experiment light intensity affect photosynthesis

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Biology Teaching Resources

Photosynthesis Virtual Lab

This lab was created to replace the popular waterweed simulator which no longer functions because it is flash-based. In this virtual photosynthesis lab , students can manipulate the light intensity, light color, and distance from the light source.

A plant is shown in a beaker and test tube which bubbles to indicate the rate of photosynthesis. Students can measure the rate over time. There is an included data table for students to type into the simulator, but I prefer to give them their own handout ,

The handout is a paper version for students to write on as the work with the simulator. The document is made with google docs so that it can be shared with remote students.

There are several experiments that can be done in the lab that would complement this virtual experiment. For example, students can use elodea and measure the number of bubbles released when the plant is under a bright light. Algae beads can also be used to measure changes in pH as the plants consume carbon dioxide.

In experiment 2, students specifically look at light color to determine which wavelength of light increases the rate of photosynthesis. Students should discover that green light has a very slow rate. Their collected data is then compared to a graph of the absorption spectrum of light.

Shannan Muskopf

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Photosynthetic Physiology of Blue, Green, and Red Light: Light Intensity Effects and Underlying Mechanisms

Associated data.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Red and blue light are traditionally believed to have a higher quantum yield of CO 2 assimilation ( QY , moles of CO 2 assimilated per mole of photons) than green light, because green light is absorbed less efficiently. However, because of its lower absorptance, green light can penetrate deeper and excite chlorophyll deeper in leaves. We hypothesized that, at high photosynthetic photon flux density ( PPFD ), green light may achieve higher QY and net CO 2 assimilation rate ( A n ) than red or blue light, because of its more uniform absorption throughtout leaves. To test the interactive effects of PPFD and light spectrum on photosynthesis, we measured leaf A n of “Green Tower” lettuce ( Lactuca sativa ) under red, blue, and green light, and combinations of those at PPFD s from 30 to 1,300 μmol⋅m –2 ⋅s –1 . The electron transport rates ( J ) and the maximum Rubisco carboxylation rate ( V c,max ) at low (200 μmol⋅m –2 ⋅s –1 ) and high PPFD (1,000 μmol⋅m –2 ⋅s –1 ) were estimated from photosynthetic CO 2 response curves. Both QY m,inc (maximum QY on incident PPFD basis) and J at low PPFD were higher under red light than under blue and green light. Factoring in light absorption, QY m,abs (the maximum QY on absorbed PPFD basis) under green and red light were both higher than under blue light, indicating that the low QY m,inc under green light was due to lower absorptance, while absorbed blue photons were used inherently least efficiently. At high PPFD , the QY inc [gross CO 2 assimilation ( A g )/incident PPFD ] and J under red and green light were similar, and higher than under blue light, confirming our hypothesis. V c,max may not limit photosynthesis at a PPFD of 200 μmol m –2 s –1 and was largely unaffected by light spectrum at 1,000 μmol⋅m –2 ⋅s –1 . A g and J under different spectra were positively correlated, suggesting that the interactive effect between light spectrum and PPFD on photosynthesis was due to effects on J . No interaction between the three colors of light was detected. In summary, at low PPFD , green light had the lowest photosynthetic efficiency because of its low absorptance. Contrary, at high PPFD , QY inc under green light was among the highest, likely resulting from more uniform distribution of green light in leaves.

Introduction

The photosynthetic activity of light is wavelength dependent. Based on McCree’s work ( McCree, 1971 , 1972 ), photosynthetically active radiation is typically defined as light with a wavelength range from 400 to 700 nm. Light with a wavelength shorter than 400 nm or longer than 700 nm was considered as unimportant for photosynthesis, due to its low quantum yield of CO 2 assimilation, when applied as a single waveband ( Figure 1 ). Within the 400–700 nm range, McCree (1971) showed that light in the red region (600–700 nm) resulted in the highest quantum yield of CO 2 assimilation of plants. Light in the green region (500–600 nm) generally resulted in a slightly higher quantum yield than light in the blue region (400–500 nm) ( Figure 1 ; McCree, 1971 ). The low absorptance of green light is partly responsible for its low quantum yield of CO 2 assimilation. Within the visible spectrum, green leaves have the highest absorptance in the blue region, followed by red. Green light is least absorbed by green leaves, which gives leaves their green appearance ( McCree, 1971 ; Zhen et al., 2019 ).

The normalized action spectrum of the maximum quantum yield of CO 2 assimilation for narrow wavebands of light from ultra-violet to far-red wavelengths ( McCree, 1971 ). Redrawn using data from Sager et al. (1988).

Since red and blue light are absorbed more strongly by photosynthetic pigments than green light, they are predominantly absorbed by the top few cell layers, while green light can penetrate deeper into leaf tissues ( Nishio, 2000 ; Vogelmann and Evans, 2002 ; Terashima et al., 2009 ; Brodersen and Vogelmann, 2010 ), thus giving it the potential to excite photosystems in deeper cell layers. Leaf photosynthesis may benefit from the more uniform light distribution throughout a leaf under green light. Absorption of photons by chloroplasts near the adaxial surface may induce heat dissipation of excess excitation energy in those chloroplasts, while chloroplasts deeper into the leaf receive little excitation energy ( Sun et al., 1998 ; Nishio, 2000 ). Blue and red photons, therefore, may be used less efficiently and are more likely to be dissipated as heat than green photons.

The misconception that red and blue light are used more efficiently by plants than green light still occasionally appears ( Singh et al., 2015 ), often citing McCree’s action spectrum or the poor absorption of green light by chlorophyll extracts. The limitations of McCree’s action spectrum were explained in his original paper: the quantum yield was measured under low photosynthetic photon flux density ( PPFD ), using narrow waveband light, and expressed on an incident light basis ( McCree, 1971 ), but these limitations are sometimes ignored. The importance of green light for photosynthesis has been well established in more recent studies ( Sun et al., 1998 ; Nishio, 2000 ; Terashima et al., 2009 ; Hogewoning et al., 2012 ; Smith et al., 2017 ).

From those studies, one trend has emerged that has not received much attention: there is an interactive effect of light quality and intensity on photosynthesis ( Sun et al., 1998 ; Evans and Vogelmann, 2003 ; Terashima et al., 2009 ). At low PPFD , green light has the lowest QY inc (quantum yield of CO 2 assimilation on incident light basis) because of its low absorptance; at high PPFD , on the other hand, red and blue light have a lower QY inc than green light, because of their high absorptance by photosynthetic pigments, which shifts much of the light absorption closer to the upper leaf surface. This reduces both the quantum yield of CO 2 assimilation in cells in the upper part of a leaf and light availability in the bottom part of a leaf.

The interactive effect between light quality and intensity was illustrated in an elegant study that quantified the differential quantum yield, or the increase in leaf CO 2 assimilation per unit of additional light ( Terashima et al., 2009 ). The differential quantum yield was measured by adding red or green light to a background illumination of white light of different intensities. At low background white light levels, the differential quantum yield of red light was higher than that of green light, due to the low absorptance of green light. But as the background light level increased, the differential quantum yield of green light decreased more slowly than that of red light, and was eventually higher than that of red light ( Terashima et al., 2009 ). The red light was absorbed efficiently by the chloroplasts in the upper part of leaves. With a high background level of white light, those chloroplasts already received a large amount of excitation energy from white light and up-regulated non-photochemical quenching (NPQ) to dissipate excess excitation energy as heat, causing the additional red light to be used inefficiently. Green light, on the other hand, was able to reach the chloroplasts deeper in the mesophyll and excited those chloroplasts that received relatively little excitation energy from white light. Therefore, with high background white light intensity, additional green light increased leaf photosynthesis more efficiently than red light ( Terashima et al., 2009 ).

In this paper, we present a comprehensive study to explore potential interactive effect of light intensity and light quality on C 3 photosynthesis and underlying processes. We quantified the photosynthetic response of plants to blue, green, and red light over a wide PPFD range to better describe how light intensity and waveband interact. In addition, we examined potential interactions among blue, green, and red light, using light with different ratios and intensities of the three narrow waveband lights. To get a better understanding of the biochemical reasons for the effects of light spectrum and intensity on CO 2 assimilation, we constructed assimilation – internal leaf CO 2 ( C i ) response curves ( A/C i curves) under blue, green, and red light, as well as combinations of the three narrow waveband lights at both high and low PPFD . We hypothesized that effects of different light spectra would be reflected in the electron transport rate ( J ) required to regenerate consumed ribulose 1,5-bisphosphate (RuBP), rather than the maximum carboxylation rate of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) ( V c,max ).

