) does not return to it. Non-cyclic photophosphorylation is carried out in collaboration of both photo system I and II. (Fig. 6.9). Electron released during photolysis of water is picked up by reaction centre of PS-II, called P 680 . The same is extruded out when the reaction centre absorbs light energy (hv). The extruded electron has an energy equivalent to 23 kcal/mole.

It passes through a series of electron carriers— Phaeophytin, PQ, cytochrome b- f complex and plastocyanin. While passing over cytochrome complex, the electron loses sufficient energy for the synthesis of ATP. The electron is handed over to reaction centre P 700 of PS-I by plastocyanin. P 700 extrudes the electron after absorbing light energy.

The extruded electron passes through FRS ferredoxin, and NADP -reductase which combines it with NADP + for becoming reduced through H+ releasing during photolysis to form NADPH 2 . ATP synthesis is not direct. The energy released by electron is actually used for pumping H + ions across the thylakoid membrane. It creates a proton gradient. This gradient triggers the coupling factor to synthesize ATP from ADP and inorganic phosphate (Pi).

Chemiosmotic Hypothesis:

How actually ATP is synthesized in the chloroplast?

The chemiosmotic hypothesis has been put forward by Peter Mitchell (1961) to explain the mechanism. Like in respiration, in photosynthesis too, ATP synthesis is linked to development of a proton gradient across a membrane. This time these are membranes of the thylakoid. There is one difference though, here the proton accumulation is towards the inside of the membrane, i.e., in the lumen. In respiration, protons accumulate in the inter-membrane space of the mitochondria when electrons move through the ETS.

Let us understand what causes the proton gradient across the membrane. We need to consider again the processes that take place during the activation of electrons and their transport to determine the steps that cause a proton gradient to develop (Figure 6.9).

(b) As electrons move through the photosystems, protons are transported across the membrane. This happens because the primary accepter of electron which is located towards the outer side of the membrane transfers its electron not to an electron carrier but to an H carrier. Hence, this molecule removes a proton from the stroma while transporting an electron. When this molecule passes on its electron to the electron carrier on the inner side of the membrane, the proton is released into the inner side or the lumen side of the membrane.

(c) The NADP reductase enzyme is located on the stroma side of the membrane. Along with electrons that come from the acceptor of electrons of PS I, protons are necessary for the reduction of NADP + to NADPH+ H + .These protons are also removed from the stroma.

Hence, within the chloroplast, protons in the stroma decrease in number, while in the lumen there is accumulation of protons. This creates a proton gradient across the thylakoid membrane as well as a measurable decrease in pH in the lumen.

Why are we so interested in the proton gradient?

This gradient is important because it is the breakdown of this gradient that leads to release of energy. The gradient is broken down due to the movement of protons across the membrane to the stroma through the trans membrane channel of the F 0 of the ATPase. The ATPase enzyme consists of two parts: one called the F 0 is embedded in the membrane and forms a trans-membrane channel that carries out facilitated diffusion of protons across the membrane. The other portion is called F 1 and protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma.

The break down of the gradient provides enough energy to cause a conformational change in the F 1 particle of the ATPase, which makes the enzyme synthesis several molecules of energy-packed ATP. Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase. Energy is used to pump protons across a membrane, to create a gradient or a high concentration of protons within the thylakoid lumen.

ATPase has a channel that allows diffusion of protons back across the membrane; this releases enough energy to activate ATPase enzyme that catalyzes the formation of ATP. Along with the NADPH produced by the movement of electrons, the ATP will be used immediately in the biosynthetic reaction taking place in the stroma, responsible for fixing CO 2 , and synthesis of sugars.

Where are the ATP and NADPH Used ?

We have seen that the products of light reaction are ATP, NADPH and O 2 . Of these O 2 diffuses out of the chloroplast while ATP and NADPH are used to drive the processes leading to the synthesis of food, more accurately, sugars. This is the biosynthetic phase of photosynthesis.

This process does not directly depend on the presence of light but is dependent on the products of the light reaction, i.e., ATP and NADPH, besides CO 2 and H 2 O. You may wonder how this could be verified; it is simple: immediately after light becomes unavailable the biosynthetic process continues for some time, and then stops. If then, light is made available, the synthesis starts again.

Can we, hence, say that calling the biosynthetic phase as the dark reaction is a misnomer?

II. Dark Reaction (Biosynthetic Phase)-The Second Phase of Photosynthesis:

The pathway by which all photosynthetic eukaryotic organisms ultimately incorporate CO 2 into carbohydrate is known as carbon fixation or photosynthetic carbon reduction (PCR.) cycle or dark reactions. The dark reactions are sensitive to temperature changes, but are independent of light hence it is called dark reaction, however it depends upon the products of light reaction of photosynthesis, i.e., NADPH 2 and ATP.

The carbon dioxide fixation takes place in the stroma of chloroplasts because it has enzymes essential for fixation of CO 2 and synthesis of sugar. Dark reaction is the pathway by which CO 2 is reduced to sugar. Since CO 2 is an energy poor compound; its conversion to an energy-rich carbohydrate involves a sizable jump up the energy ladder. This is accomplished through a series of complex steps involving small bits of energy.

The CO 2 assimilation takes place both in light and darkness when the substrates NADPH 2 and ATP are available. Because of the need for NADPH 2 as a reductant and ATP as energy equivalent, CO 2 fixation is closely linked to the light reactions. During evolution three different ecological variants have evolved with different CO 2 incorporation mechanism: C 3 , C 4 and CAM plants.

Calvin or C 3 Cycle or PCR (Photosynthetic Carbon Reduction Cycle):

It is the basic mechanism by which CO 2 is fixed (reduced) to form carbohydrates. It was proposed by Melvin Calvin. Calvin along with A.A. Benson, J. Bassham used radioactive isotope of carbon (C 14 ) in Chlorella pyrenoidosa and Scenedesmus oblique’s to determine the sequences of dark reaction. For this work Calvin was awarded Nobel prize in 1961. To synthesize one glucose molecule Calvin cycle requires 6CO 2 , 18 ATP and 12 NADPH 2 .

Calvin cycle completes in 4 major phases:

1. Carboxylation phase

2. Reductive phase

3. Glycolytic reversal phase (sugar formation phase)

4. Regeneration phase

1. Carboxylation phase:

CO 2 enters the leaf through stomata. In mesophyll cells, CO 2 combines with a phosphorylated 5-carbon sugar, called Ribulose bisphosphate (or RuBP). This reaction is catalyzed by an enzyme, called RUBISCO. The reaction results in the formation of a temporary 6 carbon compound (2-carboxy 3-keto 1,5-biphosphorbitol) Which breaks down into two molecules of 3-phosphoglyceric acid (PGA) and it is the first stable product of dark reaction (C 3 Cycle).

2. Reductive Phase:

The PGA molecules are now phosphorylated by ATP molecule and reduced by NADPH 2 (product of light reaction known as assimilatory power) to form 3-phospho-glyceraldehyde (PGAL).

3. Glycolytic Reversal (Formation of sugar) Phase:

Out of two mols of 3-phosphoglyceraldehyde one mol is converted to its isomer 3-dihydroxyacetone phosphate.

4. Regeneration Phase:

Regeneration of Ribulose-5-phosphate (Also known as Reductive Pentose Phosphate Pathway) takes place through number of biochemical steps.

Summary of Photosynthesis:

(A) Light Reaction takes place in thylakoid membrane or granum

(B) Dark Reaction (C 3 cycle) takes place in stroma of chloroplast.

C 4 Cycle (HSK Pathway or Hatch Slack and Kortschak Cycle) :

C 4 cycle may also be referred as the di-carboxylic acid cycle or the β-carboxylation pathway or Hatch and Slack cycle or cooperative photosynthesis (Karpilov, 1970). For a long time, C 3 cycle was considered to be the only photosynthetic pathway for reduction of CO 2 into carbohydrates. Kortschak, Hartt and Burr (1965) reported that rapidly photosynthesizing sugarcane leaves produced a 4-C compound like aspartic acid and malic acid as a result of CO 2 – fixation.

It was later supported by M. D. Hatch and C. R. Slack (1966) and they reported that a 4-C compound oxaloacetic acid (OAA) is the first stable product in CO 2 reduction process. This pathway was first reported in members of family Poaceae like sugarcane, maize, sorghum, etc. (tropical grasses), but later on the other subtropical plant like Atriplex spongiosa (Salt bush), Dititaria samguinolis, Cyperus rotundus, Amaranthus etc. So, the cycle has been reported not only in the members of Graminae but also among certain members of Cyperaceae and certain dicots.

Structural Peculiarities of C 4 Plants (Kranz Anatomy ):

C 4 plants have a characteristic leaf anatomy called Kranz anatomy (Wreath anatomy – German meaning ring or Helo anatomy). The vascular bundles in C 4 plant leaves are surrounded by a layer of bundle sheath cells that contain large number of chloroplast. Dimorphic (two morphologically distinct type) chloroplasts occur in C 4 plants (Fig. 6.13).

In Mesopyll cell:

(i) Chloroplast is small in size

(ii) Well developed grannum and less developed stroma.

(iii) Both PS-II and PS-I are present.

(iv) Non cyclic photophosphorylation takes place.

(v) ATP and NADPH 2 produces.

(vi) Stroma carries PEPCO but absence of RuBisCO.

(vii) CO 2 acceptor is PEPA (3C) but absence of RUBP

(viii) First stable product OAA (4C) produces.

In Bundle sheath Cell:

(i) Size of chloroplast is large

(ii) Stroma is more developed but granna is poorly developed.

(iii) Only PS-I present but absence of PS-II

(iv) Non Cyclic photophosphorylation does not takes place.

(v) Stroma carries RuBisCO but absence of PEPCO.

(vi) CO 2 acceptor RUBP (5c) is present but absence of PEPA (3C)

(vii) C3-cycle takes place and glucose synthesies.

(viii) To carry out C3-cycle both ATP and NADPH2 comes from mesophyll cell chloroplast.

Carbon dioxide from atmosphere is accepted by Phosphoenol pyruvic acid (PEPA) present in stroma of mesophyll cell chloroplast and it converts to oxaloacetic acid (OAA) in the presence of enzyme PEPCO (Phosphoenolpyruvate carboxylase). This 4-C acid (OAA) enters into the chloroplast of bundle sheath cell and there it undergoes oxidative decarboxylation yielding pyruvic acid (3C) and CO 2 .

The carbon dioxide released in bundle sheath cell reacts with RuBP (Ribulose 1, 5 bisphosphate) in presence of RUBISCO and carry out Calvin cycle to synthesize glucose. Pyruvic acid enters mesophyll cells and regenerates PEPA. In C 4 cycle two carboxylation reactions take place.

Reactions taking place in mesophyll cells are stated below: (1 st carboxylation)

C 4 plants are better photosynthesizes. There is no photorespiration in these plants. To synthesize one glucose molecule it requires 30 ATP and 12 NADPH 2 .

Significance of C 4 Cycle:

1. C 4 plants have greater rate of carbon dioxide assimilation than C 3 plants because PEPCO has great affinity for CO 2 and it shows no photorespiration resulting in higher production of dry matter.

2. C 4 plants are better adapted to environmental stress than C 3 plants.

3. Carbon dioxide fixation by C 4 plants requires more ATP than C 3 plants for conversion of pyruvic acid to PEPA.

4. Carbon dioxide acceptor in C 4 plant is PEPA and key enzyme is PEPCO.

5. They can very well grow in saline soils because of presence of C 4 organic acid.

Crassulacean Acid Metabolism (CAM Pathway):

It is a mechanism of photosynthesis which occurs in succulents and some other plants of dry habitats where the stomata remain closed during the daytime and open only at night. The process of photosynthesis is similar to that of C 4 plants but instead of spatial separation of initial PEPcase fixation and final Rubisco fixation of CO 2 , the two steps occur in the same cells (in the stroma of mesophyll chloroplasts) but at different times, night and day, e.g., Sedum, Kalanchoe, Opuntia, Pineapple (Fig. 6.13). (CAM was for the first time studied and reported by Ting (1971).

Characteristics of CAM Plants:

1. Stomatal movement is scoto-active.

2. Presence of monomorphic chloroplast.

3. Stroma of chloroplast carries both PEPCO and RUBISCO.

4. Absence of Kranz anatomy.

5. It is more similar to C 4 plants than C 3 plants.

6. In these plants pH decreases during night and increases during day time.

Mechanism of CAM Pathway :

PHASE I. During night:

Stomata of Crassulacean plants remain open at night. Carbon dioxide is absorbed from outside. With the help of Phosphoenol pyruvate carboxylase (PEPCO) enzyme the CO 2 is immediately fixed, and here the acceptor molecule is Phosphoenol pyruvate (PEP).

Malic acid is the end product of dark fixation of CO 2 . It is stored inside cell vacuole.

During day time the stomata in Crassulacean plants remain closed to check transpiration, but photosynthesis does take place in the presence of sun light. Malic acid moves out of the cell vacuoles. It is de-carboxylated with the help of malic enzyme. Pyruvate is produced. It is metabolized.

The CO 2 thus released is again fixed through Calvin Cycle with the help of RUBP and RUBISCO. This is a unique feature of these succulent plants where they photosynthesis without wasting much of water. They perform acidification or dark fixation of CO 2 during night and de-acidification during day time to release carbon dioxide for actual photosynthesis.

Ecological Significance of CAM P lants:

These plants are ecologically significant because they can reduce rate of transpiration during day time, and are well adapted to dry and hot habitats.

1. The stomata remain closed during the day and open at night when water loss is little due to prevailing low temperature.

2. CAM plants have parenchyma cells, which are large and vacuolated. These vacuoles are used for storing malic and other acids in large amounts.

3. CAM plants increase their water-use efficiency, and secondly through its enzyme PEP carboxylase, they are adapted to extreme hot climates.

4. CAM plants can also obtain a CO 2 compensation point of zero at night and in this way accomplish a steeper gradient for CO 2 uptake compared to C 3 plants.

5. They lack a real photosynthesis during daytime and the growth rate is far lower than in all other plants (with the exception of pineapple).

Photorespiration or C 2 Cycle or Glycolate Cycle or Photosynthetic Carbon Oxidation Cycle:

Photorespiration is the light dependent process of oxygenation of RUBP (Ribulose bi-phosphate) and release of carbon dioxide by photosynthetic organs of the plant. Otherwise, as we know, photosynthetic organs release oxygen and not CO 2 under normal situation.

Occurrence of photorespiration in a plant can be demonstrated by:

(i) Decrease in the rate of net photosynthesis when oxygen concentration is increased from 2-3 to 21%.

(ii) Sudden increased evolution of CO 2 when an illuminated green plant is transferred to dark.

Photorespiration is initiated under high O 2 and low CO 2 and intense light around the photosynthesizing plant. Photorespiration was discovered by Dicker and Tio (1959), while the term “Photorespiration” was coined by Krotkov (1963). Photorespiration should not be confused with photo- oxidation. While the former is a normal process in some green plants, the latter is an abnormal and injurious process occurring in extremely intense light resulting in destruction of cellular components, cells and tissues.

On the basis of photorespiration, plants can be divided into two groups:

(i) Plants with photorespiration (temperate plants) and plants without photorespiration (tropical plants).

Site of Photorespiration :

Photorespiration involves three cell organelles, viz., chloroplast, peroxisome and mitochondria for its completion. Peroxisome, the actual site of photorespiration, contains enzymes like glycolate oxydase, glutamate glyoxalate aminotransferase, peroxidase and catalase enzymes.

Mechanism of Photorespiration:

We know that the enzyme RUBISCO (Ribulose biphosphate carboxylase oxygenase) catalyzes the carboxylation reaction, where CO 2 combines with RuBP for calvin cycle (dark reaction of photosynthesis) to initiate. But this enzyme RUBISCO, under intense light conditions, has the ability to catalyse the combination of O 2 with RuPB, a process called oxygenation.

