sucrose dialysis tubing experiment

Optional Lab Activities

Osmosis and diffusion, lab objectives.

At the conclusion of the lab, the student should be able to:

• define the following terms: diffusion, osmosis, equilibrium, tonicity, turgor pressure, plasmolysis
• describe what drives simple diffusion (why do the molecules move?)
• list the factors that may affect the speed of simple diffusion
• list which molecules, in general, can freely diffuse across the plasma membrane of a cell
• describe what drives osmosis (why do water molecules move?)
• explain why water moves out of a cell when the cell is placed in a hypertonic solution
• explain why water moves into a cell when the cell is placed in a hypotonic solution
• describe what physically happens to a cell if water leaves the cell
• describe what physically happens to a cell if water enters the cell

Introduction

Understanding the concepts of diffusion and osmosis is critical for conceptualizing how substances move across cell membranes. Diffusion can occur across a semipermeable membrane; however diffusion also occurs where no barrier (or membrane) is present. A number of factors can affect the rate of diffusion, including temperature, molecular weight, concentration gradient, electrical charge, and distance. Water can also move by the same mechanism. This diffusion of water is called osmosis .

In this lab you will explore the processes of diffusion and osmosis. We will examine the effects of movement across membranes in dialysis tubing, by definition, a semi-permeable membrane made of cellulose. We will also examine these principles in living plant cells.

Part 1. Diffusion Across a Semi-Permeable Membrane: Dialysis

• Cut a piece of dialysis tubing, approximately 10 cm.
• Soak the dialysis tubing for about 5 minutes prior to using.
• Tie off one end of the tubing with dental floss.
• Use a pipette and fill the bag with a 1% starch solution leaving enough room to tie the other end of the tubing.
• Tie the other end of the tubing closed with dental floss.
• Fill a 250 mL beaker with distilled water.
• Add Lugol’s iodine to the distilled water in the beaker until the water is a uniform pale yellow color.
• Place the dialysis tubing bag in the beaker.
• The movement of starch
• The movement of iodine
• The color of the solution in the bag after 30 minutes
• The color of the solution in the beaker after 30 minutes
• Add the dialysis bag to the beaker and allow the experiment to run for 30 minutes. Record the colors of both the dialysis bag and the beaker.

Lab Questions

• Is there evidence of the diffusion of starch molecules? If so, in which direction did starch molecules diffuse?
• Is there evidence of the diffusion of iodine molecules? If so, in which direction did iodine molecules diffuse.
• What can you say about the permeability of the dialysis membrane? (What particles could move through and what particles could not?)
• What is the difference between a semi-permeable and a selectively permeable membrane

Part 2. Plasmolysis—Observing Osmosis in a Living System, Elodea

If a plant cell is immersed in a solution that has a higher solute concentration than that of the cell, water will leave/enter (circle one) the cell. The loss of water from the cell will cause the cell to lose the pressure exerted by the fluid in the plant cell’s vacuole, which is called turgor pressure. Macroscopically, you can see the effects of loss of turgor in wilted houseplants or limp lettuce. Microscopically, increased loss of water and loss of turgor become visible as a withdrawal of the protoplast from the cell wall (plasmolysis) and as a decrease in the size of the vacuole (Figure 1).

• Obtain a leaf from the tip of an Elodea Place it in a drop of water on a slide, cover it with a coverslip, and examine the material first at scanning, then low power objective and then at high power objective.
• Locate a region of health. Note the location of the chloroplasts.  Sketch a few cells. For the next step, DO NOT move the slide .
• While touching one corner of the coverslip with a piece of Kimwipe to draw off the water, add a drop of 40% salt solution to the opposite corner of the coverslip. Do this simultaneously.  Be sure that the salt solution moves under the coverslip. Wait about 5 minutes, then examine as before. Sketch these cells next to your sketch of cells in step two, note the location of the chloroplasts. Label it 40% salt solution .
• What happened to the cells in the salt solution?
• Assuming that the cells have not been killed, what should happen if the salt solution were to be replaced by water?
• Are plant cells normally hypertonic, hypotonic, or isotonic to their environment? Why?
• Can plant cells burst? Explain.