Materials and Methods

Plant material.

Lettuce “Green Towers” plants were grown from seed in 1.7 L round pots filled with soilless substrate (Fafard 4P Mix, Sun Gro Horticulture, Agawam, MA, United States). The plants were grown in a growth chamber (E15, Conviron, Winnipeg, Manitoba, Canada) at 23.2 ± 0.8°C (mean ± SD), under white fluorescent light with a 14-hr photoperiod, vapor pressure deficit (VPD) of 1.20 ± 0.43 kPa and a PPFD of 200–230 μmol⋅m –2 ⋅s –1 at the floor level, and ambient CO 2 concentration. Plants were sub-irrigated when necessary with a nutrient solution containing 100 mg⋅L –1 N, made with a complete, water-soluble fertilizer (Peter’s Excel 15-5-15 Cal-Mag fertilizer, Everris, Marysville, OH, United States).

Leaf Absorptance, Transmittance, and Reflectance

Leaf absorptance was determined using a method similar to that of Zhen et al. (2019) . Three plants were randomly selected. A newly expanded leaf from each plant was illuminated with a broad-spectrum halogen bulb (70W; Sylvania, Wilmington, MA, United States) for leaf transmittance measurement. Transmittance was measured with a spectroradiometer (SS-110, Apogee, Logan, UT, United States). The halogen light spectrum was taken as reference measurement with the spectroradiometer placed directly under the halogen bulb in a dark room. Then, a lettuce leaf was placed between the halogen bulb and spectroradiometer, with its adaxial side facing the halogen bulb and transmitted light was measured. Leaf transmittance was then calculated on 1 nm resolution. Light reflectance of the leaves was measured using a spectrometer with a leaf clip (UniSpec, PP systems, Amesbury, MA, United States). Light absorptance was calculated as 1− r e f l e c t a n c e − t r a n s m i t t a n c e . We verified that this method results in similar absorptance spectra as the use of an integrating sphere. Absorptance of each of the nine light spectra used in this study were calculated from the overall leaf absorptance spectrum and the spectra of the red, green, and blue LEDs.

Leaf Photosynthesis Measurements

All gas exchange measurements were made with a leaf gas exchange system (CIRAS-3, PP Systems). Light was provided by the LEDs built into the chlorophyll fluorescence module (CFM-3, PP Systems). This module has dimmable LED arrays of different colors, with peaks at 653 nm [red, full width at half maximum (FWHM) of 17 nm], 523 nm (green, FWHM of 36 nm), and 446 nm (blue, FWHM of 16 nm). Nine different combinations of red, green, and blue light were used in this study ( Table 1 ). Throughout the measurements, the environmental conditions inside the cuvette were controlled by the leaf gas exchange system. Leaf temperature was 23.0 ± 0.1°C, CO 2 concentration was 400.5 ± 4.1 μmol⋅mol –1 , and the VPD of air in the leaf cuvette was 1.8 ± 0.3 kPa (mean ± SD).

List of light spectrum abbreviations and their spectral composition.

Photosynthesis – Light Response Curves

To explore photosynthetic efficiency of light with different spectra, we constructed light response curves for lettuce plants using each light spectrum. Lettuce plants were exposed to 10 PPFD levels ranging from 30 to 1,300 μmol⋅m –2 ⋅s –1 (30, 60, 90, 120, 200, 350, 500, 700, 1,000, and 1,300 μmol⋅m –2 ⋅s –1 ) in ascending orders for light response curves. Photosynthetic measurements were taken on 40–66 days old lettuce plants. Lettuce plants were taken out of the growth chamber and dark-adapted for 30 min. Starting from the lowest PPFD , one newly expanded leaf was exposed to all nine spectra. Net CO 2 assimilation rate ( A n ) of the leaf was measured using the leaf gas exchange system. Under each light spectrum, three A n readings were recorded at 10 s intervals after readings were stable (about 4–20 min depending on PPFD after changing PPFD and spectrum). The three A n readings were averaged for analysis. After A n measurements under all nine light spectra were taken, the leaf was exposed to the next PPFD level and A n measurements were taken with the light spectra in the same order, until measurements were completed at all PPFD levels. Throughout the light response curves, C i decreased with increasing PPFD , from 396 ± 10 μmol⋅mol –1 at a PPFD of 30 μmol⋅m –2 ⋅s –1 to 242 ± 44 μmol⋅mol –1 at a PPFD of 1,300 μmol⋅m –2 ⋅s –1 . To account for the potential effect of plants and the order of the spectra on assimilation rates, the order of the different spectra was re-randomized for each light response curve, using a Latin square design with plant and spectrum as the blocking factors. Data were collected on nine different plants.

Regression curves (exponential rise to maximum) were fitted to the data for each light spectrum and replication (plant):

where R d is the dark respiration rate, QY m,inc is the maximum quantum yield of CO 2 assimilation (initial slope of light response curve, mol of CO 2 fixed per mol of incident photons) and A g,max is the light-saturated gross assimilation rate. The A n,max is the light-saturated net assimilation rate and was calculated as A n , m a x = A g , m a x - R d . The maximum quantum yield of CO 2 assimilation was also calculated on absorbed light basis as Q ⁢ Y m , a ⁢ b ⁢ s = Q ⁢ Y m , i ⁢ n ⁢ c l ⁢ i ⁢ g ⁢ h ⁢ t ⁢ a ⁢ b ⁢ s ⁢ o ⁢ r ⁢ p ⁢ t ⁢ a ⁢ n ⁢ c ⁢ e .

The instantaneous quantum yield of CO 2 assimilation based on incident PPFD ( QY inc ) was calculated as A g P ⁢ P ⁢ F ⁢ D for each PPFD at which A n was measured, where the gross CO 2 assimilation rate ( A g ) was calculated as A g = A n + R d . To account for differences in absorptance among the different light spectra, the quantum yield of CO 2 assimilation was also calculated based on absorbed light base, as Q ⁢ Y a ⁢ b ⁢ s = A g P ⁢ P ⁢ F ⁢ D × l ⁢ i ⁢ g ⁢ h ⁢ t ⁢ a ⁢ b ⁢ s ⁢ o ⁢ r ⁢ p ⁢ t ⁢ a ⁢ n ⁢ c ⁢ e , where light absorptance is the absorptance of lettuce leaves for each specific light spectrum. The differential QY , the increase in assimilation rate per unit of additional incident PPFD , was calculated as the derivative of Eq. 1:

Photosynthesis – Internal CO 2 Response ( A/C i ) Curves

To explore the underlying physiological mechanisms of assimilation responses to different light spectra, we constructed A/C i curves. Typically, A/C i curves are collected under saturating PPFD . We collected A/C i curves at two PPFD s (200 and 1,000 μmol⋅m –2 ⋅s –1 ) to explore interactive effects of light spectrum and PPFD on the assimilation rate. At a PPFD of 200 μmol⋅m –2 ⋅s –1 , red light has the highest A n and green light the lowest A n , while at PPFD of 1,000 μmol⋅m –2 ⋅s –1 , red and green light resulted in the highest A n and blue light in the lowest A n .

We used the rapid A/C i response (RACiR) technique that greatly accelerates the process of constructing A/C i curves ( Stinziano et al., 2017 ). We used a Latin square design, similar to the light response curves. A/C i curves were measured under the same nine spectra used for the light response curves. Nine lettuce plants were used as replicates. For each A/C i curve, CO 2 concentration in the leaf cuvette started from 0 μmol⋅mol –1 , steadily ramping to 1,200 μmol⋅mol –1 over 6 min. A reference measurement was also taken at the beginning of each replication with an empty cuvette to correct for the reaction time of the leaf gas exchange system. Post-ramp data processing was used to calculate the real A and C i with the spreadsheet provided by PP systems, which yielded the actual A/C i curves with C i range of about 100–950 μmol mol –1 . Throughout the data collection, leaf temperature was 24.4 ± 1.3°C and VPD in the cuvette was 1.4 ± 0.2 kPa.