In other words the enzyme RUBISCO can catalyse both carboxylation as well as oxygenation reactions in green plants under different conditions of light and O 2 /CO 2 ratio. Respiration that is initiated in chloroplasts under light conditions is called photorespiration. This occurs essentially because of the fact that the active site of the enzyme RUBISCO is the same for both carboxylation and oxygenation (Fig. 6.16).

The oxygenation of RuBP in the presence of O 2 is the first reaction of photorespiration, which leads to the formation of one molecule of phosphoglycolate, a 2 carbon compound and one molecule of phosphoglyceric acid (PGA). While the PGA is used up in the Calvin cycle, the phosphoglycolate is dephosphorylated to form glycolate in the chloroplast (Fig. 6.16).

From the chloroplast, the glycolate is diffused to peroxisome, where it is oxidised to glyoxylate. In the peroxisome, the glyoxylate is used to form the amino acid, glycine. Glycine enters mitochondria where two molecules of glycine (4 carbons) give rise to one molecule of serine (3 carbon) and one CO 2 (one carbon).

The serine is taken up by the peroxisome, and through a series of reactions, is converted to glycerate. The glycerate leaves the peroxisome and enters the chloroplast, where it is phosphorylated to form PGA. The PGA molecule enters the calvin cycle to make carbohydrates, but one CO 2 molecule released in mitochondria during photorespiration has to be re-fixed.

In other words, 75% of the carbon lost by oxygenation of RuBP is recovered, and 25% is lost as release of one molecule of CO 2 . Because of the features described above, photorespiration is also called photosynthetic carbon oxidation cycle.

Minimization of Photorespiration (C4 and CAM Plants):

Since photorespiration requires additional energy from the light reactions of photosynthesis, some plants have mechanisms to reduce uptake of molecular oxygen by Rubisco. They increase the concentration of CO 2 in the leaves so that Rubisco is less likely to produce glycolate through reaction with O 2 .

C 4 plants capture carbon dioxide in cells of their mesophyll (using an enzyme called PEP carboxylase), and they release it to the bundle sheath cells (site of carbon dioxide fixation by Rubisco) where oxygen concentration is low.

The enzyme PEP carboxylase is also found in other plants such as cacti and succulents who use a mechanism called Crassulacean acid metabolism or CAM in which PEP carboxylase put aside carbon at night and releases it to the photosynthesizing cells during the day.

This provides a mechanism for reducing high rates of water loss (transpiration) by stomata during the day. This ability to avoid photorespiration makes these plants more hardy than other plants in dry conditions where stomata are closed and oxygen concentration rises.

Factors Affecting Photosynthesis:

Photosynthesis is affected by both environmental and genetic (internal) factors. The environmental factors are light, CO 2 , temperature, soil, water, nutrients etc. Internal or genetic factors are all related with leaf and include protoplasmic factors, chlorophyll contents, structure of leaf, accumulation of end product etc.

Some of the important factors are discussed below:

1. Concept of Cardinal Values :

The metabolic processes are influenced by a number of factors of the environment. The rate of a metabolic process is controlled by the magnitude of each factor. Sachs (1860) recognized three critical values, the cardinal values or points of the magnitude of each factor. These are minimum, optimum and maximum. The minimum cardinal value is that magnitudes of a factor below which the metabolic process cannot proceed.

Optimum value is the one at which the metabolic process proceeds at its highest rate. Maximum is that magnitude of a factor beyond which the process stops. At magnitudes below and above the optimum, the rate of a metabolic process declines till minimum and maximum values are attained.

2. Principle of Limiting Factors :

Liebig (1843) proposed law of minimum which states that the rate of a process is limited by the pace (rapidity) of the slowest factor. However, it was later on modified by Blackman (1905) who formulated the “principle of limiting factors”. It states that when a metabolic process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace (rapidity) of the slowest factor. This principle is also known as “Blackman’s Law of Limiting Factors.”

A metabolic process is conditioned by a number of factors. The slowest factor or the limiting factor is the one whose increase in magnitude is directly responsible for an increase in the rate of the metabolic process (here photosynthesis).

To explain it further, say at a given time, only the factor that is most limiting among all will determine the rate of photosynthesis. For example, if CO 2 is available in plenty but light is limiting due to cloudy weather, the rate of photosynthesis under such a situation will be controlled by the light. Furthermore, if both CO 2 and light are limiting, then the factor which is the most limiting of the two, will control the rate of photosynthesis.

Blackman (1905) studied the effect of CO 2 concentration, light intensity and temperature on rate of photosynthesis. All other factors were maintained in optimum concentration. Initially the photosynthetic material was kept at 20°C in an environment having 0.01% CO 2 . When no light was provided to photosynthetic material, it did not perform photosynthesis. Instead, it evolved CO 2 and absorbed O 2 from its environment. He provided light of low intensity (say 150 foot candles) and found photosynthesis to occur.

When light intensity was increased (say 800 foot candles), the rate of photosynthesis increased initially but soon it leveled off. The rate of photosynthesis could be further enhanced only on the increase in availability of CO 2 . Thus, initially light intensity was limiting the rate of photosynthesis.

When sufficient light became available, CO 2 became limiting factor (Fig. 6.17). When both are provided in sufficient quantity, the rate of photosynthesis rose initially but again reached a peak. It could not be increased further. At this time, it was found that increase in temperature could raise the rate of photosynthesis up to 35°C. Further increase was not possible. At this stage, some other factor became limiting. Therefore, at one time only one factor limits the rate of physiological process.

Objections have been raised to the validity of Blackman’s law of limiting factors. For instance:

(i) It has been observed that the rate of a process cannot be increased indefinitely by increasing the availability of all the known factors;

(ii) The principle of Blackman is not operative for toxic chemicals or inhibitors and

(iii) Some workers have shown that the pace of reaction can be controlled simultaneously by two or more factors.

3. External Factors:

The environmental factors which can affect the rate of photosynthesis are carbon dioxide, light, temperature, water, oxygen, minerals, pollutants and inhibitors.

1. Effect of Carbon dioxide:

Being one of the raw materials, carbon dioxide concentration has great effect on the rate of photosynthesis. The atmosphere normally contains 0.03 to 0.04 per cent by volume of carbon dioxide. It has been experimentally proved that an increase in carbon dioxide content of the air up to about one per cent will produce a corresponding increase in photosynthesis provided the intensity of light is also increased.

2. Effect of Light:

The ultimate source of light for photosynthesis in green plants is solar radiation, which moves in the form of electromagnetic waves. Out of the total solar energy reaching to the earth, about 2% is used in photosynthesis and about 10% is used in other metabolic activities. Light varies in intensity, quality (wavelength) and duration.

The effect of light on photosynthesis can be studied under following three headings:

(i) Intensity of Light:

The total light perceived by a plant depends on its general form (viz., height of plant and size of leaves, etc.) and arrangement of leaves. Of the total light falling on a leaf, about 80% is absorbed, 10% is reflected and 10% is transmitted. Intensity of light can be measured by lux meter.

Effect of light intensity varies from plant to plant, e.g., more in heliophytes (sun loving plants) and less in sciophytes (shade loving plants). For a complete plant, rate of photosynthesis increases with increase in light intensity, except under very high light intensity where phenomenon of Solarization’ occurs, (i.e., photo-oxidation of different cellular components including chlorophyll). It also affects the opening and closing of stomata thereby affecting the gaseous exchange. The value of light saturation at which further increase is not accompanied by an increase in CO 2 uptake is called light saturation point.

(ii) Quality of Light:

Photosynthetic pigments absorb visible part of the radiation i.e., 380 mμ, to 760 mμ. For example, chlorophyll absorbs blue and red light. Usually plants show high rate of photosynthesis in the blue and red light. Maximum photosynthesis has been observed in red light than in blue light followed by yellow light (monochromatic light). The green light has minimum effect. The rate of photosynthesis is maximum in white light or sunlight (polychromatic light). On the other hand, red algae shows maximum photosynthesis in green light and brown algae in blue light.

(iii) Duration of Light:

Longer duration of light period favours photosynthesis. Generally, if the plants get 10 to 12 hrs. of light per day it favours good photosynthesis. Plants can actively exhibit photosynthesis under continuous light without being damaged. Rate of photosynthesis is independent of duration of light.

3. Effect of Temperature:

The rate of photosynthesis markedly increases with an increase in temperature provided other factors such as CO 2 and light are not limiting. The temperature affects the velocity of enzyme controlled reactions in the dark stage. When the intensity of light is low, the reaction is limited by the small quantities of reduced coenzymes available so that any increase in temperature has little effect on the overall rate of photosynthesis.

At high light intensities, it is the enzyme-controlled dark stage which controls the rate of photosynthesis and there the Q 10 = 2. If the temperature is greater than about 30°C, the rate of photosynthesis abruptly falls due to thermal inactivation of enzymes.

4. Effect of Water:

Although the amount of water required during photosynthesis is hardly one percent of the total amount of water absorbed by the plant, yet any change in the amount of water absorbed by a plant has significant effect on its rate of photosynthesis. Under normal conditions water rarely seems to be a controlling factor as the chloroplasts normally contain plenty of water.

Many experimental observations indicate that in the field the plant is able to withstand a wide range of soil moisture without any significant effect on photosynthesis and it is only when wilting sets in that the photosynthesis is retarded. Some of the effect of drought may be secondary since stomata tend to close when the plant is deprived of water. A more specific effect of drought on photosynthesis results from dehydration of protoplasm.

5. Effect of Oxygen:

Excess of O 2 may become inhibitory for the process. Enhanced supply of O 2 increases the rate of respiration simultaneously decreasing the rate of photosynthesis by the common intermediate substances. The concentration for oxygen in the atmosphere is about 21% by volume and it seldom fluctuates. O 2 is not a limiting factor of photosynthesis.

An increase in oxygen concentration decreases photosynthesis and the phenomenon is called Warburg effect. [Reported by German scientist Warburg (1920) in Chlorella algae]. This is due to competitive inhibition of RuBP-carboxylase at increased O 2 levels, i.e., O 2 competes for active sites of RuBP-carboxylase enzyme with CO 2 . The explanation of this problem lies in the phenomenon of photorespiration. If the amount of oxygen in the atmosphere decreases then photosynthesis will increase in C 3 cycle and no change in C 4 cycle.

6. Effect of Minerals:

Presence of Mn ++ and CI – is essential for smooth operation of light reactions (Photolysis of water/evolution of oxygen) Mg ++ , Cu ++ and Fe ++ ions are important for synthesis of chlorophyll.

7. Effect of Pollutants and Inhibitors:

The oxides of nitrogen and hydrocarbons present in smoke react to form peroxyacetyl nitrate (PAN) and ozone. PAN is known to inhibit Hill’s reaction. Diquat and Paraquat (commonly called as Viologens) block the transfer of electrons between Q and PQ in PS II.

Other inhibitors of photosynthesis are monouron or CMU (Chlorophenyl dimethyl urea), diuron or DCMU (Dichlorophenyl dimethyl urea), bromocil and atrazine etc., which have the same mechanism of action as that of violates. At low light intensities potassium cyanide appears to have no inhibiting effect on photosynthesis.

4. Internal Factors:

The important internal factors that regulate the rate of photosynthesis are:

1. Protoplasmic factors:

There is some unknown factor in protoplasm which affects the rate of photosynthesis. This factor affect the dark reactions. The decline in the rate of photosynthesis at temperature.above 30°C or at strong light intensities in many plants suggests the enzyme nature of this unknown factor.

2. Chlorophyll content:

Chlorophyll is an essential internal factor for photosynthesis. The amount of CO 2 fixed by a gram of chlorophyll in an hour is called photosynthetic number or assimilation number. It is usually constant for a plant species but rarely it varies. The assimilation number of variegated variety of a species was found to be higher than the green leaves variety.

3. Accumulation of end products:

Accumulation of food in the chloroplasts reduces the rate of photosynthesis.

4. Structure of leaves:

The amount of CO 2 that reaches the chloroplasts depends on structural features of the leaves like the size, position and behaviour of the stomata and the amount of intercellular spaces. Some other characters like thickness of cuticle, epidermis, presence of epidermal hairs, amount of mesophyll tissue, etc., influence the intensity and quality of light reaching the chloroplast.

5. CO 2 Compensation Point :

It is that value or point in light intensity and atmospheric CO 2 concentration when the rate of photosynthesis is just equivalent to the rate of respiration in the photosynthetic organs so that there is no net gaseous exchange. The value of light compensation point is 2.5 -100 ft. candles for shade plants and 100-400 ft. candles for sun plants. The value of CO 2 compensation point is very low in C 4 plants (0-5 ppm), where as in C 3 plants it is quite high (25-100 ppm). A plant can not survive for long at compensation point because there is net lose of organic matter due to respiration of non-green organs and dark respiration.

Related Articles:

• Differences between Respiration and Photosynthesis
• 4 Main Stages of Cellular Reaction in Plants | Metabolic Engineering
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Photosynthesis 1: An Introduction (Interactive Tutorial)

Looking for a student learning guide? It’s on the main menu for your course. Use the “Courses” menu above.

Page Outline

• Introduction
• Photosynthesis as an Endergonic Redox (Oxidation-Reduction) Reaction
• Quiz: Photosynthesis the Big Picture

1. Introducing Photosynthesis

Start by labeling the diagram below. Use a process of elimination if you get stuck.

[qwiz qrecord_id=”sciencemusicvideosMeister1961-PSN, Inputs and Outputs Diagram”]

[h]Interactive Diagram: Photosynthesis inputs and outputs

[q labels = “top”]

[l] carbohydrate

[fx] No. Please try again.

[f*] Excellent!

[l] carbon dioxide

[f*] Correct!

[f*] Great!

[fx] No, that’s not correct. Please try again.

For a minute, let’s think about life.

Living things are entities that, chemically speaking, are profoundly out of equilibrium with their environment. We (meaning people, cacti, sea anemones, redwood trees, bacteria, and every other organism) are systems that are highly organized. On a molecular level, we’re composed of highly reduced compounds, each buzzing with energetic electrons. Order and potential energy like that don’t happen spontaneously. They need an energy source to sustain them.

That energy source is the sun (“1”), and photosynthesis is the biological process that brings that energy and highly organized matter into living systems. Life’s chemical energy is stored in a variety of forms: immediately as ATP, a form of energy that, for the most part, is locked within individual cells, and can’t be transferred from one cell to another. A form that can be transferred from cell to cell is sugar, and that’s one of photosynthesis’s most direct products. The sugars made by photosynthesis are simple carbohydrates, and plants can convert them into more complex carbohydrates such as starches and cellulose. In the diagram above, carbohydrate is represented by Number 5. Essentially, it’s the plant itself.

Carbohydrates (along with proteins and fats) constitute one output of the photosynthetic process. Another one is oxygen (shown at “4”). All of the oxygen in our atmosphere is there because of photosynthesis, which has been bubbling oxygen into the air for the past 3.5 billion years. Life can exist without atmospheric oxygen. But we can’t. If you get excited by anything connected with multicellular life (which ranges from the shape of an orchid to conflict resolution among primates), then you can thank photosynthesis for creating the oxygen that made the complexity of multicellular aerobic life possible.

So, that’s what comes out. Let’s think about what goes in. Photosynthesis involves carbon fixation,  which involves taking carbon dioxide gas (“2” in the diagram above) and rendering it into a solid form as carbohydrate (and, through other reactions, proteins, lipids, and nucleic acids). You can think of fixation as fixing into place .

Carbon dioxide is a minor component of our atmosphere. At the start of the industrial revolution (let’s say 1800), carbon dioxide’s atmospheric concentration was about 280 parts per million. Now it’s above 400, a product of human combustion of fossil fuel ( click here to learn more about climate change). During photosynthesis, plants and other photosynthetic organisms (including algae and photosynthetic bacteria), absorb carbon dioxide into their cells and combine it with electrons and protons ripped away from water molecules (water is the second input, shown above at “3”).