Overall Conclusions

• Review your hypothesis for each experiment. Was your original hypothesis supported or rejected for each experiment. Explain why or why not. This should be based on the best information collected from the experiment. Explain how you arrived at this conclusion.
• If it was incorrect, give the correct answer, again based on the best information collected from the experiment.

Sources of Error

• Identify and explain two things that people may have done incorrectly that would have caused them to get different answers from the rest of the class. Be  specific .
• Biology 101 Labs. Authored by : Lynette Hauser. Provided by : Tidewater Community College. Located at : http://www.tcc.edu/ . License : CC BY: Attribution
• BIOL 211 - Majors Cellular [or Animal or Plant]. Authored by : Carey Schroyer and Diane Forson. Provided by : Open Course Library. Located at : http://opencourselibrary.org/biol-211-majors-cellular-or-animal-or-plant/ . License : CC BY: Attribution

Visking Tubing Practical ( AQA A Level Biology )

Revision note.

Biology Lead

Practical Skill: Visking Tubing

Investigating the absorption of the products of digestion using visking tubing.

• Visking tubing (sometimes referred to as dialysis tubing) is a non-living partially permeable membrane made from cellulose
• It is sometimes used to model the process of absorption that occurs in the small intestine
• Pores in the membrane are small enough to prevent the passage of large molecules (such as starch and sucrose ) but allow smaller molecules (such as glucose ) to pass through by diffusion

Image showing how a visking tubing membrane can be used to mimic the walls of the small intestine

• Fill a section of Visking tubing with a mixture of starch and amylase solutions
• Suspend the tubing in a beaker of water for a set period of time
• Starch is tested for using iodine. A blue-black colour is produced in the presence of starch
• Glucose is tested for using Benedict's reagent. An orange-red precipitate is formed in the presence of glucose
• Glucose is small enough to diffuse across the partially permeable membrane
• As a result, the amount of precipitate produced from the Benedict's reagent test will increase over time
• Comparisons between the time intervals can be made with a set of colour standards (known glucose concentrations) or a colorimeter to give a more quantitative set of results
• A graph could be drawn showing how the rate of absorption changes with the concentration gradient between the inside and outside of the tubing
• Both are selectively/partially permeable
• The small intestine has a much larger surface area due to the presence of villi
• Both have an initially low solute concentration
• The distilled water does not flow and so does not maintain the concentration gradient the way blood does

Investigating factors affecting digestive enzyme activity

• Visking tubing can also be used to study the effects of different factors on the rate of digestive enzyme activity
• Eg. multiple visking tubings are set up containing solutions of starch and amylase kept at different pH levels using buffer solutions
• Eg. multiple visking tubings are set up in water baths of different temperatures

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17.7: Osmosis and Diffusion

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• Page ID 34893

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Lab Objectives

At the conclusion of the lab, the student should be able to:

• define the following terms: diffusion, osmosis, equilibrium, tonicity, turgor pressure, plasmolysis
• describe what drives simple diffusion (why do the molecules move?)
• list the factors that may affect the speed of simple diffusion
• list which molecules, in general, can freely diffuse across the plasma membrane of a cell
• describe what drives osmosis (why do water molecules move?)
• explain why water moves out of a cell when the cell is placed in a hypertonic solution
• explain why water moves into a cell when the cell is placed in a hypotonic solution
• describe what physically happens to a cell if water leaves the cell
• describe what physically happens to a cell if water enters the cell

A SlideShare element has been excluded from this version of the text. You can view it online here: pb.libretexts.org/bio1lm/?p=140

Introduction

Understanding the concepts of diffusion and osmosis is critical for conceptualizing how substances move across cell membranes. Diffusion can occur across a semipermeable membrane; however diffusion also occurs where no barrier (or membrane) is present. A number of factors can affect the rate of diffusion, including temperature, molecular weight, concentration gradient, electrical charge, and distance. Water can also move by the same mechanism. This diffusion of water is called osmosis .

In this lab you will explore the processes of diffusion and osmosis. We will examine the effects of movement across membranes in dialysis tubing, by definition, a semi-permeable membrane made of cellulose. We will also examine these principles in living plant cells.