Curve fitting for A/C i curves was done by minimizing the residual sum of squares, following the protocol developed by Sharkey et al. (2007) . Among our nine replicates, four plants did not show clear Rubisco limitations at low PPFD and for those plants Rubisco limitation ( V c,max ) was not included in the model ( Sharkey et al., 2007 ). We therefore report V c,max values for high PPFD only. The J was determined for all light spectra at both PPFD s. We therefore report V c,max was determined for all light spectra only at high PPFD . The quantum yield of electron transport [ QY(J) ] was calculated on both incident and absorbed PPFD basis as Q ⁢ Y ⁢ ( J ) i ⁢ n ⁢ c = J P ⁢ P ⁢ F ⁢ D and Q ⁢ Y ⁢ ( J ) a ⁢ b ⁢ s = Q ⁢ Y ⁢ ( J ) i ⁢ n ⁢ c l ⁢ i ⁢ g ⁢ h ⁢ t ⁢ a ⁢ b ⁢ s ⁢ o ⁢ r ⁢ p ⁢ t ⁢ a ⁢ n ⁢ c ⁢ e , respectively. We did not estimate triose phosphate utilization, because the A/C i curves often did not show a clear plateau.

Data Analysis

The QY m,inc , QY m,abs , and A g,max were analyzed with ANOVA to determine the effects of light spectrum using SAS (SAS University Edition; SAS Institute, Cary, NC, United States). A n , QY inc , and QY abs at each PPFD level and V c,max and J estimated from A/C i curves were similarly analyzed with ANOVA using SAS. A n at different PPFD levels were analyzed with regression analysis to detect interactive effect of blue, green, and red light on leaf assimilation rates using the fractions of red, blue, and green light as explanatory variables (JMP Pro 15, SAS Institute).

Leaf Absorptance

A representative spectrum of light absorptance, reflectance and transmittance of a newly fully expanded lettuce leaf is shown in Figure 2 . In the blue region, 400–500 nm, the absorptance by “Green Towers” lettuce leaves was high and fairly constant, averaging 91.6%. The leaf absorptance decreased as the wavelength increased from 500 to 551 nm where the absorptance minimum was 69.8%. Absorptance increased again at longer wavelengths, with a second peak at 666 nm (92.6%). Above 675 nm, the absorptance decreased steadily to <5% at 747 nm ( Figure 2 ). The absorptance spectrum of our lettuce leaves is similar to what McCree (1971) obtained for growth chamber-grown lettuce, with the exception of slightly higher absorptance in the green part of the spectrum in our lettuce plants. Using this spectrum, the absorptance of the blue, green, and red LED lights were calculated to be 93.2 ± 1.0%, 81.1 ± 1.9% and 91.6 ± 1.1%, respectively. Absorptance of all nine spectra was calculated based on their ratios of red, green, and blue light ( Table 2 ).

Light absorptance, reflectance, and transmittance spectrum of a newly fully expanded “Green Towers” lettuce leaf.

Light absorptance and transmittance of new fully expanded “Green towers” lettuce leaves under nine light spectra.

Light Quality and Intensity Effects on Photosynthetic Parameters

Light response curves of lettuce under all nine spectra are shown in Figure 3 , with regression coefficients in Supplementary Table 1 . It is worth noting that a few plants showed photoinhibition under 100B (decrease in A n with PPFD > 1,000 μmol⋅m –2 ⋅s –1 ). Those data were excluded in curve fitting for light response curves to better estimate asymptotes. Photoinhibition was not observed under other spectra.

Net assimilation ( A n ) – light response curves of “Green Towers” lettuce under nine light spectra. Error bars represent the standard deviation ( n = 9). Inserts show A n against PPFD of 30-90 μmol⋅m –2 ⋅s –1 s to better show the initial slopes of curves. The composition of the nine light spectra is shown in Table 1 . The light spectra in the graphs are (A) 100B, 100G, and 100R; (B) 100B, 80B20G, 20B80G, and 100G; (C) 100G, 80G20R, 20G80R, and 100R; and (D) 20B80R, 16B20G64R, and 100G.

The QY m,inc of lettuce plants was 22 and 27% higher under red light (74.3 mmol⋅mol –1 ) than under either 100G (60.8 mmol⋅mol –1 ) or 100B (58.4 mmol⋅mol –1 ), respectively ( Figure 4A and Supplementary Table 1 ). Spectra with a high fraction of red light (64% or more) resulted in a high QY m,inc ( Figure 4A ), while 80G20R resulted in an intermediate QY m,inc ( Figure 4A ). To determine whether differences in QY m,inc were due to differences in absorptance or in the ability of plants to use the absorbed photons for CO 2 assimilation, we also calculated QY m,abs . On an absorbed light basis, 100B light still resulted in the lowest QY m,abs (62.7 mmol⋅mol –1 ) and red light resulted in the highest QY m,abs (81.1 mmol⋅mol –1 ) among narrow waveband lights ( Figure 4B ). Green light resulted in a QY m,abs (74.9 mmol⋅mol –1 ) similar to that under red light, but significantly higher than that of blue light ( Figure 4B ). We did not find any interactions (synergism or antagonism) between lights of different colors, with all physiological responses under mixed spectra being similar to the weighted average of responses under single colors. Thus, for the rest of the results we focus on the three narrow waveband spectra.

Maximum quantum yield of CO 2 assimilation of “Green Towers” lettuce based on incident ( QY m,inc ) (A) and absorbed light ( QY m,abs ) (B) under nine different light spectra. Values are calculated as the initial slope of the light response curves of corresponding light spectra (see Figure 3 ). Bars with the same letter are not statistically different ( p ≤ 0.05). Error bars represent the standard deviation ( n = 9). The composition of the nine light spectra is shown in Table 1 .

Among the three narrow waveband lights, 100G resulted in the highest A g,max (20.0 μmol⋅m –2 ⋅s –1 ), followed by red (18.9 μmol⋅m –2 ⋅s –1 ), and blue light (17.0 μmol⋅m –2 ⋅s –1 ) ( Figure 5 and Supplementary Table 1 ). As with QY m,inc and QY m,abs , combining two or three colors of light resulted in an A g,max similar to the weighted averages of individual light colors.

Maximum gross assimilation rate ( A g,max ) of “Green Towers” lettuce under different light spectra, calculated from the light response curves. Bars with the same letter are not statistically different ( p ≤ 0.05). Error bars represent standard deviation ( n = 9). The composition of the nine light spectra is shown in Table 1 .

QY inc initially increased with increasing PPFD and peaked at 90–200 μmol⋅m –2 ⋅s –1 , then decreased at higher PPFDs ( Figure 6A ). The QY inc under 100R was higher than under either green or blue light at low PPFD (≤300 μmol⋅m –2 ⋅s –1 ). Although 100G resulted in lower QY inc than 100B at low PPFD (≤300 μmol⋅m –2 ⋅s –1 ), the decrease in QY inc under 100G with increasing PPFD was slower than that with 100B or 100R. Above 500 μmol m –2 s –1 , the QY inc with 100G was comparable to the QY inc with 100R, and higher than with 100B ( Figure 6A ). The QY abs with 100R was higher than that with either 100G or 100B at PPFDs from 60 to 120 μmol⋅m –2 ⋅s –1 ( p < 0.05). The QY abs with 100G was similar to 100B at low PPFD , but decreased slower than that with either 100R or 100B as PPFD increased. At PPFD ≥ 500 μmol⋅m –2 ⋅s –1 , QY abs was lowest under 100B among the three monochromatic lights ( p < 0.05) ( Figure 6B ).

The quantum yield of CO 2 assimilation of “Green Towers” lettuce as a function of incident ( QY inc ) (A) and absorbed PPFD ( QY abs ) (B) under blue, green, and red LED light. Error bars represent the standard deviation ( n = 9).

The differential QY , which quantifies the increase in CO 2 assimilation per unit of additional PPFD , decreased with increasing PPFD . The differential QY with 100R was higher than those with 100B and 100G at low PPFD . At a PPFD of 30 μmol⋅m –2 ⋅s –1 , the differential QY was 70.5 mmol⋅mol –1 for 100R, 59.4 mmol⋅mol –1 for 100G, and 55.8 mmol⋅mol –1 for 100B ( Figure 7 ). However, the differential QY with 100R decreased rapidly with increasing PPFD and was lower than the differential QY with 100G at high PPFD ( Figure 7 ). At high PPFD , the differential QY with 100G was highest among three monochromatic light ( Figure 7 ). For instance, at a PPFD of 1,300 μmol⋅m –2 ⋅s –1 , the differential QY with 100G was 1.09 mmol⋅mol –1 , while those with 100B and 100R were 0.64 mmol⋅mol –1 and 0.46 mmol⋅mol –1 , respectively ( Figure 7 ).