Taking unorganized, randomly moving molecules in the air and organizing them into cells and multicellular organisms is a massive push against entropy. To try to envision this, just take a look at your hand: an organized structure. The bone, skin, and muscle in your hand is mostly protein. Protein, like every other biological molecule, is a carbon-based molecule. Every one of the carbon atoms making up the molecules that make up the cells that make up the tissues that make up your hand, was once, not very long ago floating freely in the air.

It’s a good thing that the sun puts out so much energy. This is not something that most people can imagine, but think about all the energy that human beings use in a year: all the electricity powering all of the lights and machines; all of the fossil fuel-derived combustion that heats our homes and moves our vehicles, all of the energy required to make everything we use. That amount, all over our planet, sustaining our civilization, is equal to an hour and a half of sunlight striking the earth (based on the Sandia Labs Solar FRQ . The calculation was based on human energy consumption in the year 2001. We undoubtedly consume more energy now).

In what follows, we’ll drop down to the cellular and molecular details of how photosynthesis work. Let’s go.

2. Photosynthesis as an endergonic redox (oxidation-reduction) reaction

Photosynthesis is a complex process, with many intermediate steps that we’ll learn about in this and the coming modules. But let’s make sure we understand the big picture first. Here’s the chemical equation for photosynthesis:

6CO 2 + 6H 2 O + light energy –> C 6 H 12 O 6 + 6O 2 .

In words, that means that six molecules of carbon dioxide are combined with six molecules of water, producing one molecule of glucose and six molecules of oxygen. Two things about this equation tell us that the reaction is endergonic (not spontaneous, requiring an input of energy to move it forward).

• The fact that energy isn’t coming out, but gets put in. That’s evident in light energy’s place on the left side of the arrow.
• The negative entropy change. Twelve molecules go into photosynthesis. Seven molecules come out. That’s a decrease in entropy. If you had twelve piles of things in your room and you organized them into seven piles, you’d have increased your room’s level of organization. Increasing organization requires energy.

For a review of the idea of free energy, read this .

Note that photosynthesis is the inverse of what happens during cellular respiration.

• Photosynthesis: 6CO 2 + 6H 2 O + light energy –> C 6 H 12 O 6 + 6O 2
• Respiration: C 6 H 12 O 6 + 6O 2 –>  energy(ATP) + 6CO 2 + 6H 2 O

During photosynthesis, carbon dioxide is reduced . That means that carbon dioxide gains energetic electrons. Those electrons come from water, which is oxidized (loses electrons). It’s important to emphasize that water doesn’t power photosynthesis. The power comes from light, and how light powers the oxidation of water is something that we’ll visit later in this series of tutorials. Note that this is, of course, also the inverse of what happens during cellular respiration, where glucose is oxidized as water is reduced.

The reduction of carbon dioxide that happens during photosynthesis is the basis of all of life’s chemical energy. Life is based on highly reduced, energetic molecules. Think of vegetable oil or a piece of bread. In fact, for an image of a reduced matter, think of a peanut butter sandwich. It’s dripping with energy. And that’s what photosynthesis does: it takes highly oxidized carbon dioxide, commonly referred to as exhaust (what comes out of a car’s tailpipe), and transforms it into highly reduced living matter (carbohydrates, which then get transformed into proteins, lipids, and nucleic acids).

The key organelles are the chloroplast (“2”), which is the site of photosynthesis; and the mitochondria (“5”) the site of most cellular respiration. Because this is a cycle, we could begin anywhere, but let’s start with the inputs of photosynthesis. These are carbon dioxide (“6”) and water (“7”). Powered by light (“1”), a chloroplast converts these inputs into the simple sugar glucose (“3”) and oxygen (“4”). These outputs of photosynthesis become the inputs for cellular respiration, the goal of which is to produce the short-term energy molecule ATP, which would appear at “8” in this diagram.

Before entering into more detail about photosynthesis, let’s make sure you’re on top of the material covered above.

3. Quiz: Photosynthesis, the big picture

[qwiz random = “false” qrecord_id=”sciencemusicvideosMeister1961-PSN, Big Picture”]

[h]Photosynthesis: the big picture.

[q labels = “top” dataset_id=”SMV_PSN_The_Big_Picture|d2867ebbbc7b4″ question_number=”8″]

The energy for all life is provided by the ______ . Within plant cells, organelles called ________________  take up ____________________ (a gas) and   __________ . Through the process of ___________________ , ______________ create the sugar _________  and release _________ as a by-product. In both plant and animal cells,   ________________  take in _________ (gas) and __________ . During cellular _____________ , mitochondria will produce   ________ , and release the gas __________________  and __________ . These, in turn, become the inputs for _________________ .

[l] chloroplasts

[l] glucose

[l] mitochondria

[l] photosynthesis

[l] respiration

[q topic= “photosynthesis_overview” question_number=”1″ dataset_id=”SMV_PSN_The_Big_Picture|d2c08ea4423b4″] Which number represents the substance that gets reduced during photosynthesis?

[textentry single_char=”true”]

[c]ID I=[Qq]

[f]IFllcy4g4oCcMuKAnSByZXByZXNlbnRzIGNhcmJvbiBkaW94aWRlLiBEdXJpbmcgcGhvdG9zeW50aGVzaXMsIGNhcmJvbiBkaW94aWRlIGdldHMgcmVkdWNlZCB0byBjYXJib2h5ZHJhdGUu[Qq]

[c]ICo=[Qq]

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[q multiple_choice=”true” dataset_id=”SMV_PSN_The_Big_Picture|d2b88dbb627b4″ question_number=”2″] In biological processes like respiration and photosynthesis and respiration, substances that are reduced have _______ chemical energy than substances that are oxidized.

[c]IG1v cmU=[Qq]

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[c]IGFib3V0IHRoZSBzYW1lIGFtb3VudCBvZg==[Qq]

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[c]IGxlc3M=[Qq]

[f]IE5vLiBUaGluayBhYm91dCBhIHJlZHVjZWQgc3Vic3RhbmNlIChsaWtlIHN1Z2FyKSBhbmQgY29tcGFyZSBpdCB0byBhbiBveGlkaXplZCBzdWJzdGFuY2UgKGxpa2UgY2FyYm9uIGRpb3hpZGUpLiBXaGljaCBvbmUgY291bGQgeW91IGNvbnN1bWUgZm9yIGZ1ZWw/

[q dataset_id=”SMV_PSN_The_Big_Picture|d2b121d57bbb4″ question_number=”3″] Which number represents the substance that gets oxidized during photosynthesis?

[c]ID M=[Qq]

[f]IFllcy4g4oCcM+KAnSByZXByZXNlbnRzIHdhdGVyLiBEdXJpbmcgcGhvdG9zeW50aGVzaXMsIHdhdGVyIGdldHMgb3hpZGl6ZWQsIGxvc2luZyBlbGVjdHJvbnMgYW5kIHByb3RvbnMsIGJlY29taW5nIG1vbGVjdWxhciBveHlnZW4gKE8= Mg== KQ==[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBUaGUgcmVkdWNlZCBwcm9kdWN0IG9mIHBob3Rvc3ludGhlc2lzIGlzIGNhcmJvaHlkcmF0ZS4gTG9vayBjYXJlZnVsbHkgYXQgdGhhdCB3b3JkLCBhbmQgc2VlIGlmIHlvdSBjYW4gZmlndXJlIG91dCBpbnB1dCBmb3IgcGhvdG9zeW50aGVzaXMgY291bGQgYmUgcmVjZWl2aW5nIGVsZWN0cm9ucyBhbmQgcHJvdG9ucyB0byBiZWNvbWUgYSBjYXJib2h5ZHJhdGUu

[q dataset_id=”SMV_PSN_The_Big_Picture|d2a9b5ef94fb4″ question_number=”4″] Which number represents the reduced product of photosynthesis?

[c]NQ ==[Qq]

[f]IFllcy4g4oCcNeKAnSByZXByZXNlbnRzIHRoZSBjYXJib2h5ZHJhdGUgKHN1Z2FycywgY2VsbHVsb3NlKSBhbmQgb3RoZXIgb3JnYW5pYyBtYXRlcmlhbCB0aGF0IHRoZSBwbGFudCBpcyBtYWRlIG9mLiBEdXJpbmcgcGhvdG9zeW50aGVzaXMsIGNhcmJvbiBkaW94aWRlIGFuZCB3YXRlciBnZXQgY29tYmluZWQgdG8gZm9ybSB0aGVzZSByZWR1Y2VkIG9yZ2FuaWMgbW9sZWN1bGVzLg==[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBUaGUgcmVkdWNlZCBwcm9kdWN0IG9mIHBob3Rvc3ludGhlc2lzIGlzIGNhcmJvaHlkcmF0ZSAoYWxvbmcgd2l0aCBvdGhlciBvcmdhbmljIG1vbGVjdWxlcykuIExvb2sgY2FyZWZ1bGx5IGF0IHRoZSBkaWFncmFtLCBhbmQgc2VlIGlmIHlvdSBjYW4gZmlndXJlIG91dCB3aGVyZSB0aGF0IGNhcmJvaHlkcmF0ZSB3b3VsZCBiZSBmb3VuZC4=

[q dataset_id=”SMV_PSN_The_Big_Picture|d2a2b9cbe8fb4″ question_number=”5″]Because photosynthesis requires energy to proceed, it’s considered to be an [hangman] process.

[c]ZW5kZXJnb25pYw==[Qq]

[f]RXhjZWxsZW50LsKgQmVjYXVzZSBwaG90b3N5bnRoZXNpcyByZXF1aXJlcyBlbmVyZ3kgdG8gcHJvY2VlZCwgaXQmIzgyMTc7cyBjb25zaWRlcmVkIHRvIGJlIGFuIA== ZW5kZXJnb25pYw== IHByb2Nlc3Mu

[q dataset_id=”SMV_PSN_The_Big_Picture|d29b98677ebb4″ question_number=”6″]Because respiration releases energy for cellular work, it’s considered to be an [hangman] process.

[c]ZXhlcmdvbmlj[Qq]

[f]RXhjZWxsZW50LsKgQmVjYXVzZSByZXNwaXJhdGlvbiByZWxlYXNlcyBlbmVyZ3kgZm9yIGNlbGx1bGFyIHdvcmssIGl0JiM4MjE3O3MgY29uc2lkZXJlZCB0byBiZSBhbiA= ZXhlcmdvbmlj IHByb2Nlc3Mu

[q labels = “top” dataset_id=”SMV_PSN_The_Big_Picture|d2911e31fcbb4″ question_number=”7″]

[l] chloroplast

[q topic= “cellular_respiration_overview” dataset_id=”SMV_PSN_The_Big_Picture|d27de8cfe3bb4″ question_number=”9″]The correct chemical reaction for cellular respiration is

[c]IEM= Ng== SA== MTI= Tw== [Qq]6 + 6CO 2 + energy(ATP) –> 6CO 2 + 6O 2

[c]IE M= Ng== SA== [Qq] 12 O 6 + 6O 2 –> energy(ATP) + 6CO 2 + 6H 2 O

[c]IDZDTw== Mg== ICsgNkg= [Qq] 2 O –> Energy(light) + C 6 H 12 O 6 + 6O 2

[c]IDZI Mg== TyArIDZP [Qq] 2  –> 6CO 2 + 6H 2 O

[f]IE5vLiBGaW5kIGEgcmVhY3Rpb24gdGhhdCBiZWdpbnMgd2l0aCBnbHVjb3NlLCBjb21iaW5lcyBpdCB3aXRoIG94eWdlbiwgYW5kIHJlbGVhc2VzIGVuZXJneSwgY2FyYm9uIGRpb3hpZGUsIGFuZCB3YXRlci4=[Qq] [f]IEV4Y2VsbGVudC4gQ2VsbHVsYXIgcmVzcGlyYXRpb24gYmVnaW5zIHdpdGggZ2x1Y29zZSwgY29tYmluZXMgaXQgd2l0aCBveHlnZW4sIGFuZCByZWxlYXNlcyBlbmVyZ3ksIGNhcmJvbiBkaW94aWRlLCBhbmQgd2F0ZXIu[Qq] [f]IE5vLiBGaW5kIGEgcmVhY3Rpb24gdGhhdCBiZWdpbnMgd2l0aCBnbHVjb3NlLCBjb21iaW5lcyBpdCB3aXRoIG94eWdlbiwgYW5kIHJlbGVhc2VzIGVuZXJneSwgY2FyYm9uIGRpb3hpZGUsIGFuZCB3YXRlci4gVGhpcyBlcXVhdGlvbiwgYnkgdGhlIHdheSwgaXMgdGhlIGNvcnJlY3QgZXF1YXRpb24gZm9yIHBob3Rvc3ludGhlc2lzLiBJZiB5b3UgcmV2ZXJzZSB0aGUgcmVhY3Rpb24sIHlvdeKAmWxsIGhhdmUgdGhlIHJlYWN0aW9uIGZvciBjZWxsdWxhciByZXNwaXJhdGlvbi4=[Qq] [f]IE5vLiBGaW5kIGEgcmVhY3Rpb24gdGhhdCBiZWdpbnMgd2l0aCBnbHVjb3NlLCBjb21iaW5lcyBpdCB3aXRoIG94eWdlbiwgYW5kIHJlbGVhc2VzIGVuZXJneSwgY2FyYm9uIGRpb3hpZGUsIGFuZCB3YXRlci4=

[Qq] [q topic= “cellular_respiration_overview” dataset_id=”SMV_PSN_The_Big_Picture|d2759d65877b4″ question_number=”10″]The function of cellular respiration is to produce

[c]IGNhcmJvbiBkaW94aWRl[Qq]

[c]IGdsdWNvc2U=[Qq]

[c]IG94eWdlbg==[Qq]

[c]IEFU UA==[Qq]

[c]IHdhdGVy[Qq]

[f]IE5vLiBDYXJib24gZGlveGlkZSBpcyBvbmUgb2YgdGhlIHdhc3RlIHByb2R1Y3RzIG9mIGNlbGx1bGFyIHJlc3BpcmF0aW9uLiBOZXh0IHRpbWUsIGxvb2sgZm9yIHNvbWV0aGluZyB0aGF0IGNlbGx1bGFyIHJlc3BpcmF0aW9uIHByb2R1Y2VzIHRoYXQgZW5hYmxlcyB0aGUgY2VsbCB0byBkbyBzb21lIHdvcmsu[Qq] [f]IE5vLiBHbHVjb3NlIGlzIHRoZSBmdWVsIGZvciBjZWxsdWxhciByZXNwaXJhdGlvbi4gTmV4dCB0aW1lLCBsb29rIGZvciBzb21ldGhpbmcgdGhhdCBjZWxsdWxhciByZXNwaXJhdGlvbiBwcm9kdWNlcyB0aGF0IGVuYWJsZXMgdGhlIGNlbGwgdG8gZG8gc29tZSB3b3JrLg==[Qq] [f]IE5vLiBPeHlnZW4gaXMgYSByZXF1aXJlZCBpbnB1dCBmb3IgY2VsbHVsYXIgcmVzcGlyYXRpb24uIFlvdeKAmXJlIGxvb2tpbmcgZm9yIGFuIG91dHB1dC4gTmV4dCB0aW1lLCBsb29rIGZvciBzb21ldGhpbmcgdGhhdCBjZWxsdWxhciByZXNwaXJhdGlvbiBwcm9kdWNlcyB0aGF0IGVuYWJsZXMgdGhlIGNlbGwgdG8gZG8gc29tZSB3b3JrLg==[Qq] [f]IFllcy4gUHJvZHVjdGlvbiBvZiBBVFAgaXMgdGhlIGdvYWwgb2YgY2VsbHVsYXIgcmVzcGlyYXRpb24uIEl04oCZcyB0aGUgbW9tZW50LXRvLW1vbWVudCBlbmVyZ3kgc291cmNlIHdpdGhpbiBjZWxscywgYW5kIGl04oCZcyB3aGF0IGVuYWJsZXMgY2VsbHMgdG8gZ2V0IHdvcmsgZG9uZS4=[Qq] [f]IE5vLiBXYXRlciBpcyBvbmUgb2YgdGhlIHdhc3RlIHByb2R1Y3RzIG9mIGNlbGx1bGFyIHJlc3BpcmF0aW9uLiBOZXh0IHRpbWUsIGxvb2sgZm9yIHNvbWV0aGluZyB0aGF0IGNlbGx1bGFyIHJlc3BpcmF0aW9uIHByb2R1Y2VzIHRoYXQgZW5hYmxlcyB0aGUgY2VsbCB0byBkbyBzb21lIHdvcmsu