Part 1. Diffusion Across a Semi-Permeable Membrane: Dialysis

• Cut a piece of dialysis tubing, approximately 10 cm.
• Soak the dialysis tubing for about 5 minutes prior to using.
• Tie off one end of the tubing with dental floss.
• Use a pipette and fill the bag with a 1% starch solution leaving enough room to tie the other end of the tubing.
• Tie the other end of the tubing closed with dental floss.
• Fill a 250 mL beaker with distilled water.
• Add Lugol’s iodine to the distilled water in the beaker until the water is a uniform pale yellow color.
• Place the dialysis tubing bag in the beaker.
• The movement of starch
• The movement of iodine
• The color of the solution in the bag after 30 minutes
• The color of the solution in the beaker after 30 minutes
• Add the dialysis bag to the beaker and allow the experiment to run for 30 minutes. Record the colors of both the dialysis bag and the beaker.

Lab Questions

• Is there evidence of the diffusion of starch molecules? If so, in which direction did starch molecules diffuse?
• Is there evidence of the diffusion of iodine molecules? If so, in which direction did iodine molecules diffuse.
• What can you say about the permeability of the dialysis membrane? (What particles could move through and what particles could not?)
• What is the difference between a semi-permeable and a selectively permeable membrane

Part 2. Plasmolysis—Observing Osmosis in a Living System, Elodea

If a plant cell is immersed in a solution that has a higher solute concentration than that of the cell, water will leave/enter (circle one) the cell. The loss of water from the cell will cause the cell to lose the pressure exerted by the fluid in the plant cell’s vacuole, which is called turgor pressure. Macroscopically, you can see the effects of loss of turgor in wilted houseplants or limp lettuce. Microscopically, increased loss of water and loss of turgor become visible as a withdrawal of the protoplast from the cell wall (plasmolysis) and as a decrease in the size of the vacuole (Figure 1).

• Obtain a leaf from the tip of an Elodea Place it in a drop of water on a slide, cover it with a coverslip, and examine the material first at scanning, then low power objective and then at high power objective.
• Locate a region of health. Note the location of the chloroplasts. Sketch a few cells. For the next step, DO NOT move the slide .
• While touching one corner of the coverslip with a piece of Kimwipe to draw off the water, add a drop of 40% salt solution to the opposite corner of the coverslip. Do this simultaneously. Be sure that the salt solution moves under the coverslip. Wait about 5 minutes, then examine as before. Sketch these cells next to your sketch of cells in step two, note the location of the chloroplasts. Label it 40% salt solution .
• What happened to the cells in the salt solution?
• Assuming that the cells have not been killed, what should happen if the salt solution were to be replaced by water?
• Are plant cells normally hypertonic, hypotonic, or isotonic to their environment? Why?
• Can plant cells burst? Explain.

Overall Conclusions

• Review your hypothesis for each experiment. Was your original hypothesis supported or rejected for each experiment. Explain why or why not. This should be based on the best information collected from the experiment. Explain how you arrived at this conclusion.
• If it was incorrect, give the correct answer, again based on the best information collected from the experiment.

Sources of Error

• Identify and explain two things that people may have done incorrectly that would have caused them to get different answers from the rest of the class. Be specific .

Contributors and Attributions

• Biology 101 Labs. Authored by : Lynette Hauser. Provided by : Tidewater Community College. Located at : http://www.tcc.edu/ . License : CC BY: Attribution
• BIOL 211 - Majors Cellular [or Animal or Plant]. Authored by : Carey Schroyer and Diane Forson. Provided by : Open Course Library. Located at : http://opencourselibrary.org/biol-211-majors-cellular-or-animal-or-plant/ . License : CC BY: Attribution

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Why is dialysis tubing impermeable to sucrose when it should retain >12,000 Da?

While writing up a lab for biology, I realized that the dialysis tubing for sale online seemed to retain large molecules (12-14,000 Da) but didn't specify an upper bound. I learned that the tubing is impermeable to Sucrose , molecular weight 342 Da, because it's too big. This doesn't make sense to me; the site says the tubing "retains >12,000 Da molecules", but Sucrose is clearly smaller than that.

So either A) Sucrose does indeed pass through dialysis tubing and I misheard/mis-learned or B) my class used tubing with much smaller pores.