The differential quantum yield of CO 2 assimilation ( differential QY ) of “Green Towers” lettuce under blue, green, and red LED light as a function of the PPFD . The differential QY is the increase in net assimilation per unit additional PPFD and was calculated as the first derivate of the light response curves ( Figure 3 ). The insert shows the differential quantum yield plotted at PPFDs of 1,000–1,300 μmol m –2 s –1 s to better show differences at high PPFD (note the different y -axis scale).

Effect of Light Spectrum and Intensity on J and V c,max

J of lettuce leaves at low PPFD was lowest under 100G (47.4 μmol⋅m –2 ⋅s –1 ), followed by 100B (56.1 μmol⋅m –2 ⋅s –1 ), and highest under 100R (64.1 μmol⋅m –2 ⋅s –1 ) ( Figure 8A ). At high PPFD , on the other hand, J of leaves exposed to 100G (115.3 μmol⋅m –2 ⋅s –1 ) and 100R (112.1 μmol⋅m –2 ⋅s –1 ) were among the highest, while J of leaves under 100B was the lowest (97.0 μmol⋅m –2 ⋅s –1 ) ( Figure 8A ). At high PPFD , V c,max of leaves under blue light (59.3 μmol⋅m –2 ⋅s –1 ) was lower than V c,max of leaves under 16B20G64R light (72.1 μmol⋅m –2 ⋅s –1 ), but none of the other treatments differed significantly ( Figure 8 ). When PPFD increased from 200 to 1,000 μmol⋅m –2 ⋅s –1 , J under green light increased by 143%, while J under blue and red light increased by 73% and 75%, respectively ( Figure 8A ). J and V c,max at high PPFD were strongly correlated ( R 2 = 0.82) ( Supplementary Figure 3 ).

Electron transport rate ( J ) at PPFD s of 200 (left bars) and 1,000 μmol m –2 s –1 (right bars) (A) and maximum Rubisco carboxylation rate ( V c,max ) at a PPFD of 1,000 μmol m –2 s –1 (B) of “Green Towers” lettuce, as estimated from A/C i curves under different light spectra. Bars with the same letter are not statistically different ( p ≤ 0.05). Error bars represent the standard deviation ( n = 9). The light composition of the nine light spectra is shown in Table 1 .

Interactive Effect of Light Spectrum and PPFD on Photosynthesis

There was an interactive effect of light spectrum and PPFD on photosynthetic properties of lettuce. Under low light conditions (≤200 μmol⋅m –2 ⋅s –1 ), the QY inc of lettuce leaves under green light was lowest among blue, green, and red light ( Figure 6A ), due to its lower absorptance by lettuce leaves. After accounting for absorptance, green photons were used at similar efficiency as blue photons, while red photons were used most efficiently ( Figure 6B ). The QY m,abs under green and red light were higher than under blue light ( Figure 4B ). At high PPFD , green and red light had similar quantum yield, higher than that of blue light, both on an absorbed and incident light basis ( Figure 6A ). Multiple factors contributed to the interactive effect of light spectrum and PPFD on quantum yield and photosynthesis.

Light Absorptance and Non-Photosynthetic Pigments Determine Assimilation at Low PPFD

QY m,inc with blue and green light was lower than with red light ( Figure 4A ), consistent with McCree’s action spectrum ( McCree, 1971 ). But when taking leaf absorptance into account, QY m,abs was similar under green and red light and lower under blue light ( Figure 4B ). Similarly, at low PPFD (≤200 μmol⋅m –2 ⋅s –1 ), QY inc of lettuce leaves was highest under red, intermediate under blue, and lowest under green light. When accounting for leaf absorptance, QY abs under red light remained highest and QY abs under both green and blue light were similar at low PPFD ( Figure 6A ). Consistent with our data, previous studies also documented that, once absorbed, green light can drive photosynthesis efficiently at low PPFD ( Balegh and Biddulph, 1970 ; McCree, 1971 ; Evans, 1987 ; Sun et al., 1998 ; Nishio, 2000 ; Terashima et al., 2009 ; Hogewoning et al., 2012 ; Vogelmann and Gorton, 2014 ). For example, the QY m,abs of spinach ( Spinacia oleracea ) and cabbage ( Brassica oleracea L. ) was highest under red light, followed by that under green light and lowest with blue light. But on incident light basis, QY m,inc of under green light was lower than under red or blue light ( Sun et al., 1998 ).

Both our data ( Figure 4B ) and those of Sun et al. (1998) show that QY m,abs with blue light is lower than that with red and green light, indicating that blue light is used intrinsically less efficiently by lettuce. Blue light, and, to a lesser extent, green light is absorbed not just by chlorophyll, but also by flavonoids and carotenoids ( Sun et al., 1998 ). Those pigments can divert energy away from photochemistry and thus reduce the QY abs under blue light. Flavonoids (e.g., anthocyanins) are primarily located in the vacuole and cannot transfer absorbed light energy to photosynthetic pigments ( Sun et al., 1998 ). Likewise, free carotenoids do not contribute to photochemistry ( Hogewoning et al., 2012 ). Carotenoids in light-harvesting antennae and reaction centers channel light energy to photochemistry, but with lower transfer efficiency than chlorophylls ( Croce et al., 2001 ; de Weerd et al., 2003a , b ; Wientjes et al., 2011 ; Hogewoning et al., 2012 ). Therefore, absorption of blue light by flavonoids and carotenoids reduces the quantum yield of CO 2 assimilation. Thus, even with the high absorptance of blue light by green leaves, QY m,abs of leaves under blue light was the lowest among the three monochromatic lights ( Figure 4B ). It is likely that the lower QY abs under green light than that under red light was also due to absorption of green light by carotenoids and flavonoids ( Hogewoning et al., 2012 ). At high PPFD , absorption of blue light by flavonoids and carotenoids still occurs, but this is less of a limiting factor for photosynthesis, since light availability is not limiting under high PPFD .

Light Dependence of Respiration and Rubisco Activity May Reduce the Quantum Yield at Low PPFD

At PPFD s below 200 μmol⋅m –2 ⋅s –1 , the QY inc and QY abs of lettuce showed an unexpected pattern in response to PPFD ( Figure 6 ). Unlike the quantum yield of PSII, which decreases exponentially with increasing PPFD ( Weaver and van Iersel, 2019 ), QY inc and QY abs increased initially with increasing PPFD ( Figure 6 ). A similar pattern was previously observed by Craver et al. (2020) in petunia ( Petunia × hybrida ) seedlings. This pattern could result from light-dependent regulation of respiration ( Croce et al., 2001 ), alternative electron sinks such as nitrate reduction ( Skillman, 2008 ; Nunes-Nesi et al., 2010 ), or Rubisco activity ( Campbell and Ogren, 1992 ; Zhang and Portis, 1999 ). In our calculations, we assumed that the leaf respiration in the light was the same as R d . However, leaf respiration in the light is lower than in the dark, in a PPFD -dependent manner ( Brooks and Farquhar, 1985 ; Atkin et al., 1997 ), which can lead to overestimation of A g with increasing PPFD . When we accounted for this down-regulation of respiration, using the model by Müller et al. (2005) to correct A g , QY inc , and QY abs , we found that depression of respiration by light did not explain the initial increase in QY inc and QY abs we observed ( Supplementary Figure 4 ). Alternative electron sinks in the chloroplasts that are upregulated in response to light can explain the low QY inc , and QY abs at low PPFD , because they compete with the Calvin cycle for reducing power (ferredoxin/NADPH). Such processes include photorespiration ( Krall and Edwards, 1992 ), nitrate assimilation ( Nunes-Nesi et al., 2010 ), sulfate assimilation ( Takahashi et al., 2011 ) and the Mehler reaction ( Badger et al., 2000 ) and their effect on QY inc , and QY abs would be especially notable under low PPFD ( Supplementary Figure 5 ).