[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d26d9c7ca7bb4″ question_number=”11″]The equation 6CO 2 + 6H 2 O + Energy(light) C 6 H 12 O 6 + 6O 2 is for

[c]IGNlbGx1bGFyIHJlc3BpcmF0aW9u[Qq]

[c]IGxhY3RpYyBhY2lkIGZlcm1lbnRhdGlvbg==[Qq]

[c]IHBob3Rvc3 ludGhlc2lz[Qq]

[c]IGFsY29ob2wgZmVybWVudGF0aW9u[Qq]

[f]IE5vLiBJbiBjZWxsdWxhciByZXNwaXJhdGlvbiwgZ2x1Y29zZSBpcyBjb21iaW5lZCB3aXRoIG94eWdlbiB0byBwcm9kdWNlIEFUUCwgd2l0aCBjYXJib24gZGlveGlkZSBhbmQgd2F0ZXIgcHJvZHVjZWQgYXMgYSBieXByb2R1Y3QuIFRoZSBlcXVhdGlvbiBhYm92ZSBzaG93cyBob3cgY2FyYm9uIGRpb3hpZGUgaXMgY29tYmluZWQgd2l0aCB3YXRlciB0byBjcmVhdGUgZ2x1Y29zZSwgd2l0aCBveHlnZW4gYmVpbmcgcmVsZWFzZWQgYXMgYSBieXByb2R1Y3QuIFdoYXQgcHJvY2VzcyBjcmVhdGVzIGdsdWNvc2U/IEFzIGEgaGludCwgbm90ZSB0aGF0IGxpZ2h0IGRyaXZlcyB0aGUgcmVhY3Rpb24u[Qq]

[f]IE5vLiBMYWN0aWMgYWNpZCBmZXJtZW50YXRpb24gaXMgYSBraW5kIG9mIGFuYWVyb2JpYyByZXNwaXJhdGlvbi4gVGhlIGVxdWF0aW9uIGFib3ZlIHNob3dzIGhvdyBjYXJib24gZGlveGlkZSBpcyBjb21iaW5lZCB3aXRoIHdhdGVyIHRvIGNyZWF0ZSBnbHVjb3NlLCB3aXRoIG94eWdlbiBiZWluZyByZWxlYXNlZCBhcyBhIGJ5cHJvZHVjdC4gV2hhdCBwcm9jZXNzIGNyZWF0ZXMgZ2x1Y29zZT8gQXMgYSBoaW50LCBub3RlIHRoYXQgbGlnaHQgZHJpdmVzIHRoZSByZWFjdGlvbi4=[Qq]

[f]IFllcy4gVGhlIGVxdWF0aW9uIGFib3ZlIHNob3dzIHRoZSByZWFjdGlvbiBmb3IgcGhvdG9zeW50aGVzaXM6IGNhcmJvbiBkaW94aWRlIGJlaW5nIGNvbWJpbmVkIHdpdGggd2F0ZXIgdG8gY3JlYXRlIGdsdWNvc2UsIHdpdGggb3h5Z2VuIGJlaW5nIHJlbGVhc2VkIGFzIGEgYnlwcm9kdWN0Lg==[Qq]

[f]IE5vLiBBbGNvaG9sIGZlcm1lbnRhdGlvbiBpcyBhIGtpbmQgb2YgYW5hZXJvYmljIHJlc3BpcmF0aW9uLiBUaGUgZXF1YXRpb24gYWJvdmUgc2hvd3MgaG93IGNhcmJvbiBkaW94aWRlIGlzIGNvbWJpbmVkIHdpdGggd2F0ZXIgdG8gY3JlYXRlIGdsdWNvc2UsIHdpdGggb3h5Z2VuIGJlaW5nIHJlbGVhc2VkIGFzIGEgYnlwcm9kdWN0LiBXaGF0IHByb2Nlc3MgY3JlYXRlcyBnbHVjb3NlPyBBcyBhIGhpbnQsIG5vdGUgdGhhdCBsaWdodCBkcml2ZXMgdGhlIHJlYWN0aW9uLg==

[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d2652bd18d3b4″ question_number=”12″]The energy that powers photosynthesis comes from

[c]IGxp Z2h0[Qq]

[f]IE5vLiBHbHVjb3NlIGlzIHRoZSBwcm9kdWN0IG9mIHBob3Rvc3ludGhlc2lzLiBUaGUgZW5lcmd5IHNvdXJjZSBmb3IgcGhvdG9zeW50aGVzaXMgaXMgYnVpbHQgaW50byB0aGUgd29yZCDigJhwaG90b3N5bnRoZXNpcy7igJkg4oCYUGhvdG/igJkgcmVmZXJzIHRvIHdoYXQ/[Qq]

[f]IE5vLiBPeHlnZW4gaXMgYSBieXByb2R1Y3Qgb2YgcGhvdG9zeW50aGVzaXMuIFRoZSBlbmVyZ3kgc291cmNlIGZvciBwaG90b3N5bnRoZXNpcyBpcyBidWlsdCBpbnRvIHRoZSB3b3JkIOKAmHBob3Rvc3ludGhlc2lzLuKAmSDigJhQaG90b+KAmSByZWZlcnMgdG8gd2hhdD8=[Qq]

[f]IE5vLiBDYXJib24gZGlveGlkZSBpcyBhbiBpbnB1dCBmb3IgcGhvdG9zeW50aGVzaXMsIG5vdCBvbmUgb2YgdGhlIHByb2R1Y3RzLiBUaGUgZW5lcmd5IHNvdXJjZSBmb3IgcGhvdG9zeW50aGVzaXMgaXMgYnVpbHQgaW50byB0aGUgd29yZCDigJhwaG90b3N5bnRoZXNpcy7igJkg4oCYUGhvdG/igJkgcmVmZXJzIHRvIHdoYXQ/[Qq]

[f]IFllcy4gTGlnaHQgZW5lcmd5IGlzIHdoYXQgZHJpdmVzIHRoZSByZWFjdGlvbnMgb2YgcGhvdG9zeW50aGVzaXMu

[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d25d50296bbb4″ question_number=”13″]The correct chemical equation for photosynthesis is

[c]IEM= Ng== SA== MTI= Tw== [Qq]6 + 6CO 2 + energy(ATP) -> 6CO 2 + 6O 2

[c]IEM= Ng== SA== MTI= Tw== [Qq]6 + 6O 2 -> energy(ATP) + 6CO 2 + 6H 2 O

[c]IDZD Tw== Mg== ICsgNkg= Mg== TyArIEVuZXJneShsaWdodCkgLSZndDsgQw== [Qq]6 H 12 O 6 + 6O 2

[c]IDZI Mg== TyArIDZP MiA= IC0mZ3Q7IDZDTw== [Qq]2 + 6H 2 O

[f]IE5vLiBUaGUgcmVhY3Rpb24gYWJvdmUgc2hvd3MgZ2x1Y29zZSBiZWluZyBjb21iaW5lZCB3aXRoIGNhcmJvbiBkaW94aWRlLiBJbiBwaG90b3N5bnRoZXNpcywgY2FyYm9uIGRpb3hpZGUgaXMgY29tYmluZWQgd2l0aCB3YXRlciB0byBjcmVhdGUgZ2x1Y29zZSwgd2l0aCBveHlnZW4gcmVsZWFzZWQgYXMgYSBieXByb2R1Y3Qu[Qq]

[f]IE5vLiBJbiBwaG90b3N5bnRoZXNpcywgY2FyYm9uIGRpb3hpZGUgaXMgY29tYmluZWQgd2l0aCB3YXRlciB0byBjcmVhdGUgZ2x1Y29zZSwgd2l0aCBveHlnZW4gcmVsZWFzZWQgYXMgYSBieXByb2R1Y3Qu[Qq]

[f]IFllcy4gSW4gcGhvdG9zeW50aGVzaXMsIGNhcmJvbiBkaW94aWRlIGlzIGNvbWJpbmVkIHdpdGggd2F0ZXIgdG8gY3JlYXRlIGdsdWNvc2UsIHdpdGggb3h5Z2VuIHJlbGVhc2VkIGFzIGEgYnlwcm9kdWN0Lg==[Qq]

[f]IE5vLiBGaW5kIGEgcmVhY3Rpb24gdGhhdCBiZWdpbnMgd2l0aCBjYXJib24gZGlveGlkZSBhbmQgd2F0ZXIsIGFuZCBjb21iaW5lcyB0aG9zZSBpbnB1dHMgdG8gY3JlYXRlIGdsdWNvc2UsIHdpdGggb3h5Z2VuIHJlbGVhc2VkIGFzIGEgYnlwcm9kdWN0Lg==

[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d255bf02c6bb4″ question_number=”14″]The organelle that carries out photosynthesis in a plant cell is

[c]IHRoZSB2YWN1b2xl[Qq]

[c]IHRoZSBjaGxv cm9wbGFzdA==[Qq]

[c]IHRoZSBudWNsZXVz[Qq]

[c]IHRoZSBtaXRvY2hvbmRyaWE=[Qq]

[f]IE5vLiBUaGUgdmFjdW9sZeKAmXMgcm9sZSBpcyB0byBzdG9yZSB3YXRlciBhbmQgb3RoZXIgc3Vic3RhbmNlcy4gRm9yIGEgaGludCBhYm91dCB0aGUgb3JnYW5lbGxlIHRoYXQgZG9lcyBwaG90b3N5bnRoZXNpcywgdGhpbmsgb2YgdGhlIGdyZWVuIHBpZ21lbnQg4oCYY2hsb3JvcGh5bGwu4oCZIFdoaWNoIG9yZ2FuZWxsZSBoYXMgYSBzaW1pbGFyIHNvdW5kPw==[Qq]

[f]IFllcy4gVGhlIGNobG9yb3BsYXN0LCB3aGljaCBnZXRzIGl0cyBuYW1lIGZyb20gdGhlIHBpZ21lbnQgY2hsb3JvcGh5bGwsIGlzIHRoZSBvcmdhbmVsbGUgdGhhdCBjYXJyaWVzIG91dCBwaG90b3N5bnRoZXNpcy4=[Qq]

[f]IE5vLiBUaGUgbnVjbGV1cyBpcyB0aGUgY2VsbOKAmXMgY29udHJvbCBjZW50ZXIuIEZvciBhIGhpbnQgYWJvdXQgdGhlIG9yZ2FuZWxsZSB0aGF0IGRvZXMgcGhvdG9zeW50aGVzaXMsIHRoaW5rIG9mIHRoZSBncmVlbiBwaWdtZW50IOKAmGNobG9yb3BoeWxsLuKAmSBXaGljaCBvcmdhbmVsbGUgaGFzIGEgc2ltaWxhciBzb3VuZD8=[Qq]

[f]IE5vLiBUaGUgbWl0b2Nob25kcmlhIHBsYXkgYSBrZXkgcm9sZSBpbiBjZWxsdWxhciByZXNwaXJhdGlvbi4gRm9yIGEgaGludCBhYm91dCB0aGUgb3JnYW5lbGxlIHRoYXQgZG9lcyBwaG90b3N5bnRoZXNpcywgdGhpbmsgb2YgdGhlIGdyZWVuIHBpZ21lbnQg4oCYY2hsb3JvcGh5bGwu4oCZIFdoaWNoIG9yZ2FuZWxsZSBoYXMgYSBzaW1pbGFyIHNvdW5kPw==

[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d24dbe19e6fb4″ question_number=”15″]From a plant’s perspective, the purpose of photosynthesis is production of

[c]IGxpZ2h0[Qq]

[c]IGNhcmJvaHlkcmF0 ZSAoZ2x1Y29zZSk=[Qq]

[f]IE5vLiBDYXJib24gZGlveGlkZSBpcyBhbiBpbnB1dCBmb3IgcGhvdG9zeW50aGVzaXMuIEZyb20gYSBwbGFudOKAmXMgcGVyc3BlY3RpdmUsIHBob3Rvc3ludGhlc2lzIGlzIGFib3V0IG1ha2luZyBmb29kLiBXaGljaCBzdWJzdGFuY2UgaXMgYSB0eXBlIG9mIGZvb2Q/[Qq]

[f]IE5vLiBMaWdodCBpcyB0aGUgZW5lcmd5IHRoYXQgZHJpdmVzIHBob3Rvc3ludGhlc2lzLiBGcm9tIGEgcGxhbnTigJlzIHBlcnNwZWN0aXZlLCBwaG90b3N5bnRoZXNpcyBpcyBhYm91dCBtYWtpbmcgZm9vZC4gV2hpY2ggc3Vic3RhbmNlIGlzIGEgdHlwZSBvZiBmb29kPw==[Qq]

[f]IE5vLiBXYXRlciBpcyBhbiBpbnB1dCBmb3IgcGhvdG9zeW50aGVzaXMuIEZyb20gYSBwbGFudOKAmXMgcGVyc3BlY3RpdmUsIHBob3Rvc3ludGhlc2lzIGlzIGFib3V0IG1ha2luZyBmb29kLiBXaGljaCBzdWJzdGFuY2UgaXMgYSB0eXBlIG9mIGZvb2Q/[Qq]

[f]IFllcy4gRnJvbSBhIHBsYW504oCZcyBwZXJzcGVjdGl2ZSwgcGhvdG9zeW50aGVzaXMgaXMgYWJvdXQgbWFraW5nIGZvb2Q7IHRoYXQgZm9vZCBpcyBhIGNhcmJvaHlkcmF0ZSwgdHlwaWNhbGx5IGdsdWNvc2Uu[Qq]

[f]IE5vLiBPeHlnZW4gaXMgYSBieXByb2R1Y3Qgb2YgcGhvdG9zeW50aGVzaXMuIFdoaWxlIHdl4oCZcmUgdmVyeSBncmF0ZWZ1bCBmb3IgcGxhbnRzIGZvciBjcmVhdGluZyBveHlnZW4sIGl04oCZcyBub3QgdGhlIGdvYWwgb2YgdGhlIHByb2Nlc3MuIEZyb20gYSBwbGFudOKAmXMgcGVyc3BlY3RpdmUsIHBob3Rvc3ludGhlc2lzIGlzIGFib3V0IG1ha2luZyBmb29kLiBXaGljaCBzdWJzdGFuY2UgaXMgYSB0eXBlIG9mIGZvb2Q/

[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d245bd31073b4″ question_number=”16″]The carbon that photosynthesis puts into carbohydrates comes from

[c]IGNhcmJvbi BkaW94aWRl[Qq]

[c]IEFUUA==[Qq]

[f]IFllcy4gQ2FyYm9uIGRpb3hpZGUgaXMgdGhlIHNvdXJjZSBvZiB0aGUgY2FyYm9uIHRoYXQgcGhvdG9zeW50aGVzaXMgcHV0cyBpbnRvIGNhcmJvaHlkcmF0ZXMu[Qq]

[f]IE5vLiBMaWdodCBpcyB0aGUgZW5lcmd5IHRoYXQgZHJpdmVzIHBob3Rvc3ludGhlc2lzLiBGb3IgdGhlIHNvdXJjZSBvZiB0aGUgY2FyYm9uIHRoYXQgZ29lcyBpbnRvIGNhcmJvaHlkcmF0ZXMsIGxvb2sgZm9yIHRoZSBjaG9pY2UgdGhhdCBpcyBib3RoIGFuIGlucHV0IGZvciBwaG90b3N5bnRoZXNpcywgYW5kIHdoaWNoIGhhcyB0aGUgZWxlbWVudCBjYXJib24gaW4gaXQu[Qq]

[f]IE5vLiBBVFAgaXMgYW4gZW5lcmd5IHNvdXJjZSBmb3IgY2VsbHMsIGJ1dCBpdOKAmXMgbm90IGEgY2FyYm9uIHNvdXJjZS4gRm9yIHRoZSBzb3VyY2Ugb2YgdGhlIGNhcmJvbiB0aGF0IGdvZXMgaW50byBjYXJib2h5ZHJhdGVzLCBsb29rIGZvciB0aGUgY2hvaWNlIHRoYXQgaXMgYm90aCBhbiBpbnB1dCBmb3IgcGhvdG9zeW50aGVzaXMsIGFuZCB3aGljaCBoYXMgdGhlIGVsZW1lbnQgY2FyYm9uIGluIGl0Lg==[Qq]

[f]IE5vLiBHbHVjb3NlIGlzIG9uZSBvZiB0aGUgcHJvZHVjdHMgb2YgcGhvdG9zeW50aGVzaXMsIG5vdCB0aGUgZW5lcmd5IHNvdXJjZSBvciBjYXJib24gc291cmNlLiBGb3IgdGhlIHNvdXJjZSBvZiB0aGUgY2FyYm9uIHRoYXQgZ29lcyBpbnRvIGNhcmJvaHlkcmF0ZXMsIGxvb2sgZm9yIHRoZSBjaG9pY2UgdGhhdCBpcyBib3RoIGFuIGlucHV0IGZvciBwaG90b3N5bnRoZXNpcywgYW5kIHdoaWNoIGhhcyB0aGUgZWxlbWVudCBjYXJib24gaW4gaXQu[Qq]

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[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d23de188e5bb4″ question_number=”17″]Photosynthesis has changed our atmosphere by adding _______ to the air.