• $\begingroup$ So it seems that we used tubing with smaller pores and there is a discrepancy between the metric I was using to compare and the size of the molecules. Thank you to both answers. $\endgroup$ –  Maddy Byahoo Commented Oct 13, 2012 at 4:06

2 Answers 2

There exist many types of semi-permeable membranes (the ones used for dialysis tubing), with various pore sizes. One of the very common lab experiment on the topic of osmosis is using a sucrose solution (sucrose is cheap) and small-pores membrane, such that water and small ions (typically Na + and Cl – ) can pass, but not sucrose. The one which you link to simply happens to be a variety with much wider pores.

Dialysis tubing (and most semi-permeable membranes) operate based on differences of size and not molecular weight. While the size of a molecule or ion does increase as its molecular weight increases, there is some nonintuitive size-mass discrepancies that can occur.

Many of these discrepancies are well known to polymer chemists. Most of the ways that we attempt to measure the average molecular weight of a polymer sample are actually measures of average molecular size . The shape that a molecule adopts in solution greatly impacts its apparent size. A polymer of MW in the 40 kDa range that adopts a rigid-rod conformation in solution will usually have a larger apparent size than a polymer of MW in the 100 kDa range that folds up into a tight globular conformation (spheres have a smaller surface area to volume ratio than cylinders).

Another factor in the "size" of a molecule is the number of solvent molecules that get dragged along with it. If the molecule is dissolved in a good solvent (for example sucrose in water), it will always have a number os solvent molecules associated with it. The solvodynamic radius of a molecule may be much greater that the molecule's actual size.

If indeed you had dialysis tubing rated to retain only greater than 12,000 Da (and like @F'x, I'm not sure that you did) it is still possible for sucrose to not pass through if the sucrose is trying to drag too many water molecules (from hydrogen bonding) with it.

• $\begingroup$ It's a very valid point… but in the case presented, it would be one or two orders of magnitude difference between weight and size. $\endgroup$ –  F'x Commented Oct 12, 2012 at 12:56

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• Classroom Practicals

Osmosis and Diffusion

AUSTRALIAN CURRICULUM ALIGNMENT: 

• Movement of materials across membranes occurs via diffusion, osmosis, active transport and/or endocytosis

BACKGROUND: 

The cell membrane maintains the cell a separate entity; it holds the cell contents within, and acts as a barrier to the external environment. It is selectively permeable and has various mechanisms to allow for the exchange of gases and nutrients. These mechanisms allow for the intake of anything that is required and allows for the expulsion of waste and toxins. This membrane does not resemble a sheet or bag; rather, it is many molecules of Phospholipid Bilayers held together by the combined forces of attraction and repulsion. They are comprised of a Phosphate head; which is hydrophilic (water-loving), and a Lipid (fatty acid) tail which is hydrophobic (repelled by water). As the internal and external environments of a cell are aqueous, these molecules arrange themselves into two layers; one with the Phosphate heads oriented out into the external fluid, and the other with the heads oriented inwards into the internal fluid (the Cytoplasm). The Lipid tails are between the two layers of Phosphate heads; thereby, protected from the water, and the strength of this attraction/repulsion mechanism keeps the molecules together as though the membrane were a single entity.

In this practical, dialysis tubing is used as a surrogate cell membrane for a visual demonstration of osmosis and diffusion. A solution containing large molecules (Starch) and small molecules (Glucose) is placed inside the tubing; which is then placed in a solution containing iodine. Students are able to observe as the solution inside the tubing turns dark blue, while the surrounding solution it is submerged in does not. From this, students can use their prior knowledge of the Starch-Iodine complex to surmise that Iodine is able to pass through the membrane while starch is not. The Glucose-testing strips indicate that glucose has been able to pass out of the tubing and into the external fluid. Thus proving the tubing allows movement in both directions. 

This inexpensive and simple experiment provides students with a clear visual result that effectively demonstrates how the size of a molecule can affect its ability to be transported into or out of a cell. It also illustrates the mechanics of diffusion and osmosis by which a cell will attempt to create homeostasis, or equilibrium between its inner and outer environments.