Upregulation of Rubisco activity by Rubisco activase in the light may also have contributed to the increase in QY inc and QY abs at low PPFD ( Campbell and Ogren, 1992 ; Zhang and Portis, 1999 ). In the dark, 2-carboxy-D-arabinitol-1-phosphate (CA1P) or RuBP binds strongly to the active sites of Rubisco, preventing carboxylation activity. In the light, Rubisco activase releases the inhibitory CA1P or RuBP from the catalytic site of Rubisco, in a light-dependent manner ( Campbell and Ogren, 1992 ; Zhang and Portis, 1999 ; Parry et al., 2008 ). At PPFD < 120 μmol⋅m –2 ⋅s –1 , low Rubisco activity may have limited photosynthesis.

Light Distribution Within Leaves Affects QY at High PPFD

Except for the initial increase at low PPFD , both QY inc and QY abs decreased with increasing PPFD . QY inc decreased slower under green than under red or blue light ( Figure 6A ). At a PPFD ≥ 500 μmol⋅m –2 ⋅s –1 , QY inc under green light was higher than that under blue light ( Figure 6A ). Accordingly, A n under blue light was lower than under green and red light at PPFD s above 500 μmol⋅m –2 ⋅s –1 ( Figure 3A ). The lower QY inc under blue light than under green and red light at high PPFD can be explained by disparities in the light distribution within leaves.

Blue and red light were strongly absorbed by lettuce leaves (93.2 and 91.6%, respectively), while green light was absorbed less (81.1%) ( Table 2 ). Similar low green absorptance was found in sunflower ( Helianthus annuus L.), snapdragon ( Antirrhínum majus L.) ( Brodersen and Vogelmann, 2010 ), and spinach ( Vogelmann and Han, 2000 ). In leaves of those species, absorption of red and blue light peaked in the upper 20% of leaves, and declined sharply further into the leaf. Absorption of red light decreased slower with increasing depth than that of blue light ( Vogelmann and Han, 2000 ; Brodersen and Vogelmann, 2010 ). Green light absorption peaked deeper into leaves, and was more evenly distributed throughout leaves, because of low absorption of green light by chlorophyll ( Vogelmann and Han, 2000 ; Brodersen and Vogelmann, 2010 ). The more even distribution of green light within leaves, as compared to red and blue light, can explain the interactive effects between PPFD and light spectrum on leaf photosynthesis. It was estimated that less than 10% of blue light traveled through the palisade mesophyll and reached the spongy mesophyll in spinach, while about 35% of green light and 25% of red light did so ( Vogelmann and Evans, 2002 ). It was also estimated that chlorophyll in the lowermost chloroplasts of spinach leaves absorbed about 10% of green and <2% of blue light, compared to chlorophyll in the uppermost chloroplasts ( Vogelmann and Evans, 2002 ; Terashima et al., 2009 ).

The more uniform green light distribution within leaves may be a key contributor to higher leaf level QY inc under high PPFD because less heat dissipation of excess light energy is needed ( Nishio, 2000 ; Terashima et al., 2009 ). Reaction centers near the adaxial leaf surface receive more excitation energy under blue, and to a lesser extent under red light, than under green light, because of the differences in absorptance. Consequently, under high intensity blue light, NPQ is up-regulated in the chloroplasts near the adaxial leaf surface to dissipate some of the excitation energy ( Sun et al., 1998 ; Nishio, 2000 ), lowering the QY inc under blue light. Since less green light is absorbed near the adaxial surface, less heat dissipation is required. When incident light increased from 150 to 600 μmol⋅m –2 ⋅s –1 , the fraction of whole leaf CO 2 assimilation that occurred in the top half of spinach leaves remained the same under green light (58%), but decreased from 87 to 73% under blue light. This indicates more upregulation of heat dissipation in the top of the leaves under blue, than under green light ( Evans and Vogelmann, 2003 ). On the other hand, the bottom half of the leaves can still utilize the available light with relatively high QY inc , since the amount of light reaching the bottom half is relatively low, even under high PPFD ( Nishio, 2000 ). By channeling more light to the under-utilized bottom part of leaves, leaves could achieve higher QY inc even under high intensity green light. In our study, high QY inc under green light and low QY inc under blue light at high PPFD ( Figure 6 ) can be thus explained by the large disparities in the light environment in chloroplasts from the adaxial to the abaxial side of leaves due to differences in leaf absorptance. Similarly, differential QY of lettuce leaves was highest under green light and lower under blue and red light at high PPFD (>300 μmol⋅m –2 ⋅s –1 ) ( Figure 7 ), also potentially because of the more uniform distribution of green light and the uneven distribution of blue and red light in leaves.

Along the same line, A n of lettuce leaves was the lowest under blue light at PPFD > 500 μmol⋅m –2 ⋅s –1 ( Figure 3 ). Also, A n of lettuce leaves approached light saturation at lower PPFD s under blue and red light, than under green light ( Figure 3A ). Under blue, green, and red light, lettuce leaves reached 95% of A n,max at PPFD s of 954, 1,110 and 856 μmol⋅m –2 ⋅s –1 , respectively. This can be seen more clearly in the differential QY at high PPFD ( Figure 7 ). At a PPFD of 1,300 μmol⋅m –2 ⋅s –1 , green light had a differential QY of 1.09 mmol⋅mol –1 , while that of red and blue light was only 0.46 and 0.69 mmol⋅mol –1 , respectively ( Figure 7 ). Green light also resulted in a higher A g,max (22.9 μmol⋅m –2 ⋅s –1 ) than red and blue light (21.8 and 19.3 μmol⋅m –2 ⋅s –1 , respectively) ( Figure 5 ). As discussed before, the high A g,max under green light resulted from the more uniform light distribution under green light, allowing deeper cell layers to photosynthesize more. Previous research similarly found that at high PPFD (>500 μmol⋅m –2 ⋅s –1 ), A n of both spinach and cabbage were lower under blue light than under white, red and green light ( Sun et al., 1998 ). Overall, under high PPFD , the differences in light distribution throughout a leaf are important to quantum yield and assimilation rate, since it affects NPQ up-regulation ( Sun et al., 1998 ; Nishio, 2000 ). However, light distribution within a leaf is less important at low than at high PPFD , because upregulation of NPQ increases with increasing PPFD ( Zhen and van Iersel, 2017 ).

Light Spectrum Affects J , but Not V c,max

We examined the effect of light quality and intensity on J and V c,max ( Figure 8 ). For the light-dependent reactions, the interactive effect between light spectra and PPFD found for CO 2 assimilation and quantum yield was also observed for J ( Figure 8A ). At low PPFD (200 μmol⋅m –2 ⋅s –1 ), green light resulted in the lowest J and red light in the highest J among single waveband spectra. But at a PPFD of 1,000 μmol⋅m –2 ⋅s –1 , red and green light resulted in the highest J and blue light in the lowest J ( Figure 8A ), similar to the differences in A g .

There was no clear evidence of Rubisco limitations to photosynthesis at a PPFD of 200 μmol⋅m –2 ⋅s –1 , so the rate of the light-dependent reactions likely limited photosynthesis. This is corroborated by the strong correlation between A g and J at a PPFD of 200 μmol⋅m –2 ⋅s –1 . Although Rubisco limitations to photosynthesis were observed at a PPFD of 1,000 μmol⋅m –2 ⋅s –1 , there were no meaningful differences in V c,max in response to light spectrum, in contrast to J ( Figure 8 ).

When PPFD increased 5×, from 200 to 1,000 μmol⋅m –2 ⋅s –1 , there was only a 1.7 to 2.4× increase in J , indicating a lower QY(J) inc at higher PPFD . This matches the lower QY inc and the asymptotic increase in A n in response to increasing PPFD ( Figure 3 ). The relative increase of J under green light (143%) was greater than that under both blue and red light (73 and 75%, respectively) as PPFD increased. This similarly can be attributed to a more uniform energy distribution of green light among reaction centers throughout a leaf and weaker upregulation of non-photochemical quenching with increasing green light intensity ( Sun et al., 1998 ; Nishio, 2000 ; Evans and Vogelmann, 2003 ), as discussed before.

There was a strong correlation between J and A g under the nine light spectra at both PPFD levels ( Figure 9A ). QY abs and QY(J) abs are similarly strongly correlated ( Figure 9B ). Unlike J , V c,max was largely unaffected by light spectra ( Figure 8B ) and was not correlated with A g (data not shown). There was, however, a strong correlation between J and V c,max at a PPFD of 1,000 μmol⋅m –2 ⋅s –1 ( R 2 = 0.82, Supplementary Figure 3 ), suggesting that J and V c,max are co-regulated. Similarly, Wullschleger (1993) noted a strong linear relationship between J and V c,max across 109 C 3 species. The ratio between J and V c,max in our study (1.5–2.0) similar to the ratio found by Wullschleger (1993) . These results suggest that the interactive effect of light spectra and PPFD resulted from effects on J , which is associated with light energy harvesting by reaction centers, rather than from V c,max .