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[q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d22e4f7960fb4″ question_number=”19″]In the diagram below, which number shows a mitochondrion?

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[f]IE5vLiBOdW1iZXIgMSByZWZlcnMgdG8gdGhlIHN1bi4=[Qq]

[f]IE5vLiBOdW1iZXIgMiByZWZlcnMgdG8gYSBjaGxvcm9wbGFzdC4=[Qq]

[f]IFllcy4gTnVtYmVyIDUgc2hvd3MgYSBtaXRvY2hvbmRyaW9uLg==[Qq]

[q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d22673d13f7b4″ question_number=”20″]In the diagram below, number 4 would have to be

[c]wqB3YXRlcg==[Qq]

[f]IFllcy4gTnVtYmVyIDQgaXMgYSBnYXMgdGhhdOKAmXMgY29taW5nIG91dCBvZiBhIGNobG9yb3BsYXN0LiBUaGUgZ2FzIHJlbGVhc2VkIGZyb20gYSBjaGxvcm9wbGFzdCB3b3VsZCBoYXZlIHRvIGJlIG94eWdlbi4=[Qq]

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[f]IE5vLiBOdW1iZXIgNCBpcyBsYWJlbGVkIGFzIGEgZ2FzLCBhbmQgZ2x1Y29zZSBpcyBhIHNvbGlkLiBIb3dldmVyLCB5b3XigJlyZSBvbiB0aGUgcmlnaHQgdHJhY2ssIGluc29mYXIgYXMgeW914oCZcmUgbG9va2luZyBmb3Igb25lIG9mIHRoZSBvdXRwdXRzIG9mIHBob3Rvc3ludGhlc2lzLg==[Qq]

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[q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d21e2866e33b4″ question_number=”21″]In the diagram below, number 6 would have to be

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[f]IFllcy4gTnVtYmVyIDYgaXMgY2FyYm9uIGRpb3hpZGUsIHdoaWNoIGdvZXMgaW50byBjaGxvcm9wbGFzdHMgYXMgYW4gaW5wdXQgb2YgcGhvdG9zeW50aGVzaXMsIGFuZCBjb21lcyBvdXQgb2YgdGhlIG1pdG9jaG9uZHJpb24gYXMgYSB3YXN0ZSBwcm9kdWN0IG9mIGNlbGx1bGFyIHJlc3BpcmF0aW9uLg==[Qq]

[f]IE5vLiBHbHVjb3NlIGlzIGEgc29saWQgdGhhdCBjb21lcyBvdXQgb2YgcGhvdG9zeW50aGVzaXMgYW5kIGdvZXMgaW50byB0aGUgbWl0b2Nob25kcmlvbiAoNSkuIE51bWJlciBzaXggaXMgYSBnYXMgdGhhdCBpcyBhbiBpbnB1dCBvZiBwaG90b3N5bnRoZXNpcyBhbmQgb3V0cHV0IG9mIHJlc3BpcmF0aW9uLiBXaGF0IHN1YnN0YW5jZSBjb3VsZCBwbGF5IHRob3NlIHR3byByb2xlcz8=[Qq]

[f]IE5vLiBPeHlnZW4gY29tZXMgb3V0IG9mIHRoZSBjaGxvcm9wbGFzdCAoMikgYW5kIGdvZXMgaW50byB0aGUgbWl0b2Nob25kcmlvbiAoNSkuIE51bWJlciBzaXggaXMgYSBnYXMgdGhhdCBpcyBhbiBpbnB1dCBvZiBwaG90b3N5bnRoZXNpcyBhbmQgb3V0cHV0IG9mIHJlc3BpcmF0aW9uLiBXaGF0IHN1YnN0YW5jZSBjb3VsZCBwbGF5IHRob3NlIHR3byByb2xlcz8=[Qq]

[q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d216277e037b4″ question_number=”22″]In the diagram below, number 7 would have to be

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[f]IE5vLiBPeHlnZW4gaXMgc2hvd24gYXQgNC4gSXTigJlzIGEgZ2FzIHRoYXQgY29tZXMgb3V0IG9mIGEgY2hsb3JvcGxhc3QgKDIpIGFuZCBnb2VzIGludG8gdGhlIG1pdG9jaG9uZHJpb24gKDUpLiBXaGF0IGxpcXVpZCBnb2VzIGludG8gcGhvdG9zeW50aGVzaXMgYW5kIGNvbWVzIG91dCBvZiByZXNwaXJhdGlvbj8=[Qq]

[f]IE5vLiBDYXJib24gZGlveGlkZSBpcyBzaG93biBhdCA2LiBXaGF0IGxpcXVpZCBnb2VzIGludG8gcGhvdG9zeW50aGVzaXMgYW5kIGNvbWVzIG91dCBvZiByZXNwaXJhdGlvbj8=[Qq]

[f]IE5vLiBHbHVjb3NlIGlzIGEgc29saWQgdGhhdCBjb21lcyBvdXQgb2YgcGhvdG9zeW50aGVzaXMgYW5kIGdvZXMgaW50byB0aGUgbWl0b2Nob25kcmlvbi4gSXTigJlzIHNob3duIGF0IDMuIFdoYXQgbGlxdWlkIGdvZXMgaW50byBwaG90b3N5bnRoZXNpcyBhbmQgY29tZXMgb3V0IG9mIHJlc3BpcmF0aW9uPw==[Qq]

[f]IFllcy4gV2F0ZXIgaXMgdGhlIGxpcXVpZCB0aGF0IGNvbWVzIG91dCBvZiByZXNwaXJhdGlvbiAod2hpY2ggb2NjdXJzIGluIHRoZSBtaXRvY2hvbmRyaW9uKSBhbmQgZ29lcyBpbnRvIHBob3Rvc3ludGhlc2lzICh3aGljaCBvY2N1cnMgaW4gdGhlIGNobG9yb3BsYXN0KS4=[Qq]

[q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d20ddc13a73b4″ question_number=”23″]In the diagram below, ATP would have to be

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[f]IE5vLiBXYXRlciBpcyBzaG93biBhdCA3LiBBVFAgaXMgcHJvZHVjZWQgaW4gYSBtaXRvY2hvbmRyaW9uIGR1cmluZyBjZWxsdWxhciByZXNwaXJhdGlvbiAoYW5kIGl04oCZcyBub3QgYW4gaW5wdXQgZm9yIHBob3Rvc3ludGhlc2lzKQ==[Qq]

[f]IE5vLiBOdW1iZXIgNCBpcyBveHlnZW4uIEFUUCBpcyBwcm9kdWNlZCBpbiBhIG1pdG9jaG9uZHJpb24gZHVyaW5nIGNlbGx1bGFyIHJlc3BpcmF0aW9uIChhbmQgaXTigJlzIG5vdCBhbiBpbnB1dCBmb3IgcGhvdG9zeW50aGVzaXMp[Qq]

[q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d205b5ea093b4″ question_number=”24″]In the diagram below, number 3 would have to be

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• Photosynthesis 2: The Two Phases of Photosynthesis (the next tutorial in this series)
• Photosynthesis Main Menu

• Biology Article

Chloroplasts

Plants form the basis of all life on earth and are known as producers. Plant cells contain structures known as plastids which are absent in animal cells. These plastids are double-membraned cell organelles which play a primary role in the manufacturing and storing of food. There are three types of plastids –

• Chromoplasts- They are the colour plastids, found in all flowers, fruits and are mainly responsible for their distinctive colours.
• Chloroplasts- They are green coloured plastids, which comprise green-coloured pigments within the plant cell and are called chlorophyll.
• Leucoplasts- They are colourless plastids and are mainly used for the storage of starch, lipids and proteins within the plant cell.

Table of Contents

• Explanation

Let us have a detailed look at the chloroplast structure and function.

Chloroplast Definition

“Chloroplast is an organelle that contains the photosynthetic pigment chlorophyll that captures sunlight and converts it into useful energy, thereby, releasing oxygen from water. “

What is a Chloroplast?

Chloroplasts are found in all green plants and algae. They are the food producers of plants. These are found in mesophyll cells located in the leaves of the plants. They contain a high concentration of chlorophyll that traps sunlight. This cell organelle is not present in animal cells.

Chloroplast has its own extra-nuclear DNA and therefore are semiautonomous, like mitochondria. They also produce proteins and lipids required for the production of chloroplast membrane.

Also Read :  Plastids

Diagram of Chloroplast

The chloroplast diagram below represents the chloroplast structure mentioning the different parts of the chloroplast. The parts of a chloroplast such as the inner membrane, outer membrane, intermembrane space, thylakoid membrane, stroma and lamella can be clearly marked out.

Chloroplast Diagram representing Chloroplast Structure

 Structure of Chloroplast

Chloroplasts are found in all higher plants. It is oval or biconvex, found within the mesophyll of the plant cell . The size of the chloroplast usually varies between 4-6 µm in diameter and 1-3 µm in thickness. They are double-membrane organelle with the presence of outer, inner and intermembrane space. There are two distinct regions present inside a chloroplast known as the grana and stroma.

• Grana are made up of stacks of disc-shaped structures known as thylakoids or lamellae. The grana of the chloroplast consists of chlorophyll pigments and are the functional units of chloroplasts.
• Stroma is the homogenous matrix which contains grana and is similar to the cytoplasm in cells in which all the organelles are embedded. Stroma also contains various enzymes, DNA, ribosomes, and other substances. Stroma lamellae function by connecting the stacks of thylakoid sacs or grana.

The chloroplast structure consists of the following parts:

Membrane Envelope

It comprises inner and outer lipid bilayer membranes. The inner membrane separates the stroma from the intermembrane space.

• Intermembrane Space

The space between inner and outer membranes.

Thylakoid System (Lamellae)

The system is suspended in the stroma. It is a collection of membranous sacs called thylakoids or lamellae. The green coloured pigments called chlorophyll are found in the thylakoid membranes. It is the sight for the process of light-dependent reactions of the photosynthesis process. The thylakoids are arranged in stacks known as grana and each granum contains around 10-20 thylakoids.

It is a colourless, alkaline, aqueous, protein-rich fluid present within the inner membrane of the chloroplast present surrounding the grana.

Stack of lamellae in plastids is known as grana. These are the sites of conversion of light energy into chemical energy.

Chlorophyll

It is a green photosynthetic pigment that helps in the process of photosynthesis.

Also read:  Light-dependent Reactions

Functions of Chloroplast

Following are the important chloroplast functions:

• The most important function of the chloroplast is to synthesise food by the process of photosynthesis.
• Absorbs light energy and converts it into chemical energy.
• Chloroplast has a structure called chlorophyll which functions by trapping the solar energy and is used for the synthesis of food in all green plants.
• Produces NADPH and molecular oxygen (O 2 ) by photolysis of water.
• Produces ATP – Adenosine triphosphate by the process of photosynthesis.
• The carbon dioxide (CO2) obtained from the air is used to generate carbon and sugar during the Calvin Cycle or dark reaction of photosynthesis.

Also Refer:   Calvin Cycle

Frequently Asked Questions

Where does the photosynthesis process occur in the plant cell.

In all green plants, photosynthesis takes place within the thylakoid membrane of the Chloroplast.

List out the different parts of Chloroplast?

Chloroplasts are cell organelles present only in a plant cell and it includes:

• Inner membrane
• Outer membrane
• Thylakoid membrane

What is the most important function of chloroplast?

The most important function of chloroplast is the production of food by the process of photosynthesis.

Why is the chloroplast green?

Chloroplast contains a green pigment called chlorophyll which gives it a green colour.

How many types of plastids are there?

There are three types of plastids-chloroplast, chromoplast and leucoplast.

What is the stack of lamellae inside a plastid called?

The stack of lamellae or thylakoids inside a plastid is called grana.

Put your understanding of this concept to test by answering a few MCQs. Click ‘Start Quiz’ to begin!

Select the correct answer and click on the “Finish” button Check your score and answers at the end of the quiz

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What is the definition of chloroplast

The chloroplast is an organelle that contains the photosynthetic pigment chlorophyll which uses sunlight to create energy which can be used by the plant.

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Biology Wise

Photosynthesis Explained with a Diagram

It is extremely important to know the meaning and process of photosynthesis, irrespective of the fact that whether it the part of one's curriculum or not. The diagram given in this BiologyWise article is a small pictorial elaboration of the process of photosynthesis that will prove helpful for kids and teenagers to understand this vital process of the plant kingdom.

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It is extremely important to know the meaning and process of photosynthesis, irrespective of the fact that whether it the part of one’s curriculum or not. The diagram given in this BiologyWise article is a small pictorial elaboration of the process of photosynthesis that will prove helpful for kids and teenagers to understand this vital process of the plant kingdom.

The world, our planet, and the life on it are merely a magic trick by God. Mother nature contains all living things and some non-living factors too. All the living things need some food to stay alive. We consume many different foods in a single day, cakes, burgers … well the list is almost endless. Similarly, other animals also consume some or the other type of foods. Deer eat grass, fish eat plants or small fish and microbes, that are present in the water. Animals such as lions and tigers eat other animals. But have you ever wondered, what do plants eat?

What is Photosynthesis?

The process that plants carry out in the presence of radiant energy in order to create their food is known as photosynthesis. This process is one of the reasons because of which man and other forms of life are alive on the Earth today. This process basically occurs in the green parts of leaves. This process requires the following ingredients.