PREPARATION - BY LAB TECHNICIAN

• Cut the dialysis tubing into 15cm lengths and soak for 15 minutes in a beaker filled with room temperature distilled water. Prepare one length of tubing per student or group. However, it is best to prepare extra strips for students, as some strips may tear or leak through handling.  
• To create the Starch solution, dissolve 2g of Starch in 100mL of boiling hot water (2% solution) on a hot plate until the Starch powder has been fully dissolved. Stir as required.  
• To create the Glucose solution, dissolve 30g of Glucose in 100mL water (30% solution) and continue stirring until the glucose has been fully dissolved. 
• Combine the Starch and Glucose solutions in a single beaker. Use a stirring rod to mix well.

METHOD - STUDENT ACTIVITY

Glucose/ Starch Solution

• Measure 5-10 mL of the Glucose/Starch mixture in a small beaker or test tube.
• To determine the initial glucose concentration within the Starch/ Glucose solution, you will first need to dilute a sample of the mixture in water.  To do this, collect 1mL of your mixture using a transfer pipette and add to a test tube filled with 9mL of water. Mix using a clean stirring rod.
• Measure the diluted Starch/Glucose by placing a Glucose-testing strip in the solution, immediately removing it and waiting 60 seconds to observe any colour change. Using the colour guide on the testing strip container, determine the approximate Glucose levels, and record the results.

Iodine Solution

• Fill a large beaker with 100mL water, and add 1mL of Iodine/KI solution. The solution should appear a yellowish colour. 
• Measure the Glucose levels of the Iodine solution with another strip; following the same procedure as before. Ensure you record the results.

  Preparing the "cell" tubing

• Retrieve your soaked piece of dialysis tubing and tie a knot in one end as though you are tying a balloon.
• Using a transfer pipette, half-fill the tubing with your undiluted Starch/Glucose solution and tie the other end to create a “cell”.
• Submerge the “cell” tubing into the Iodine solution.

    Observing changes in the “cell”

• After 15 minutes, observe any colour changes in the tubing and in the beaker solution.
• Measure the Glucose levels in the Iodine solution.
• Carefully open the tubing and pour the contents into a clean beaker.
• To dilute the tubing contents for Glucose testing, collect 1mL of the contents using a pipette and deposit into a test tube filled with 9mL of water.
• Measure the Glucose levels in the diluted contents using a Glucose testing strip following the same procedure as before.
• Record the results of the Glucose testing. 
• Compare the changes in Glucose levels before and after the 15 minute interval.

OBSERVATION AND RESULTS

INVESTIGATION 

• Provide students with the information that you prepared a 100mL solution of 2% starch and a 100mL solution of 30% Glucose. Based on this information, ask your students to calculate the concentration of each in the combined solution. Students should understand that double the volume without extra solute means half the concentration, so what was 2g of Starch in 100mL (2%) is now 2g of Starch in 200mL (1%); and what was 30g of Glucose in 100mL (30%) is now 30g of Starch in 200mL (15%).
• Ask students to identify what occurred the Starch, based on the fact that the blue colour is found inside the cell but not outside of it, students should be able to identify that the Starch has not been able to pass through the tubing, while the Iodine has. Students should understand that the Starch-Iodine complex has therefore been confined to the area where both Starch and Iodine are found, that is, the inside of the cell. 
• Ask students to describe what is suggested by the Glucose results. The appearance of Glucose into the previously Glucose-free solution in the beaker should inform students that Glucose has been able to pass through the membrane.
• To provide students with a deeper understanding surrounding the molecular size of Glucose and Iodine, you may provide students with the information that our dialysis tubing typically allows passage to molecules of up to 12,000 to 14,000 daltons (g/mol). This should provide some guidance of the sizes that Starch molecules can reach. Remind students, however, that the shape of a molecule may affect the passage as a large linear molecule may be able to pass through more easily than a smaller but globular molecule.

TEACHER NOTES

The concentration of Glucose in this practical is quite high to enable shorter waiting times for students. This allows them to more readily measure the glucose which has diffused out of the “cell” using their test strips. However, this also means that the initial concentration is too high to show that the concentration inside the cell has decreased in line with the increase outside the cell. To manage this, students are asked to take a sample of the original combined Glucose/ Starch solution prior to being placed in the “cell” and also a sample of the now-blue solution inside the “cell” at the end of the prac. Both solutions are diluted by a factor of ten to bring the Glucose concentration into the range of the Uriscan strips.