The correlation between gross CO 2 assimilation rate ( A g ) estimated from light response curves and electron transport rate ( J ) estimated from A/C i curves (A) , and between the quantum yield of CO 2 assimilation ( QY abs ) and the quantum yield of electron transport on an absorbed light basis [ QY(J) abs ] (B) , under low PPFD (200 μmol m –2 s –1 ) and high PPFD (1,000 μmol m –2 s –1 ) under nine light spectra averaged over nine “Green Towers” lettuce plants. The color scheme representing the nine spectra is the same as Figure 8 .

No Interactive Effects Among Blue, Green, and Red Light

The Emerson enhancement effect describes a synergistic effect between lights of different wavebands (red and far-red) on photosynthesis ( Emerson, 1957 ). McCree (1971) attempted to account for interactions between light with different spectra when developing photosynthetic action spectra and applied low intensity monochromatic lights from 350 to 725 nm with white background light to plants. His results showed no interactive effect between those monochromatic lights and white light ( McCree, 1971 ). We tested different ratios of blue, green, and red light and different PPFD s, and similarly did not find any synergistic or antagonistic effect of different wavebands on any physiological parameters measured or calculated.

Importance of Interactions Between PPFD and Light Quality and Its Applications

The interactive effect between PPFD and light quality demonstrates a remarkable adaptation of plants to different light intensities. By not absorbing green light strongly, plants open up a “green window,” as Terashima et al. (2009) called it, to excite chloroplasts deeper into leaves, and thus facilitating CO 2 assimilation throughout the leaf. While red light resulted in relatively high QY inc , QY abs and A n at both high and low PPFD ( Figures 3 , ​ ,6), 6 ), it is still mainly absorbed in the upper part of leaves ( Sun et al., 1998 ; Brodersen and Vogelmann, 2010 ). Green light can penetrate deeper into leaves ( Brodersen and Vogelmann, 2010 ) and help plants drive efficient CO 2 assimilation at high PPFD ( Figures 3 , ​ , 5 5 ).

Many early photosynthesis studies investigated the absorptance and action spectrum of photosynthesis of green algae, e.g., Haxo and Blinks (1950) or chlorophyll or chloroplasts extracts, e.g., Chen (1952) . Extrapolating light absorptance of green algae and suspension of chlorophyll or chloroplast to whole leaves from can lead to an underestimation of absorptance of green light by whole leaves and the belief that green light has little photosynthetic activity ( Moss and Loomis, 1952 ; Smith et al., 2017 ). Photosynthetic action spectra developed on whole leaves of higher plants, however, have long shown that green light effectively contributes to CO 2 assimilation, although with lower QY inc than red light ( Hoover, 1937 ; McCree, 1971 ; Inada, 1976 ; Evans, 1987 ). The importance of green light for photosynthesis was clearly established in more recent studies, emphasizing its role in more uniformly exciting all chloroplasts, which especially important under high PPFD ( Sun et al., 1998 ; Nishio, 2000 ; Terashima et al., 2009 ; Hogewoning et al., 2012 ; Smith et al., 2017 ). The idea that red and blue light are more efficient at driving photosynthesis, unfortunately, still lingers, e.g., Singh et al. (2015) .

Light-emitting diodes (LEDs) have received wide attention in recent years for use in controlled environment agriculture, as they now have superior efficacy over traditional lighting technologies ( Pattison et al., 2018 ). LEDs can have a narrow spectrum and great controllability. This provides unprecedented opportunities to fine tune light spectra and PPFD to manipulate crop growth and development. Blue and red LEDs have higher efficacy than white and green LEDs ( Kusuma et al., 2020 ). By coincidence, McCree’s action spectrum ( Figure 1 ; McCree, 1971 ) also has peaks in the red and blue region, although the peak in the blue region is substantially lower than the one in the red region. Therefore, red and blue LEDs are sometimes considered optimal for driving photosynthesis. This claim holds true only under low PPFD . Green light plays an important role in photosynthesis, as it helps plants to adapt to different light intensities. The wavelength-dependent absorptance of chlorophylls channels green light deeper into leaves, resulting in more uniform light absorption throughout leaves and providing excitation energy to cells further from the adaxial surface. Under high PPFD , this can increase leaf photosynthesis. Plant evolved under sunlight for hundreds of millions of years, and it seems likely that the relatively low absorptance of green light contributes to the overall photosynthetic efficiency of plants ( Nishio, 2000 ).

There was an interactive effect of light spectrum and PPFD on leaf photosynthesis. Under low PPFD , QY inc was lowest under green and highest under red light. The low QY inc under green light at low PPFD was due to low absorptance. In contrast, at high PPFD , green and red light achieved similar QY inc , higher than that of blue light. The strong absorption of blue light by chlorophyll creates a large light gradient from the top to the bottom of leaves. The large amount of excitation energy near the adaxial side of a leaf results in upregulation of nonphotochemical quenching, while chloroplasts near the bottom of a leaf receive little excitation energy under blue light. The more uniform distribution of green light absorption within leaves reduces the need for nonphotochemical quenching near the top of the leaf, while providing more excitation energy to cells near the bottom of the leaf. We also found that the interactive effect of light spectrum and PPFD on photosynthesis was a result of the light-dependent reactions; gross assimilation and J were strongly correlated. We detected no synergistic or antagonistic interactions between blue, green, and red light.

Data Availability Statement

Author contributions.

JL and MI designed the experiment, discussed the data, and revised the manuscript. JL performed the experiment, analyzed data, and prepared the first draft. Both authors contributed to the article and approved the submitted version.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

Funding. This study was funded by the USDA-NIFA-SCRI award number 2018-51181-28365, project Lighting Approaches to Maximize Profits.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fpls.2021.619987/full#supplementary-material

Supplementary Figure 1

(Related to Figure 6 ) Quantum yield of CO 2 assimilation of “Green Towers” lettuce as a function of incident ( QY inc ) (A,C,E,G) and absorbed PPFD ( QY abs ) (B,D,F,H) under nine light spectra (see Table 1 ). Error bars represent standard deviation ( n = 9).

Supplementary Figure 2

(Related to Figure 7 ) Differential quantum yield of CO 2 assimilation ( differential QY ) of “Green Towers” lettuce under nine light spectra as a function of the PPFD . Inserts show differential QY at PPFD s of 1,000–1,300 μmol⋅m –2 s –1 s to better show differences at high PPFD (note the different y -axis scale). The composition of the nine light spectra is shown in Table 1 . The light spectra in the graphs are (A) 100B, 100G and 100R; (B) 100B, 80B20G, 20B80G and 100G; (C) 100G, 80G20R, 20G80R and 100R; and (D) 20B80R, 16B20G64R and 100G.

Supplementary Figure 3

(Related to Figure 6 ) The correlation between electron transport ( J ) and maximum Rubisco carboxylation rate ( V c,max ) of “Green Towers” lettuce estimated from A/C i curves under PPFD (1000 μmol m –2 s –1 ) under nine light spectra ( p < 0.001).

Supplementary Figure 4

(Related to Figure 6 ) The comparison between QY inc before (A) and after (B) correcting for light-suppression of respiration under blue, green, and red LED light. Note that the initial increase in QY inc became more pronounced after correction of light suppressed respiration.

Supplementary Figure 5

The comparison between QY abs before (A) and after (B) correcting for alternative electron sinks under blue, green, and red LED light. Assuming a simplified electron sink that diverts energy of 15 μmol m –2 s –1 of absorbed photons (an arbitrary value used for illustrative purposes only) away from the Calvin cycle under all PPFD s, the corrected QY abs was calculated based on remaining photons available to support Calvin cycle processes (B) . Note that the pattern of QY inc after correcting of alternative electron sink (B) is similar to quantum yield of PSII measured by chlorophyll fluorescence by Weaver and van Iersel (2019) .