Soil does not become directly involved in the process of photosynthesis, but the plant absorbs some important ingredients, that are present in the soil. Chlorophyll is actually a chemical that is found in most of the plants and imparts green color to them. The process of photosynthesis actually becomes possible due to the chlorophyll that is present in plant leaves. During this process of food generation, the following reaction takes place:

6 CO 2 + 6 H 2 O → (in the presence of sunlight) C 6 H 12 O 6 + 6 O 2

Photosynthesis Diagram

According to the diagram of photosynthesis, the process begins with three most important non-living elements: water, soil, and carbon dioxide. Plants begin making their ‘food’, which basically includes large quantities of sugars and carbohydrate, when sunlight falls on their leaves. The ‘food’ is then stored aside by the plant and some of it is consumed during the day. This process goes on till the end of the day (until sunlight is available). The ‘food’ that is prepared by the plants is always in excess and humans and other animals consume it through different sources such as fruits and vegetables. Animals and human beings in return breathe out carbon dioxide during the process of cellular respiration. This carbon dioxide is in turn used by the plants to make more food. The rains and accumulated water table provides water to the plants and the sun provides light (radiant energy) every day. This process is thus, nothing but a cycle that goes on and on. According to the facts of this phenomenon, this cycle has been going on for almost 3,500 million years, which is quite a long time.

The process of photosynthesis is the reason why all animals and human beings are alive today. Hence, it is absolutely necessary to help plants, to complete this food-producing process. We can simply follow this by not plucking their leaves and watering them every day.

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Characteristics of chloroplasts

The photosynthetic machinery, chloroplast genome and membrane transport.

What is a chloroplast?

Where are chloroplasts found, do chloroplasts have dna.

• Why is photosynthesis important?
• What is the basic formula for photosynthesis?

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• Frontiers - Chloroplast Degradation: Multiple Routes Into the Vacuole
• Biology LibreTexts - Chloroplast
• Khan Academy - Mitochondria and chloroplasts
• National Center for Biotechnology Information - Chloroplasts and Photosynthesis
• Florida State University - Molecular Expressions - Chloroplasts
• Table Of Contents

A chloroplast is an organelle within the cells of  plants  and certain algae  that is the site of  photosynthesis , which is the process by which energy from the Sun is converted into chemical energy for growth. A chloroplast is a type of plastid (a saclike organelle with a double membrane) that contains  chlorophyll to absorb light energy.

Chloroplasts are present in the cells of all green tissues of plants and algae . Chloroplasts are also found in photosynthetic tissues that do not appear green, such as the brown blades of giant kelp or the red leaves of certain plants. In plants, chloroplasts are concentrated particularly in the parenchyma cells of the leaf mesophyll (the internal cell layers of a leaf ).

Why are chloroplasts green?

Chloroplasts are green because they contain the pigment chlorophyll , which is vital for photosynthesis . Chlorophyll occurs in several distinct forms. Chlorophylls  a  and  b  are the major pigments found in higher plants and green algae.

Unlike most other organelles , chloroplasts and mitochondria have small circular chromosomes known as extranuclear DNA. Chloroplast DNA contains genes that are involved with aspects of  photosynthesis and other chloroplast activities. It is thought that both chloroplasts and mitochondria are descended from free-living cyanobacteria , which could explain why they possess DNA that is distinct from the rest of the cell.

chloroplast , structure within the cells of plants and green algae that is the site of photosynthesis , the process by which light energy is converted to chemical energy , resulting in the production of oxygen and energy-rich organic compounds . Photosynthetic cyanobacteria are free-living close relatives of chloroplasts; endosymbiotic theory posits that chloroplasts and mitochondria (energy-producing organelles in eukaryotic cells ) are descended from such organisms.

Chloroplasts are a type of plastid—a round, oval, or disk-shaped body that is involved in the synthesis and storage of foodstuffs. Chloroplasts are distinguished from other types of plastids by their green colour, which results from the presence of two pigments, chlorophyll a and chlorophyll b . A function of those pigments is to absorb light energy for the process of photosynthesis . Other pigments, such as carotenoids , are also present in chloroplasts and serve as accessory pigments, trapping solar energy and passing it to chlorophyll. In plants, chloroplasts occur in all green tissues, though they are concentrated particularly in the parenchyma cells of the leaf mesophyll.

Chloroplasts are roughly 1–2 μm (1 μm = 0.001 mm) thick and 5–7 μm in diameter . They are enclosed in a chloroplast envelope, which consists of a double membrane with outer and inner layers, between which is a gap called the intermembrane space. A third, internal membrane, extensively folded and characterized by the presence of closed disks (or thylakoids ), is known as the thylakoid membrane. In most higher plants, the thylakoids are arranged in tight stacks called grana (singular granum ). Grana are connected by stromal lamellae, extensions that run from one granum, through the stroma, into a neighbouring granum . The thylakoid membrane envelops a central aqueous region known as the thylakoid lumen. The space between the inner membrane and the thylakoid membrane is filled with stroma , a matrix containing dissolved enzymes , starch granules, and copies of the chloroplast genome.

The thylakoid membrane houses chlorophylls and different protein complexes, including photosystem I, photosystem II, and ATP ( adenosine triphosphate ) synthase, which are specialized for light-dependent photosynthesis. When sunlight strikes the thylakoids, the light energy excites chlorophyll pigments, causing them to give up electrons . The electrons then enter the electron transport chain, a series of reactions that ultimately drives the phosphorylation of adenosine diphosphate (ADP) to the energy-rich storage compound ATP. Electron transport also results in the production of the reducing agent nicotinamide adenine dinucleotide phosphate (NADPH).

ATP and NADPH are used in the light-independent reactions (dark reactions) of photosynthesis, in which carbon dioxide and water are assimilated into organic compounds . The light-independent reactions of photosynthesis are carried out in the chloroplast stroma, which contains the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco). Rubisco catalyzes the first step of carbon fixation in the Calvin cycle (also called Calvin-Benson cycle), the primary pathway of carbon transport in plants. Among so-called C 4 plants, the initial carbon fixation step and the Calvin cycle are separated spatially—carbon fixation occurs via phosphoenolpyruvate (PEP) carboxylation in chloroplasts located in the mesophyll, while malate, the four-carbon product of that process, is transported to chloroplasts in bundle-sheath cells, where the Calvin cycle is carried out. C 4 photosynthesis attempts to minimize the loss of carbon dioxide to photorespiration. In plants that use crassulacean acid metabolism (CAM), PEP carboxylation and the Calvin cycle are separated temporally in chloroplasts, the former taking place at night and the latter during the day. The CAM pathway allows plants to carry out photosynthesis with minimal water loss.

The chloroplast genome typically is circular (though linear forms have also been observed) and is roughly 120–200 kilobases in length. The modern chloroplast genome, however, is much reduced in size: over the course of evolution , increasing numbers of chloroplast genes have been transferred to the genome in the cell nucleus . As a result, proteins encoded by nuclear DNA have become essential to chloroplast function. Hence, the outer membrane of the chloroplast, which is freely permeable to small molecules, also contains transmembrane channels for the import of larger molecules, including nuclear-encoded proteins. The inner membrane is more restrictive, with transport limited to certain proteins (e.g., nuclear-encoded proteins) that are targeted for passage through transmembrane channels.

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Leaf Structures Involved in Photosynthesis

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When it comes to photosynthesis, the most important parts of the plant are the leaves. Their cells and structures are specialized to take in light and allow for gas exchange with the air around them. They also contain vascular structures that transport water from the roots into the cells that carry out photosynthesis.

1. The plant’s vascular tissues—xylem and phloem—transport water to the leaves and carry glucose away from the leaves.

Anyone who cares for plants could probably tell you that pouring water directly onto the leaves isn’t the best idea. Plants absorb water from the soil, using their roots.

As you probably already know, water is necessary for photosynthesis, which primarily occurs in the plant’s leaves. You might wonder how the water gets from the roots into the leaves, and the answer is through the plant’s vascular system! Just like the veins and arteries that circulate blood throughout our bodies, the plant’s vascular tissues move water, nutrients, and the products of photosynthesis throughout the plant.

When a plant’s roots absorb water and nutrients from the soil, these materials move up the stem and into the leaves through the xylem. Capillary action—which relies on liquid’s properties of cohesion, surface tension, and adhesion—is what allows water to “defy gravity” as it travels through the xylem and into the leaves.

Once photosynthesis has occurred, the produced sugars move through the phloem to other parts of the plant to be used in cellular respiration or stored for later.

2. Stomata, regulated by guard cells, allow gases to pass in and out of the leaf.

We may not be able to see them with the naked eye, but the leaves of plants contain tons of tiny holes, or pores, called stomata (sing. stoma). They play a central role in photosynthesis, allowing carbon dioxide to enter the leaf and oxygen to exit the leaf. The stomata also facilitate transpiration, the process by which water vapor is released through a plant’s leaves.

The stomata can be opened and closed, depending on the turgor pressure—the pressure of a cell’s contents against the cell wall—in the two guard cells that border each stoma. High turgor pressure causes these cells to bend outward, opening the stomatal pore. Low turgor pressure, due to loss of water, keeps the stomatal pores closed.

3. Cells in the mesophyll of the leaf have numerous chloroplasts.

In leaves, cells in the mesophyll (the tissue between the upper and lower epidermis) are uniquely suited to carry out photosynthesis on a large scale. This is due to their high concentration of chloroplasts, which are the sites of photosynthesis. More chloroplasts means more photosynthetic capability.

Certain types of plants (dicots and some net-veined monocots) have two different types of mesophyll tissue. Palisade mesophyll cells are densely packed together, whereas spongy mesophyll cells are arranged more loosely to allow gases to pass through them. Palisade mesophyll cells also have more chloroplasts than spongy mesophyll cells.

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External Sources

A fun and easy activity from Scientific American that allows you to observe capillary action.

An OSU page explaining turgor pressure inside plant cells.

An article on transpiration and the water cycle from the USGS.

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Photosystems Labeling

*Label the steps that occur in the process shown.

1)  _________________________________________________________  2) __________________________________________________________ 3) __________________________________________________________               4) __________________________________________________________ 5) __________________________________________________________

Other Resources on Photosynthesis

Plant Pigments  – Use chromatography to observe plant pigments separate

Photosynthesis Simulation  – this simulator uses light and varying levels of carbon dioxide to explore rates of photosynthesis, replaces the waterweed simulator

Inquiry Investigation – what factors affect rate of transpiration  – use leaves and evaporation to explore how plants take up water

Stomate and Leaf Investigation  – view stomata under the microscope and determine density on upper and lower surface of the leaf

Investigate Photosynthesis with Vernier Probes  – explore changes in oxygen levels in plants exposed to light

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Biology Dictionary

[LS1-5] Photosynthesis Modeling

This standard focuses on photosynthesis, specifically on the inputs and outputs that allow sunlight energy to be capture and used to create new bonds stored in sugar molecules.

Resources for this Standard:

For teachers & students.

• Overview (This article)
• Quick Concept Quiz

For Teachers Only

Coming Soon!

Here’s the Actual Standard:

Use a model to illustrate how photosynthesis transforms light energy into stored chemical energy.

Standard Breakdown

To understand this standard, we will start with the view from the top and work our way down to specifics.

Photosynthesis

Organelles of Photosynthesis

A little clarification:

The standard contains this clarification statement: Emphasis is on illustrating inputs and outputs of matter and the transfer and transformation of energy in photosynthesis by plants and other photosynthesizing organisms. Examples of models could include diagrams, chemical equations, and conceptual models. Let’s look at this clarification a little closer:

Inputs and Outputs of Photosynthesis

What to Avoid

This NGSS standard also contains the following Assessment Boundary: Assessment does not include specific biochemical steps. Here’s a little more specificity on what that means:

Biochemical Steps of Photosynthesis:

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Microbe Notes

Plant Cell: Structure, Parts, Functions, Labeled Diagram

Plant cells are eukaryotic cells, that are found in green plants, photosynthetic eukaryotes of the kingdom Plantae which means they have a membrane-bound nucleus.

They have a variety of membrane-bound cell organelles that perform various specific functions to maintain the normal functioning of the plant cell.

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Structure of Plant cell

Generally, plant cells are a lot bigger than animal cells, coming in more similar sizes and they are typically cubed or rectangular in shape.  Plant cells also have structural organelles that are not found in the animals’ cells including the cell wall, vacuoles, plastids e. g Chloroplast. Animal cells also contain structures that are not found in the plant cells such as, cilia and flagella, lysosomes, and centrioles.

The typical characteristics that define the plant cell include cellulose, hemicellulose and pectin, plastids which play a major role in photosynthesis and storage of starch, large vacuoles responsible for regulating the cell turgor pressure. They also have a very unique cell division process whereby there is the formation of a phragmoplast (a complex made up of microtubules, microfilaments, and the endoplasmic reticulum) all assembling during cytokinesis, to separate the daughter cells.

These organelles most of them are similar to the animal organelles performing the same functions as those of the animal cell.  Organelles  have a wide range of responsibilities that include everything from producing hormones and enzymes to providing energy for a plant cell.

Plants cells have DNA that helps in making new cells, hence enhancing the growth of the plant. the DNA is enclosed within the nucleus, an enveloped membrane structure at the center of the cell. The plant cell also has several cell organelle structures performing a variety of functions to maintain cellular metabolisms, growth, and development.

Plant Cell Free Worksheet

List of 14 Plant Cell Organelles

• Cytoskeleton
• Cell (Plasma) membrane

Plasmodesmata

• The cytoplasm

Plant Vacuoles

Mitochondria, endoplasmic reticulum (er).

• Storage granules

Golgi bodies

Peroxisomes, plant cell wall.

Plant Cell Wall is the rigid outer cover of the plant cell with a major role of protecting the plant cell, giving it, its shape.

Structure of plant cell wall

• It is a specialized matrix that covers the surface of the plant cell. Every plant cell has a cell wall layer which is a major distinguishing factor between a plant cell and an animal cell.
• The cell wall is made up of two layers, a middle lamella, and a primary cell wall and sometimes a secondary cell wall.
• The middle lamella acts as the strengthening layer between the primary walls of the neighboring cells.
• The primary wall is made up of cellulose underlying the cells that are dividing and maturing. The primary wall is a lot thinner and less rigid as compared to those of the cells that have reached complete maturation. The thinness allows the cell wall to expand.
• After full cell growth, some plants get rid of the primary wall but most, they thicken the primary wall or it makes another layer with rigidity but a different arrangement, known as the secondary wall.
• The secondary wall offers permanent stiff mechanical support to the plant cell especially the support found in wood.
• In contrast to the permanent stiffness and load-bearing capacity of thick secondary walls.

The function of the plant cell wall

The primary role of the cell wall is defined to be a mechanical and structural function, that is highly effective in serving the plant cell. These functions include:

• Providing the cell with mechanical protection and shielding the cell from the chemically harsh environment, provided by the secondary wall layer.
• It is semipermeable hence it allows in and out, the circulation of materials such as water, molecular nutrients, and minerals.
• It also forms provides a rigid building block to stabilize the plant to produce some of its structures, for example, the stem and leaves of the plants.
• It also provided a site for the storage of some elements such as the regulatory molecules that detect pathogens in the plant, hindering the development of diseased tissue.
• The thin primary walls serve as structural and supportive functional layers when the cell vacuoles are filled with water, exerting turgor pressure on the cell wall, thus maintaining the plants’ stiffness and preventing plants from losing water and withering.

The basic building block is made of cellulose fibers, of both the primary and secondary walls, despite having different compositions and structures. Cellulose is a polysaccharide matrix that offers tensile strength to the cells. This strength is entrenched within the highly concentrated matrix of water and glycoproteins.

Plant  cytoskeleton

This is a network of microtubules and filaments that plays a primary role in maintaining the plant cell shape and giving the cell cytoplasm support and maintaining its structural organization. These filaments and tubules normally extend all over the cell, through the cell cytoplasm. Besides giving support and maintaining the cell and the cell cytoplasm, its also involved in the transportation of cellular molecules, cell division, and cell signaling activities.

Structure of the plant cytoskeleton

The cytoskeleton has an essential definition of the structure of eukaryotic cells, describing the support system of these cells, the maintenance factors and transport involvements within the cell. These functions are defined by the structure of the cytoskeleton which is made up of three filaments i. e actin filament (microfilaments), microtubules and intermediate filaments.