EXTENSION EXERCISE

To observe the process of cell diffusion and osmosis over an extended period of time, make an extra “cell” and keep it in solution until the next class. By the beginning of next class, the Glucose inside and outside the cell should have somewhat equalised. This could be conducted as a class demonstration, or each student may make an extra cell. Once again, dilute both solutions by a factor of ten prior to measuring.

TEACHER TIPS: 

Prepare extra dialysis strips for students, as some strips may tear or leak through handling as students attempt to tie them. 

Time Requirements

• 45 mins  

Material List

Dialysis tubing

• Starch  
• Iodine/KI solution  
• Glucose 
• Glucose-testing strips  
• Test tubes 
• Test tube rack
• Beakers 500mL
• Beakers 100mL
• Transfer pipettes 
• Stirring rod

 Safety Requirements

• Wear appropriate personal protective equipment (PPE); particularly gloves and a lab coat as Iodine will stain clothing and skin on contact. 
• Exercise caution when handling the chemicals used in this prac. 
• Avoid any direct contact with the solution and wash hands thoroughly.   

||Biology||Classroom Practicals||Cells||Year 11&12||

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Related Resources

• Solution Sheets

Diffusion Experiment: Osmosis in Sucrose Solutions

To being the experiment each dialysis tubing was tied off at the bottom and filled with a sucrose solution. Bag A and Bag B were both filled with a 10mL 1% sucrose solution. Bag C was filled with a 10mL 25% sucrose solution and Bag D was filled with a 10mL 50% sucrose solution. Each bag had all the air pushed out of it and the open end was tied off. Each bag was also label with a piece of tape attached to a piece of dental floss marking each individual bag so as not to confuse them. The bags were then weighed and the weight was written down. After that Bag A was placed in a 50% sucrose solution and Bags B, C and D were all placed in a 1% sucrose solution. Once all the bags were placed in their respected solutions, the weight changes were recorded in fifteen minute intervals from the initial starting time and weight change was recorded.

When the experiment was done, Bag A was the only bag to have a drastic weight decrease. Bag A’s initial weight was 11.2 grams and in the end had a weight of 5.9 grams after the full sixty minutes. Bag B had a slight loss in weight, from its initial weight of 11.8 grams to 11.2 grams. On the other hand, Bags C and D each gained weight over time with Bag C starting out with 11.8 grams and ending at 13.5 grams. Bag D’s weight began at 13.2 grams and finished up at 17.5 grams. The charts representing the changes can be found attached at the end of this report.

From the results taken, the more concentrated solutions of sucrose seemed to draw the water towards it. This supports my hypothesis and also shows that sucrose is also a hypertonic solution since it there was more water brought into the solution than outside of it. Bag A had a stronger concentration outside its membrane which caused the sucrose on the outside to draw the water out of the dialysis tubing and into the solution that was in the bowl. Bag D has the greatest weight in the end primarily due to the fact that it also contained the highest percentage of solution within the membrane. Bag B had little change in weight due to the low concentration of sucrose in the bag and in the solution outside the bag. Since Bags C and D had the higher concentrations they pull the majority of the water out of the solution and into their membranes. Had Bag B been placed in the solution that Bag A was in, it would more than likely have the same affect that Bag A had.

This could be applied in the real world with people who suffer from dehydration. By raising the sugar levels in their body, they are more likely to take in more water into their cells. One thing that could be tried in future experiments could be to add additional types of solutions to the experiment that would simulate more of the human internal cell system. By adding more substances, the chances of seeing how osmosis truly works in the body can be seen.

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Selective Permeability of Dialysis Tubing Lab: Explained

• Selective Permeability of Dialysis Tubing…

This experiment was conducted to investigate the selective permeability of dialysis tubing. The permeability of the tubing to glucose, starch and iodine (potassium iodide) was tested. The dialysis tubing was clipped to form a bag so that glucose and starch was fed into the bag through the other end, and was also clipped to avoid the seeping of the solution.

Water with several drops of iodine added to it until it was visibly yellow-amber was added to a 400ml beaker. The bag was then placed in the beaker, which was stirred with a magnetic stirrer. It was left there for 30 minutes. It was seen that the color of the solution in the bag changed to blue-black color, this showed that iodine was able to pass through the membrane into the bag.