Supplementary Table 1

Dark respiration rate (R d ), maximum quantum yield of CO 2 assimilation (QY m,inc ) and maximum gross assimilation rate (A g,max ) of “Green towers” lettuce derived from the light response curves for nine different spectra using Eq. 1. The light response curves are shown in Figure 3 . *See light composition of nine lights presented here in Table 1 .

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Biology archive

Course: biology archive   >   unit 11.

• Conceptual overview of light dependent reactions
• Light dependent reactions actors
• Photosynthesis: Overview of the light-dependent reactions

Light and photosynthetic pigments

• The light-dependent reactions

Introduction

What is light energy, pigments absorb light used in photosynthesis, chlorophylls, carotenoids, what does it mean for a pigment to absorb light, attribution:.

• “ The light-dependent reactions of photosynthesis ,” by OpenStax College ( CC BY 3.0 ). Download the original article for free at http://cnx.org/contents/f829b3bd-472d-4885-a0a4-6fea3252e2b2@11 .
• " Bis2A 06.3 Photophosphorylation: the light reactions of photosynthesis ," by Mitch Singer ( CC BY 4.0 ). Download the original article for free at http://cnx.org/contents/c8fa5bf4-1af7-4591-8d76-711d0c1f05f9@2 .

Works cited:

• Chlorophyll a. (2015, October 11). Retrieved October 22, 2015 from Wikipedia: https://en.wikipedia.org/wiki/Chlorophyll_a .
• Speer, B.R., (1997, July 9) Photosynthetic pigments. In UCMP glossary . Retrieved from http://www.ucmp.berkeley.edu/glossary/gloss3/pigments.html .
• Bullerjahn, G. S. and A. F. Post. (1993). The prochlorophytes: are they more than just chlorophyll a/b-containing cyanobacteria? Crit. Rev. Microbiol. 19(1), 43. http://dx.doi.org/10.3109/10408419309113522 .
• Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). Photosynthesis. In Campbell biology (10th ed.). San Francisco, CA: Pearson, 193.

Additional references:

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Practical Biology

A collection of experiments that demonstrate biological concepts and processes.

Observing earthworm locomotion

Practical Work for Learning

Published experiments

Investigating factors affecting the rate of photosynthesis, class practical.

In this experiment the rate of photosynthesis is measured by counting the number of bubbles rising from the cut end of a piece of Elodea or Cabomba .

Lesson organisation

The work could be carried out individually or in groups of up to 3 students (counter, timekeeper and scribe).

Apparatus and Chemicals

Students may choose to use:.

Thermometer, –10 °C –110°C

Coloured filters or light bulbs

Push-button counter

Potassium hydrogencarbonate powder or solution (Hazcard 95C describes this as low hazard)

For each group of students:

Student sheets, 1 per student

Beaker, 600 cm 3 , 1

Metre ruler, 1

Elodea ( Note 1 ) or other oxygenating pond plant ( Note 2 )

Electric lamp

Clamp stand with boss and clamp

Health & Safety and Technical notes

Normal laboratory safety procedures should be followed. There is a slight risk of infection from pond water, so take sensible hygiene precautions, cover cuts and wash hands thoroughly after the work is complete.

Read our standard health & safety guidance

1 Elodea can be stored in a fish tank on a windowsill, in the laboratory or prep room. However it is probably a good idea to replace it every so often with a fresh supply from an aquarist centre or a pond. (It’s worth finding out if any colleague has a pond.) On the day of the experiment, cut 10 cm lengths of Elodea , put a paper-clip on one end to weigh them down and place in a boiling tube of water in a boiling tube rack, near a high intensity lamp, such as a halogen lamp or a fluorescent striplight. Check the Elodea to see if it is bubbling. Sometimes cutting 2–3 mm off the end of the Elodea will induce bubbling from the cut end or change the size of the bubbles being produced.

2 Cabomba (available from pet shops or suppliers of aquaria – used as an oxygenator in tropical fish tanks) can be used as an alternative to Elodea , and some people find it produces more bubbles. It does, though tend to break apart very easily, and fish may eat it very quickly.

3 If possible, provide cardboard to allow students to shield their experiment from other lights in the room.

Ethical issues

Look out for small aquatic invertebrates attached to the pond weed used, and remove them to a pond or aquarium.

Explore How Light Affects Photosynthesis

Algae are aquatic, plant-like organisms that can be found in oceans, lakes, ponds, rivers, and even in snow. But don’t worry, if you’re not near a waterway, it can easily be ordered from Amazon or Carolina Biological. Algae range from single-celled phytoplankton (microalgae) to large seaweeds (macroalgae). Phytoplanktons can be found drifting in water and are usually single-celled. They can also grow in colonies (group of single-cells) that are large enough to see with the naked eye. The specific types of algae that can be used in this experiment are  Scenedesmus, Chlamydomonas, or  Chlorella , all of which are phytoplanktons or microalgae. 

Experimental variables

• Color filter paper
• Table/desk lamp
• Light bulbs (varying intensities and colors)

Laboratory Supplies

• Transfer pipettes
• Vials with caps
• Freshwater Algae ( Scenedesmus , Chlorella , or Chlamydomonas )
• Small beakers or cups

Laboratory Solutions

• 2% Calcium Chloride
• 2% Sodium alginate
• Cresol red/thymol blue pH indicator solution

Solution Preparations

2% calcium chloride (cacl 2 ).

• 20 g of CaCl 2
• Fill to 1000 mL with water

2% CaCl 2 is stable at room temperature indefinitely.

2% Sodium alginate (prepared in advance)

• 2 g sodium alginate
• Fill to 100 mL with water

It takes a while for the alginate to go into solution. We recommend to dissolve by stirring using a magnetic stir bar overnight at room temperature. Store at 4 °C for up to 6 months or use immediately.

Cresol red/Thymol blue pH indicator solution (10x)

• 0.1 g cresol red
• 0.2 g thymol blue
• 0.85 g sodium bicarbonate (NaHCO 3 )
• 20 mL ethanol
• Fill to 1L with fresh boiled water

Measure indicators and mix with ethanol. Measure sodium bicarbonate and mix with warm/hot water. Mix the solutions together and fill with remaining freshly boiled water up to 1L final solution. The 10x stock solution is stable for at least a year.

In preparation for doing the experiment, prepare 1x indicator solution by diluting the 10x indicator solution with distilled water (e.g. 20 ml 10x into 200 mL final solution).

Experimental Bench Set-Up

• ~10 mL of 2% CaCl 2 in a cup or beaker
• ~3-5 mL of sodium alginate in cup or beaker
• Cup with ~10 mL of water
• Empty cup or beaker that holds a minimum of 30 mL

Preparing Algae for Experiment

• Prepare a concentrated suspension of algae. Without centrifuge : leave ~50 mL of algae suspension to settle (preferably overnight), then carefully pour off the supernatant to leave ~3-5 mL of concentrated algae. With centrifuge : Centrifuge ~50 mL of algae suspension at low speed for 10 minutes and then carefully pour off the supernatant, leaving behind ~3-5 mL of concentrated algae.
• In a small beaker, add equal volumes of sodium alginate and then add in the concentrated algae. Gently mix algae and sodium alginate together using a transfer pipette until its evenly distributed.
• Using the transfer pipette, carefully add single drops of the algae/sodium alginate mixture into the CaCl 2 to make little “algae balls”
• Once all of the “algae balls” are in the CaCl 2 solution, allow them to harden for 5 minutes
• Place the strainer over the empty cup or beaker, and pour over the entire solution of “algae balls” and CaCl 2 into the strainer allowing the CaCl 2 to pass through, leaving just the algae in the strainer
• Keeping the strainer over the container, pour the water over the “algae balls” to rinse the remain CaCl 2
• Transfer your newly made “algae balls” to a new cup or beaker

Setting up Photosynthesis Experiment

• Distance from light (using ruler) – group can set up vials different distances from one light source
• Different color lights (using color filter paper or different color light bulbs) – group can set up by covering the vials with different colored films and arrange them the same distance away from the light source or set up 1 vial in front of a different colored lamp same distance away.
• With or without light – group places 1 vial in front of an illuminated lamp and another has the vial or lamp covered with black paper the same distance away

• When starting your experiment, be sure to take note of the time that you placed your vial in front of the light source. Vials should be left for ~1-2 hours.

What would happen if the algae photosynthesizes (increase O2) in a solution that started at pH8.2?