• Microfilaments, also known as actin filaments, are a meshwork of fibers running parallel to each other. They are made up of the thin strands of actin proteins hence the name actin filaments. They are the thinnest filaments of the cytoskeleton with a thickness of 7 nanometers.
• Intermediate filaments have a diameter of about 8-12 nm; They lie between the actin filaments and the microtubules. Its function in plant cells is not clearly understood
• Microtubules are hollow tubes made up of tubulins, with a diameter of 23nm. They are the largest filament compared to the other two filaments.

Functions of the plant cytoskeleton

Microfilaments.

• They play a primary role is a division of the cell cytoplasm by a mechanism known as cytokinesis, forming two daughter cells.
• They also participate in cytoplasmic streaming, a process of cytosol flow all over the cell, transporting nutrients and cell organelles.

Intermediate Filaments

• The intermediate filaments’ role in the plant cells is not clearly understood but has a role to play in maintaining the cell shape, structural support and retain tension within the cell.

Microtubules

• Unlike the role of the microtubule in cell division in the animal cell, the plant cell uses the microtubules to transport materials within the vell and they are also used in forming the plant cell, cell wall.

Other functions of the cytoskeleton in plants include:

• Giving the plant cell shape, maintaining the cell shape and transportation of some cell organelles throughout the cell, molecules, and nutrients across the cell cytoplasm.
• It also plays a role in mitotic cell division.
• In summary, the cytoskeleton is the frame of building the cell, hence it maintains the cell structure, provides cell structural support and defines the cell structure.

Plant  Cell (Plasma) membrane

Structure of the plant cell (plasma) membrane.

• This is a bilipid membrane that is made up of protein subunits and carbohydrates, with a characteristic semi permeability factor.
• It surrounds the cell cytoplasm, thus enclosing its content.

Functions of the  plant cell (plasma) membrane

• In-plant cells the cell membrane separated the cytoplasm from the cell wall.
• It has a selective permeability hence it regulates the contents that move in and out of the cell.
• It also protects the cell from external damage and provides support and stability to the cell.
• It has embedded proteins which are conjugated with lipids and carbohydrates, along the membrane, used to transport cellular molecules.

Plasmodesmata are microscopic channels that assist in communicating and transporting materials across plant cells.

They connect the cellular plant spaces allowing intracellular movement of cellular nutrients, water, minerals, and other molecules. They also allow signaling of cellular molecules. There are two types of plasmodesmata

• Primary plasmodesmata , formed during cell division.
• Secondary plasmodesmata , formed between mature plant cells.

Primary plasmodesmata are formed when part of the endoplasmic reticulum is caught in the middle lamella as the new cell wall is processed during cell division. As they form, they create a connection between each adjacent, and at the connection site, they form thin spaces known as pits on the walls. The plasmodesmata may get inserted to already mature cells just between their cell wall and these are termed as the secondary plasmodesmata. These are found in plant cells and algal cells, evolving independently. Plasmodesmata structure is regulated by callose polymer formed during cell cytokinesis.

Structure of plasmodesmata of plant cells

Plasmodesmata have a diameter of 50–60 nm in diameter. They have three layers i.e. plasma membrane, cytoplasmic sleeve, and the desmotubules. these layers can thicken the cell wall up to about 90nm.

• Plasma membrane –  it is a continuous extension on the plasmalemma that is made up of phospholipids layered structure.
• Cytoplasmic sleeves –  are fluid-filled spaces enclosed by the plasmalemma forming an endless pouch of the cytosol.
• Desmotubules –   this is a flat tube originating from the endoplasmic reticulum, running between two adjacent cells.

Functions of the plasmodesmata

• Transportation of transcription proteins, short units of RNA, mRNA, viral genomes and viral particles from one cell to another. Such as the movement of MP-30 proteins of the Tobacco mosaic virus, which binds to the viral genome moving it from infected cell to non-infected cell, through the plasmodesmata.MP-30 is thought to bind to the virus’s own genome and shuttle it from infected cells to uninfected cells through plasmodesmata.
• They are used to regulate the sieve tube cells with the help of the companion cells.
• They are also used by the phloem cells to facilitate the transportation of nutrients.
• This is a gel-like matrix lying just below the cell membrane, housing most of the cell organelles.
• Its made up of water, enzymes, salts, organelles, and various organic molecules.
• It is not classified as one of the cell’s organelles because it doesn’t possess major roles except being a physical medium for holding and housing most of the complex cell’s interior organelles and being a medium for transporting and processing cell molecules for maintaining cell life.
• This is because some of these organelles have their own membranes that protect them, for example, the mitochondria and the Golgi bodies have at least 2 layers offering several functions to the organelles.
• The nucleus is not classified as part of the cytoplasm because of its double-layered centrally placed features and it has its own organelles and sub-organelles enclosed within it.
• The cytoplasm of the plant houses several organelles including Plastids, Mitochondria, Central vacuoles, Endoplasmic reticulum, Golgi bodies, Storage granules, lysosomes.

Plastids are specialized organelles found specifically in plant and algal cells. They have a double-layered membrane.

• They have characteristic pigments that aid their mechanisms majorly in food processing and storage. these pigments also determine the color of the plant.
• Generally, plastids are used to manufacture and store food for plants double-membrane organelle which is found in the cells of plants and algae.
• Plastids have the ability to differentiate in between there forms and they can multiply rapidly by binary fission, depending on the cell, forming over 1000 plastid copies. In mature cells, plastids reduce in number to about 100 per mature cell.
• Plastids are derivates of proplastids (undifferentiated plastids), found in the meristematic tissues of the plant.

Development of plastids

Plastids associated with the inner membrane of the cell, existing as large protein-DNA complexes known as plastid nucleoids. The nucleoids have at least 10 copies of plastid DNA. Undifferentiated plastids are known as proplastids, and each proplastid has one nucleoid. These differentiate into the plastid which has more nucleoids found at the edges of the membranes bound to the inner envelope membrane.

During differentiation and development, the proplastid nucleoid undergoes remodeling, changing its shape, size and moves to a different location within the organelle. This mechanism of remodeling is mediated by the nucleoid proteins.

General functions of plastids

• They are actively involved in manufacturing food for the plant by photosynthesis due to the presence of chlorophyll pigment in the chloroplast.
• They also store food in the form of starch.
• They have the ability to synthesize fatty acids and terpenes that produces energy for the cell’s mechanisms.
• Palmitic acid, a component synthesized by chloroplasts is used in manufacturing the plant cuticle and waxy materials.

Types of Plastids

Plastids are classified based on their functions and the presence of the characteristic pigments. They include:

• Chloroplasts  – green plastids used in photosynthesis
• Chromoplasts  – colored plastids used to synthesize and store plant pigments
• Gerontoplasts  – they dismantle photosynthetic apparatus during aging of plants
• Leucoplasts  – they are colorless plastids used to manufacture terpene substance that protects the plants. they can differentiate, forming specialized plastids performing a variety of functions. i. e amyloplast. elaioplasts. proteinoplast, tannosomes.

Chloroplast

Structure of the plant cell chloroplast.

• These are organelles found in plant cells and algal cells.
• They are oval-shaped.
• They are made up of two surface membranes, i.e outer and inner membrane and an inner layer known as the thylakoid layer has two membranes.
• The outer membrane forms the external lining of the chloroplast while the inner membrane is below the outer layer.
• The membranes are separated by thin membranous space and within the membrane, there is also a space known as the stroma. The stroma houses the chloroplast.
• The third layer known as the thylakoid layer is extensively folded making the appearance of a flattened disk known as thylakoids which have large numbers of chlorophyll and carotenoids and the electron transport chain, defined as the l ight-harvesting complex, used during photosynthesis.
• Thylakoids are piled on top of each other in stacks known as grana.

Functions of the plant cell chloroplast

• The chloroplast is the site of food synthesis for plant cells, by a mechanism known as photosynthesis .
• Chloroplasts contain chlorophyll, a green pigment that absorbs light energy from the sun for photosynthesis.
• The photosynthesis process converts water, carbon dioxide, and light energy into nutrients for utilization by the plants .
• Thylakoids contain chlorophyll pigments and carotenoids for trapping light energy for use in photosynthesis.
• the chlorophyll pigment gives plants their green color.

Chromoplast plastid

Chromoplasts define all the plant pigments stored and synthesized in plants. They are found in a variety of plants of all kinds of ages.

• They are normally formed from the chloroplasts is the name given to an area for all the pigments to be kept and synthesized in the plant.
• The have carotenoid pigments that allow the differentiation in color seen in flowers and fruits. Its color attracts pollination mechanisms by pollinators.

Structure of plant chromoplast

Microscopic observation indicates that chromoplast has at least four types:

• Proteic stroma which contains granules
• Amorphous pigment with granules
• Protein and pigment crystals
• Crystalised chromoplast

Although, the more specialized feature has been observed classifying it further into 5 types:

• Globular chromoplasts which appear as globules
• Crystalline chromoplast which appears crystalized
• Fibrillar chromoplast which appears like fibers
• Tubular chromoplast which looks like tubes
• Membranous chromoplast

These chromoplasts live amongst each other though some plants have specific types such as mangoes have the globular chromoplast while carrots have crystallized chromoplast, tomatoes have both crystalline and membranous chromoplast because they accumulate carotenoids.

Functions of plant chromoplast

• They give distinctive colors to plant parts such as flowers, fruits, roots, and leaves. Differentiation of chloroplast to chromoplast makes the fruits of plant ripen.
• They synthesize and store plant pigments such as yellow pigments for xanthophylls, orange for carotenes. This gives the plant and its parts the color.
• They attract pollinators by the colors they produce, which helps in the reproduction of the plant seed.
• Chromoplats found in roots enable the accumulation of water-insoluble elements especially in tubers such as carrots and potatoes.
• They contribute to color change during plant aging, for flowers, fruits, and leaves.

Gerontoplast plastids of the plant cell

• These plastids found in plant leaves are the organelles responsible for cell aging. They differentiate from chloroplast when the plants start to age, and they can not perform photosynthesis anymore.
• They appear as unstacked chloroplasts without a thylakoid membrane and accumulation of plastoglobuli that is used in producing energy for the cell.
• The primary function of Gerontoplast is to aid the aging of the plant parts giving them a distinct color to indicate a lack of photosynthesis process.

Leucoplast plastids of the plant cell

• These are the non-pigmented plastids. Since they lack the chloroplast pigments, they are found in non-photosynthetic parts of the plants like the roots and seeds.
• They are smaller than the chloroplasts, which varying morphologies others appearing ameboid shaped.
• They are interconnected with a network of stromules in roots, flower petals.
• They can be specialized to store starch, lipids, and proteins in large quantities hence named as amyloplasts, elaioplast, and proteinoplast, depending on what they store respectively.

The main function of the leucoplast includes:

• Storage of starch, lipids, and proteins.
• They are also used to convert amino acids and fatty acids.
• Plant cells have large vacuoles as compared to animal cells.
• The central vacuoles are found in the cytoplasmic layer of cells of a variety of different organisms, but larger in the plant cells.

Structure of plant cell vacuoles

• These are large, vesicles filled with fluid, within the cytoplasm of a cell.
• It is made up of 30% fluid of the cell volume but can fill up to 90% of the cell’s intracellular space.

Functions of the central vacuole

• The central vacuoles are used to adjusted the size of the cell and to maintains the turgor pressure of the plant cells, preventing wilting and withering of plants especially the leaves.
• When the cytoplasmic volume is constant, the vacuoles account majorly for the size of the plant cell.
• Turgor pressure is maintained when the vacuoles are full of water. When there is no turgor pressure, it is an indication of the plant losing water, hence the plant leaves and stems wither.
• Plant cells thrive in high water levels (Hypotonic solutions), taking up water by osmosis from the environment, thus maintaining turgidity.
• A plant cell can have more than one type of vacuole. some specialized vacuoles especially those structurally related to lysosomes contain degradative enzymes used to break down macromolecules.
• Vacuoles are also responsible for the storage of cellular nutrients including sugars, organic salts, inorganic salts, proteins, cellular pigments, lipids. these elements are stored until when the cell requires them for cellular metabolisms. For example, vacuoles store proteins for seeds and opium metabolites.

Mitochondria are also known as chondriosomes, are the power generating organelles of a cell, hence they are commonly known as the powerhouse of the cell.

• The mitochondria convert stored nutrients by the help of oxygen to produce energy in for of (ATP )Adenosine TriPhosphate, hence they are the site for non-photosynthetic energy transduction.
• There are hundreds of mitochondria within a single plant cell.
• Mitochondria are found in high numbers within the phloem pigment of the plant cell, and the neighboring cells have high metabolism rates. This is to supply energies that support various needing mechanisms, like the transportation of food through the sieve tubes.
• As they perform their mechanisms, mitochondria continuously move and change their shapes, depending on its interactions with light trapped for photosynthesis, level of cytosolic sugars and the endoplasmic reticulum mediated interactions.
• The animal and plant mitochondria are very similar except for a few notable differences e.g. mitochondria in plants have reduced nicotinamide adenine dinucleotide (NADH) dehyg=drogenase used for oxidation of exogenous NADH which animal cell lack.
• Mitochondria from many plant sources are relatively insensitive to cyanide inhibition, a feature not found in animal mitochondria. On the other hand, the b -oxidation pathway of fatty acids is located in animal mitochondria, whereas in plants, the enzymes of fatty acid oxidation occur in the glyoxysomes. (https://publishing.cdlib.org/ucpressebooks/view?docId=ft796nb4n2&chunk.id=d0e6787&toc.depth=1&toc.id=d0e6787&brand=ucpress)

Structure of plant mitochondria

• Plant cell mitochondria have high pleomorphism.
• Mitochondria in green plants are discrete, spherical-oval shaped organelles of diameter ranging from 0.2to1.5μm
• The mitochondria have a double-layered system i. e a smooth outer membrane and an inner complex membrane that encloses the organelle matrix.
• The two layers are lipid bilayers complexed with a hydrophobic fatty acid chain. These lipids are a class of phospholipids that are highly dynamic with a strong attraction to the fatty acid regions.
• They have a mitochondrial gel-matrix in the central mass.
• The mitochondria also possess all the enzymes for the Tricarboxylic cycle (TCA) including citrate synthetase, Pyruvate oxidase, Isocitrate Dehydrogenase, Malate Dehydrogenase, Malic Enzyme.

Functions of mitochondria in plants

• The mitochondria are the powerhouse of the cell, hence their major function is generating energy for use by the cell.
• To have a high rate of metabolism because they supply energy for the unknown mechanism by which foods, mainly sucrose, are transported in the sieve tubes.
• Within the mitochondria, the potential energy in food that is manufactured by photosynthesis is what is used for the metabolisms of the cells. For example, energy used for the formation of new cell content, enzyme production and moving of sugar molecules are produced by the mitochondria.
• This is the cite for the Tricarboxylic cycle (TCA), also known as the Krebs cycle. The TCA cycle uses the cell’s nutrients, converting them into by-products that the mitochondria use for producing energy. These processes take place in the inner membrane because the membrane bends into folds called the cristae , where the protein components used for the main energy production system cells, known as the Electron Transport Chain (ETC). ETC is the main source of ATP production in the body.

The ER is a continuous network of folded membranous sacs housed in the cell cytosol. It is a complex organelle taking up a sizable part of the cell’s cytosol .