The solution in the beaker became pale yellow-amber, this showed that starch didn’t pass through the membrane into the beaker. To confirm the presence of glucose in the beaker and also the bag, a Benedict test was performed on the solutions including tap water (control) too.

The beaker solution turned into light brown color after Benedict solution was added to it and suspended in water bath for 10 minutes. The bag solution also changed to brown color, while tap water remained blue. This experiment showed that dialysis tubing is selective in its permeability to molecules. It was permeable to glucose and iodine but not starch.

INTRODUCTION:

PURPOSE: The purpose of the experiment was to test the permeability of dialysis tubing to glucose, starch and iodine.

Living cells need to obtain nutrients from their environment and get rid of waste materials to their surroundings. This exchange of materials between the cell and its surroundings is crucial to its existence. Cells have membranes composed of a phospholipid bilayer embedded with proteins.

This cell membrane can distinguish between different substances, slowing or hindering the movement of other substances and allowing others to pass through readily. This property of the cell is known as selective permeability (Ramlingam, 2008).

Selective permeability is a property of a cell membrane that allows it to control which molecules can pass (moving into and out of the cell) through the pores of the membrane. Selective permeable membranes only allows small molecules such as glucose, amino acids to readily pass through, and inhibits larger molecules like protein, starch, from passing through it.

The dialysis tubing is a semi-permeable membrane tubing used in separation techniques and demonstration of diffusion, osmosis, and movement of molecules across a restrictive membrane (Todd, 2012). It separates dissolved substances of different molecular sizes in a solution, and some of the substances may readily pass through the pores of the membrane while others are excluded. The dialysis tubing is made up of cellulose fibers. This is shaped in a flat tube.

In this experiment, the selective permeability of dialysis tubing to glucose, starch and iodine (potassium iodide) will be tested. This experiment consists of two tests; the test for starch and the test for reducing sugar. When iodine (potassium iodide) is added to a solution in which starch is present, the solution turns blue-black or purple otherwise it remains yellow-amber.

And when Benedict’s reagent is added to a solution in which reducing sugar is present and it is heated in a water bath, the solution turns green, yellow, orange, red, and then brick red or brown (with high concentration of sugar present). Otherwise, the solution remains blue.

Will glucose, starch and iodine (potassium iodide) readily pass through the pores of the dialysis tubing?

HYPOTHESIS:

Glucose, starch and iodine (potassium iodide) will readily pass through the membrane of the dialysis tubing.

PREDICTION:

The solution in the bag and the beaker will both turn blue-black due to the presence of iodine and starch; the presence of glucose in the bag and beaker will be investigated using Benedict test.

• Dialysis Tubing
• Test Tubes rack
• Benedict’s reagent
• Iodine (Potassium Iodide)

EXPERIMENT PROCEDURE:

1) 250 ml of tap water was added to a beaker. Several droppers of Iodine (Potassium Iodide) solution was added to the water until it was visibly yellow-amber in color. The color was then recorded.

2) The dialysis tubing was soaked in water for a few minutes until it began to open. One end of the bag was folded and clipped in order to secure it so that no solution seeped through.

3) The other end of the tubing was opened so that it forms a bag and 4ml of glucose and 3ml of starch was fed into it. The bag was also closed and its content was mixed. The color of the solution was then recorded.

4) The outside of the bag was rinsed in tap water.

5) The magnetic stirrer and then the bag was placed in the beaker. The other end of the bag was made to hang over the edge of the beaker.

6) The bag was left in the beaker for about 30 minutes, as the beaker was being stirred.

7) After 30 minutes, the bag was carefully removed and made to stand in a dry beaker. The final color of the solutions was recorded.

8) Benedict test was performed to test for the presence of reducing sugar in the solution in the bag, beaker and tap water (serves as control).

• a) 3 test tubes were labelled control, bag and beaker.
• b) 2 ml of water was added to the control test tube. 2 ml of the bag solution was added to the bag test tube and 2 ml of the beaker solution was added to the beaker test tube.
• c) 2 ml of Benedict’s reagent was added to each test tube and was suspended in a boiling water bath for 10 minutes. The color change was recorded.

The solution in the bag turned blue-black in color owing to the movement of molecules of iodine from the beaker to the bag which contains starch. The solution in the beaker turned brown after Benedict’s test.

This indicated the presence of glucose in the beaker. This means that the tubing was permeable to both glucose and iodine but not starch. It is known that starch didn’t pass because the solution in the beaker which contains iodine didn’t turn blue-black in color, but remained yellow-amber.

DISCUSSION:

1) How can you explain your results?

From the results of the experiment represented in a tabular form above, the hypothesis suggested before carrying out the experiment turned out to be incorrect. The dialysis tubing was not permeable to all the three solutions- glucose, starch and Iodine (Potassium Iodide). Rather, the tubing was permeable to glucose and iodine but not starch.

This could be known from the color change in the solutions in the beaker and the bag. The tubing was permeable to iodine and so the content of the bag turned blue-black in color indicating the presence of starch. Glucose also readily passed through the pores of the membrane.

After performing Benedict’s test on the solutions, the bag’s solution as well as the beaker’s solution turned brown in color. This shows the presence of reducing sugar in both solutions, meaning that glucose passed into the beaker from the bag.

2) From your results, predict the size of Iodine (Potassium Iodide) relative to Starch.

From the results of this experiment, it is obvious that glucose and iodine (potassium iodide) has smaller molecular size than starch. Because starch had larger molecular size, the dialysis tubing was not permeable to it (it didn’t allow it to readily pass through the pores of its membrane).

3) What colors would you expect if the experiment started with glucose and iodine (potassium iodide) inside the bag and starch in the beaker? Explain

* The solution in the bag will remain yellow-amber in color at the end of the experiment.

* The solution in the beaker will turn blue-black in color at the end of the experiment.

* After performing benedict test, both solutions will turn brown in color.

The solution in the bag remained yellow-amber in color at the end of the experiment because the dialysis tubing is not permeable to starch and so starch didn’t pass through from the beaker into the bag.

The solution in the beaker turned blue-black in color at the end of the experiment because iodine passed from the bag into the beaker through the membrane.

After performing Benedict’s test on the bag and beaker solution, both solutions turned brown in color because the tubing was permeable to glucose, so glucose readily passed from the bag into the beaker through the membrane.

PRECAUTIONS:

• It was ensured that the right quantity of solutions was used in every part of the experiment.
• It was also ensured that the time required for the successful complement of the experiment was adhered to.
• It was ensured that all apparatus used were handled with caution.
• And also, the dialysis tubing was clipped well on both ends to secure it so that no solution seeped through.

CONCLUSION:

It was concluded that the dialysis tubing doesn’t allow all kinds of substances to pass readily through the pores of its membrane. This means that it is selective in its permeability to substances. The dialysis tubing was permeable to glucose and iodine but not to starch. Starch was excluded because it has a larger molecular size than glucose and iodine.

Ramlingam, S. T. (2008). Modern Biology. Onitsha: African First Publishers.

Todd, I. S. (2012). Dialysis: History, Development and Promise. World Scientific Publishing Co Pte Ltd.

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18 Comments

Could oxygen pass through the dialysis tubing

so what was the chemical formula for this experiment?

After 24 hours of leaving the bag in the iodine solution; -The dialysis bag turned dark blue/purple, Explain. -The fructose test strip turned positive when dipped in the solution, Explain.  

if the dialysis represent the membrane of a root air cell, and the sugar solution inside represent the cells cytoplasm, which is hypotonic, hypertonic or isotonic. is there any movement of iodine molecules?

What is the purpose of the Iodine Solution?

you added starch and glucose to dialysis tubing, a semipermeable membrane that mimics the plasma membrane of cells. The filled tubing which was placed in a beaker of water containing iodine. What is the purpose of the iodine?

Is the iodine entering the dialysis tube an example of diffusion or osmosis? or can osmosis only occur with water?

what was the purpose of placing the dialysis tubing containing starch solution into the beaker of distilled water

What were the limitations of your experiment ?

What about the NaCl? I did this lab but we had a question if NaCl moved out of the tube.

what did not diffuse through the membrane

Starch and Benedict’s solution.

Maybe the starch and its size.

How can you explain the change in weight of the cells?

osmosis of water

Include an analysis maybe? all around good job though!

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