• It is made up of two regions known as the rough endoplasmic reticulum (they have ribosomes attached to their surface membrane) and the smooth endoplasmic reticulum (they lack ribosomal attachment).
• The endoplasmic reticulum known for its high dynamics functions in eukaryotic cells, play major roles in synthesizing, processing, transporting and storing proteins, lipids, and chemical elements. These elements are used by the plant cell and other organelles such as the vacuoles and the apoplast (Plasma membrane).
• The inner space of the ER is known as the lumen.
• It is attached to the nuclear envelope, providing a link between the nucleus and the cell cytosol, and also giving a link between the cell to the plasmodesmata tubes, which connect to the plant cells. It accounts for 10% of the volume of the cytosol.
• On the other hand, rough ER almost always appears as stacks of double membranes that are heavily dotted with ribosomes. Based on the consistent appearance of rough ER, it most likely consists of parallel sheets of membrane, rather than the tubular sheets that characterize smooth ER.
• These flattened, interconnected sacs are called cisternae, or cisternal cells. The cisternal cells of rough ER are also referred to as luminal cells. Rough ER and the Golgi complex are both composed of cisternal cells.

Structure of plant cell endoplasmic reticulum

• This is a consistently folded membranous organelle found in the cytoplasm of the cell, that is made up of a thin network of flattened interconnected compartments (sacs) that connects from the cytoplasm to the cell nucleus.
• Within its membranes, there are membranous spaces called the  cristae spaces  and the membrane folding are called  cristae .
• There are two types of ER based on their structure and the function they perform including  Rough Endoplasmic reticulum  and the  Smooth endoplasmic reticulum .

Functions of the endoplasmic reticulum

Functions of the Rough and smooth endoplasmic reticulum

• The Rough endoplasmic reticulum is covered by ribosomes around its surface membrane, making a rough bumpy appearance. the primary role of the Rough ER in synthesizing proteins, which are transported from the cell to the Golgi bodies, which carry them to other parts of the plant to help in its growth. These proteins are an assembly of amino acid sequences that combine to form antibodies, hormones, digestive enzymes. the assembling is accomplished by the ribosomes attached to the rough ER.
• Some proteins are processed outside the cell, they can also be transported into the Rough ER where they undergo assembling into the right shape and dimensions for cell utilization and conjugated with sugar elements to form a complete protein. these complexes are then transported and distributed to parts of the ER known as the transitional ER, for packaging in cell vesicles and passed to the Golgi bodies which export them to other parts of the plant.
• The smooth ER is smooth due to a lack of attached surface ribosomes. They look as though they are budding off from the lumen of the rough endoplasmic reticulum. Its role is synthesizing, secreting and storing lipids, metabolizing carbohydrates and manufacturing of new membranes. This is enhanced by the presence of several enzymes bound to its surface.
• When a plant has enough energy for utilization for photosynthesis and still possess excess lipids manufactured by the cell, these lipids are stored in the smooth Endoplasmic reticulum in the form of triglycerides. And when the cell needs more energy, the triglycerides are broken down to produce the energy required by the plants.
• Minimally,  the smooth endoplasmic reticulum has also been linked to the formation of the cellulose on the cell wall.

Other functions of the endoplasmic reticulum in the plant cell

• Calcium is used in the growth and development of plant cells which enhances plant growth but in some cases, calcium may be produced in excessive quantities that harm the plant cell by causing cell death. Therefore the Endoplasmic reticulum has been linked to regulating the excess calcium by converting it to calcium oxalate crystals.  Specialized cells in the endoplasmic reticulum known as crystal idioblast play a major role in this conversion and also in storing these crystals.
• In the event of sensitivity, the sensory ER move and collect at the top and the bottom of the cell, making them be squeezed together thus causing a constraint on them. This leads to the release of accumulated calcium, which in turn produces the sense of touch.
• The cortical ER is highly linked with the plasmodesmata (a narrow thread of cytoplasm that passes through the cell walls of adjacent plant cells and allows communication between them). The Plasmodesmata acts as a channel of communication among the cells thus linking to the motor cells triggering the cells and the plant to respond accordingly.
• This is the organelle responsible for protein synthesis of the cell.
• Its found in the cell cytoplasm in large numbers and a few of them called functional ribosomes can be found in the nucleus, mitochondria, and the cell chloroplast.
• Its made up of ribosomal DNA (rDNA) and cell proteins
• The process of protein synthesis by the ribosomes is known as translation, by using the messenger RNA, which delivers the  nucleotides  to the ribosomes.
• The ribosomes then guide and translate the message in the form of nucleotides, contained by the mRNA.

Structure of ribosomes of the plant cell

• The ribosomes’ structure is the same in all cells but smaller in prokaryotic cells. Generally, ribosomes in eukaryotic cells are large and they can only be measured in Svedberg units (S). S unit is a measure of aggregation of large molecules to sediments on centrifugation. High S value means fast sedimentation rate hence greater mass.
• Eukaryotic cell sediment in the 90s while prokaryotic cell sediment in the 70s.
• Ribosomes found in the mitochondria and chloroplasts are as small as the prokaryotic ribosomes.
• Naturally, ribosomes are made up of two subunits i. e small and large subunits, both classified according to their sedimentation rates by the S unit.
• The plant cell, being a eukaryotic cell, has large complex ribosomes with higher S units, with four rRNAs with over 80 proteins. The large subunit has the S unit of the 60s (28s rRNA, 5.8s rRNA, and 5s rRNA) with 42 proteins. The small subunit has a sedimentation rate of the 40s, made up one rRNA and 33 proteins.
• The ribosomal subunits combine in the nucleolus of the cell, which is then transported into the cytoplasm through the nuclear pores. The cytoplasm is the primary site for protein synthesis (translation).

Functions of ribosomes in plant cells

• Containing a subunit of RNA, ribosomes major functions is to synthesize proteins for the cellular functions such as cell repair mechanism.
• Ribosomes act as catalysts in producing strong binding for portion extension using peptidyl transfer and peptidyl hydrolysis.
• Ribosomes found in the cell cytoplasm are responsible for the conversion of genetic codes to amino acid sequences and building protein polymers from amino acid monomers.
• they are also used in protein assembling and folding.

Storage granules  of plant cell

• These are aggregates found within the cytoplasmic membrane and the plant cell plastids.
• They are inert organelles found in plants whose primary function is to store starch.

Functions of storage granules in plant cell

• They are used as food reservoirs
• They store carbohydrates for the cell in the form of glycogen or carbohydrate polymers
• They naturally store starch granules for the plant cell
• They also fuel metabolisms in the cell that involved chemical reactions thus producing energy for the production of new cellular materials.

Golgi bodies are complex membrane-bound cell organelles found in the cytoplasm of a eukaryotic cell, which is also known as the Golgi complex or Golgi apparatus. They lie just next to the endoplasmic reticulum and near the nucleus.

Structure of the Golgi bodies in a plant cell

• Golgi bodies are maintained together by cytoplasmic microtubules and clasped by a protein matrix
• They are made up of flattened stacked pouches known as  cisternae.
• Plant cells have a few hundreds of the Golgi bodies moving along the cell’s cytoskeleton, over the endoplasmic reticulum as compared to the very few found in animal cells (1-2).
• Cis Golgi network  is also known  as Goods inwards,  are the cisternae the is closest to the endoplasmic reticulum. Also called the cis Golgi reticulum it is the entry area to the Golgi apparatus.
• The medial or the Golgi stack-  this is the Main processing area, placed at the central layer of the cisternae
• Trans Golgi network  is also known as  the Goods outwards cisternae.  This is the farthest cisternae endoplasmic reticulum from the endoplasmic reticulum.

Functions of the Golgi bodies in a plant cell

• The Golgi bodies have several functions linked to them, from being an adjacent organelle to the endoplasmic reticulum to where they deliver the cell products to. They are found in the middle of the cells’ secretory pathway, as a membranous complex that primarily functions to process, distribute and store proteins for use by the plant during stress responses and others in leguminous plants such as cereals and grains.
• Cleaving the protein molecules to oligosaccharides chains
• Attaching of sugar moieties of different side chains to the protein elements
• Addition of fatty acids and phosphate groups to the elements and removal of monosaccharides.
• The cell vesicles carrying protein molecules from the endoplasmic reticulum into the cis compartment, where the product is modified, and then packaged into other vesicles which then transports it to the next compartment. The transportation is enhanced by marking the vesicle with a tag like a phosphate group or special protein molecules, leading it to its next endpoint.
• Finally, when the vesicles have transported the proteins and lipid molecules, the Golgi bodies are responsible for assembling the product and transporting it to the final destination. This is enhanced by the presence of enzymes in the plants’ Golgi bodies, which attache to the sugar moieties to the proteins, packing them and transporting them to the cell wall.

The nucleus is the information center of a cell. It is a specialized complex organelle whose primary function is to store the cell’s genetic information.

• It is also responsible for coordinating the cell’s activities including cell metabolism, cell growth, synthesis of proteins and lipids and generally the cell reproduction by cell division mechanisms.
• The nucleus contains the cells’ genetic information known as Deoxyribonucleic Acid (DNA), on the Chromosomes (special thread-like strands of nucleic acids and protein found in the nucleus, carrying genetic information).

Structure of the nucleus of the plant cell

• The nucleus is spherically shaped, centrally placed in the cell. It occupies about 10% of the cell volume content.
• It as a double-layered membrane known as the nuclear envelope which separates the contents in the nucleus from those in the cell cytoplasm.
• The nuclear materials included chromatins, DNA which forms the cell chromosomes during cell division, the nucleolus which is responsible for synthesizing the cell ribosomes.

Functions of the nucleus of the plant cell

• The Primary role of the cell nucleus is, it functions as the cell’s control center.
• The presence of the nuclear membrane, it encloses the nucleus and its contents from the cytoplasmic organelles. This nuclear membrane has the nuclear envelope, which has several nuclear pores, which offers selective permeability to and from the nucleus and the cytoplasm.
• The nucleus is also linked to the site for protein synthesis, i.e the endoplasmic reticulum by a network of microfilaments and microtubules. These tubules extend all over the cell manufacturing elements and molecules depending on the specificity of the cell.
• Chromosomes:  they are also known as the chromatids. They are found in the cell nucleus of almost all cells. They have 6 long strands of DNA which divide into 46 separate molecules which pair up into two, made of 23 molecules per chromosome. To form a functional DNA unit, it is combined with cel proteins to form a compact structure of dense fiber-like strands known as the  chromatins.
• The 6 DNA strands, each wraps around small protein molecules produced by the ER known as Histones. These form the beadlike structures known as nucleosomes. DNA strands have a negative charge which is neutralized by the histones’ positive charge. Unused DNA is folded and stored for future use.

Chromatins are classified into two types:

• Euchromatin : It is the active part of the DNA that is used for RNA transcription producing cellular protein for cell growth and functioning.
• Heterochromatin : it is the inactive part of DNA that has the compressed and condensed DNA that is not in use.

During Chromatin formation, the chromatins change into other forms of the nucleus during cell division. Throughout the life of a cell, chromatin fibers take on different forms inside the nucleus. During the interphase stage of cell division, the euchromatin is expressed to start transcription. Into the metaphase stage, the chromatins divide making its own copies during replication exposing the chromatins more to form more specialized structures known as c hromosomes . These chromosomes then divide and separate, forming two new complete cells, with their own genetic information.

• It is a sub-organelle in the cell nucleus, which lacks a membrane.
• Its primary function is to synthesize the cell ribosomes, the organelles used to produce cellular proteins.
• The cell has about 4 nucleoli.
• The nucleolus is formed when chromosomes are brought together, just before cell division is initiated.
• The nucleolus disappears from during cell division.
• The nucleolus is linked to cell aging which affects the aging of living things.

Nuclear Envelope

• Its made up of two membranes separated from each other by perinuclear space. the space links into the endoplasmic reticulum.
• With its perforated wall, it regulates the molecules that enter and leave the nucleus into and out of the cytoplasm respectively.
• The inner membrane has a lining of proteins known as nuclear lamina, binding chromatins, and other nuclear elements.
• The envelope disintegrates and disappears during cell division.

Nuclear Pores

• They are perforate the cell envelope and their function is to regulate the passage of cellular molecules such as proteins, histones through into and out of the nucleus and the cytoplasm respectively.
• They also allow DNA and RNA into the nucleus, providing energy for making up the genetic materials.

Peroxisomes are highly dynamic tiny structures that have a single membrane containing enzymes responsible for the production of hydrogen peroxide.

They play major roles in primary and secondary metabolisms, responding to abiotic and biotic stress in regulating photorespiration and cell development.

Structure of the peroxisomes

• Peroxisomes are small with a diameter of 0.1-1 µm diameter.
• It is made up of compartments having a granulated matrix.
• They also have a single membrane layer.
• They are found in the cytoplasm of a cell.
• The compartments assist in various metabolic processes of the cell to help sustain the cellular activities within the cell.

Functions of the peroxisomes

• Production and degradation of hydrogen peroxide
• oxidation and metabolism of fatty acids
• Metabolizing carbon elements
• Photorespiration and absorption of Nitrogen for specific functions of the plant.
• Providing defense mechanisms against pathogens.

Lysosomes  in plant cells?

The presence of lysosomes in plants has been long debated over with little evidence on their structural presence. In plants, Its believed that lysosomes partially differentiate into vacuoles and partially into the Golgi bodies, which perform the functions stipulated for lysosomes in plants. Unlike in animals where lysosomes distinctively posses hydrolytic enzymes and digestive enzymes, for breaking down toxic materials and removing them from the cell and digestion of proteins respectively, in plants these enzymes combined are found in the vacuoles and the Golgi bodies.

The partial differentiation has been liked to the multiprocess that contribute to the formation of Golgi bodies from the endoplasmic reticulum, whereby, there is a short phase of lysosomal exudation just before Golgi bodies are fully formed.

• Plant peroxisomes by Mano S., Nishimura M.
• nature.com/scitable/topicpage/plant-cells-chloroplasts-and-cell-walls-14053956/
• https://www.quora.com/Do-plant-cells-have-lysosomes-Why-or-why-not
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4556774/
• https://www.ncbi.nlm.nih.gov/books/NBK9930/
• https://www.ncbi.nlm.nih.gov/books/NBK9927/
• https://www.ncbi.nlm.nih.gov/books/NBK9845/
• https://www.ncbi.nlm.nih.gov/books/NBK26928/
• https://www.ncbi.nlm.nih.gov/books/NBK26857/
• https://www.sciencedirect.com/science/article/pii/B9780128132784000117
• https://www.researchgate.net/publication/51769784_Crystal_Structure_of_the_Eukaryotic_60S_Ribosomal_Subunit_in_Complex_with_Initiation_Factor_6
• https://www.researchgate.net/publication/11624368_Primary_and_secondary_plasmodesmata_Structure_origin_and_functioning
• https://sciencing.com/type-energy-produced-photosynthesis-5558184.html
• https://labs.wsu.edu/knoblauch/sieve-element-plasids/

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Faith Mokobi

13 thoughts on “Plant Cell: Structure, Parts, Functions, Labeled Diagram”

Hello this was very useful for my bio assignment. I used these illustrations and some information. Hope it is okay. Thank you so much. This helped me a lot. 🙂

This is very useful, thank you. I can use this for my class

I hope it is alright if I use some of your marvelous illustrations in my biology course. I will cite these in my presentation.

Hi John, Sure you can use the illustrations for the biology course and presentations. Thanks, – Sagar

Hi Sagar! I found this very useful. Can I use this for my class? Thank you aand more power.

Hello, thank you so much. You can use this for your class.

Hi Sagar, Thank you and sorry for delay in reply. Okay I will stick to using only 5 figures. And I will include “created with biorender”. and other citations.Thanks a lot

Nice and very detailed article…

Hi, please can you tell me what are the conditions for using the plant cell images here. Who do I need to contact? I am seeking permission to use the images here in a textbook I am writing. Thank you.

Hello, sure you can use the image with citations and references. If you need further information, you can email me at [email protected] Thank you, Sagar

Thank you very much Sagar. I will use the images with citations and references. I did not mention in the previous mail that the textbook will be commercial. Just thinking you should know that. Thanks again

Sure, no problem with that the textbook will be commercial, but biorender let me use only 5 figures for textbooks. How many figures are you using? And you need to add a text somewhere, “created with biorender” in the figure with citations. All the best for your textbook. Hope to read it soon. Sagar,

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