Analyzing photosynthesis results

• After 1-2 hours, return to the experiment. Without disturbing the vials, analyze and take pictures of results. Have students write down the time that their experiment ended.
• Using the color chart above, determine which pH matches your sample the closest.
• Have students determine if they got what they expected and discuss amongst their group members.

Explain how the rate of photosynthesis is affected by their different variables.

What were your conclusions from this experiment? If you were to repeat the experiment, what would you change and why? What’s the relationship with O2 and CO2 during the process of photosynthesis? Is there a “best” source of light that allowed the algae to photosynthesize better?

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11.2 Light and Photosynthesis

Learning objectives

• Understand the meaning of photoautotroph in reference to plants.
• Explain how the energy from light is converted into carbon-based chemical energy and building blocks in plants.
• Identify where in the plant the various photosynthetic reactions take place.
• Explain how the carbon-based building blocks move to other parts of the plant and are used for energy, storage, and structures.

Photoautotrophs

Plants are autotrophs, meaning that they are self-nourishing (Greek autos = self and trophe = nutrition). Specifically, plants are photoautotrophs , because they use the energy from light to produce organic molecules with which they build their cells and store energy.

Organic molecules are compounds associated with living organisms that contain carbon atoms. It was once thought that organic molecules could only be synthesized in nature by living organisms through the intervention of a “life force.” This hypothesis was disproved in 1828 when urea, a simple organic compound, was synthesized in a laboratory. Since that time, a major branch of chemistry, organic chemistry, has arisen to study and synthesize organic molecules. In contrast to organic compounds, inorganic compounds were historically defined as those lifeless minerals that are dug up from the ground.

Note that this chemical definition of organic (containing carbon atoms) has little or no relationship to the contemporary use of the word to describe a method of producing food. Organic food production, by regulation, relies strictly on inputs of organic molecules that come from life (like manure) and also on inorganic compounds like minerals, and eschews the use of organic molecules that have been synthesized by humans.

The organic molecules that a plant produces must be:

• Storable within the plant.
• Metabolized by the plant to yield energy for use in growth, maintenance, and producing other required organic molecules.
• Reasonably compact so that enough energy can be stored for growth.
• Transportable within the plant.
• Stable and non-toxic to the plant.

Since plants are photoautotrophs, they must have a mechanism for capturing energy from the sun or other sources of light and using that energy to produce organic molecules with the characteristics noted above. Photosynthesis is the process on which photoautotrophs rely to capture that light energy and to produce carbon-based organic molecules. The carbon used to make these molecules comes from the carbon dioxide (CO 2 ) in the atmosphere. Because photosynthesis removes carbon from the atmosphere and incorporates it into organic molecules which eventually become the plant’s leaves, stems, roots, and fruits, photosynthesis is sometimes said to fix carbon. Fix , in this sense, means to secure or sequester rather than to repair.

If you follow the public discourse on climate change, you are aware that global warming is accelerated by the accumulation of greenhouse gasses in the atmosphere which trap and re-radiate sunlight and heat back to the earth. CO 2 is one of these greenhouse gasses. Removal of CO 2 from the atmosphere, for instance by planting trees that photosynthesize, fix carbon, and store the carbon-rich product as wood, is one method of carbon sequestration. An emerging and increasingly popular strategy for remediating greenhouse gas emissions is through the buying and selling of carbon credits, where industries that discharge CO 2 into the atmosphere purchase credits from organizations whose activities (such as tree planting) sequester carbon. Photosynthesis and sequestration of carbon by trees is one tool used to offset the industrial release of CO 2 .

Review questions

• In what sense does photosynthesis fix carbon?
• Where does the carbon come from that is used by photosynthesis, and where does it go within the plant?

Light reaction

Let’s start with light, because that’s where the plant gets the energy for photosynthesis. Here are some characteristics of light:

• Light travels in waves.

• Within the visible wavelengths of light, the longest wavelengths are red light; outside the visible range of wavelengths, even longer wavelengths include infrared radiation, microwaves, and radio waves.
• Shorter visible wavelengths include blue and purple light, and beyond the visible range even shorter wavelengths include UV light, X-rays, and Gamma rays
• Light also has a particulate nature, and those particles are called photons .

The photons in light provide the energy that drives photosynthesis. This energy is used to incorporate carbon found in CO 2 from the atmosphere into organic molecules and, in particular, into simple sugars used by the plant. The chemical formula is the same for the two types of simple sugars produced by photosynthesis: glucose and fructose :  C 6 H 12 O 6 . The equation that summarizes photosynthesis is:

water + carbon dioxide -> oxygen, water, and simple sugars

12H 2 0 + 6CO 2 -> 6O 2 + 6H 2 O + C 6 H 12 O 6

This balanced equation tells us that 12 molecules of water plus 6 molecules of carbon dioxide, in the presence of chlorophyll , accessory pigments , and light, produces 6 molecules of oxygen gas, returns 6 molecules of water back to the cell, and produces one molecule of a simple sugar like glucose or fructose.

Two reactions make up photosynthesis: the Light Reaction (abbreviated LR) and the Light Independent Reaction (abbreviated LIR). As the names suggest, the LR requires light while the LIR does not. The LR uses light energy to split water, which transforms the energy from the sun into hydrogen ions and electrons. The LIR uses that energy to grab the carbon from carbon dioxide and use the carbon to build simple sugars.

Let’s start with the light reaction. You’ve heard of chlorophyll, and may recognize this molecule as a green pigment that captures light for photosynthesis. There are two chlorophyll pigments in plants that are critical for absorbing light: Chlorophyll a and Chlorophyll b .

The graph above shows % absorbance of different wavelengths by these two chlorophylls. The Y axis (the vertical one) shows the percentage of the light that is absorbed (rather than reflected). High levels of absorption mean that the chlorophyll molecule uses that wavelength of light for energy. Low absorption means that the molecule does not use that wavelength, and is thus reflected away. The X axis indicates the wavelength of light in nanometers (nm) (the wavelength of green light, roughly 500 nm). The bar at the top represents the color of the light at the wavelength shown. The blue line is a typical absorption curve for chlorophyll a, while the green line shows chlorophyll b .

High absorbance at a particular wavelength means that pigment is collecting that light at that wavelength to harvest energy. Low absorbance means that the plant is reflecting that light back. Both chlorophyll a and b absorb blue and red light wavelengths and reflect green. Chlorophyll a has a peak in the violet and red regions and chlorophyll b in the blue and orange regions. Notice how their absorbance is very low in the green region. That’s why we think of chlorophyll as green, and why we perceive leaves, which have chlorophyll as the predominant pigment, as green. Also notice that chlorophyll reflects some yellow wavelengths, but when the yellow and deep green wavelengths are mixed, we see the green leaf color.

The graph above shows the absorbance of carotenoid pigments , which are present throughout the growing season. Carotenoids are called an accessory pigment in photosynthesis. They assist chlorophyll in light capture and energy transfer, and contribute to the regulation and moderation of excessive excitation of pigment molecules during intense sunlight, including exposure to UV light. Carotenoids absorb light in the green range, but reflect in yellow and red. We don’t see these pigments during the growing season because they are much lower in concentration than the chlorophylls, so the green reflected light overwhelms the orange, and we see green. But when the chlorophyll fades in the fall, due to decomposition of chlorophyll, the orange can be seen in beautiful fall leaf colors.

Chlorophyll a and b , as well as the accessory pigments, are found in the chloroplasts , which are membrane-bound organelles within cells. The highest concentration of chloroplasts is most commonly found in the palisade mesophyll cells of the leaf.

The above illustration of a chloroplast labels the internal structures. The chloroplast has a double membrane. The interior of the chloroplast is called the stroma . Within the stroma are coin-like thylakoids . The stacks of thylakoids are called grana . The thylakoids are also surrounded by a membrane, called the thylakoid membrane . The green chlorophyll pigment that you associate with photosynthesis, as well as the accessory pigments, are embedded in the thylakoid membranes and arranged in a structure called the antenna complex — given this name because it captures and routes the energy from sunlight to a collector called a reaction center .

As shown above, when light hits a pigment molecule in the antenna complex, the energy from the light photon promotes (pushes up) an electron in one of the pigment’s atoms to a higher orbital as seen in the cartoon and energy is gained.

The electron can’t stay in that higher orbital indefinitely, and when it drops back to its home orbital it releases the energy it absorbed from light, denoted as energy loss . This released energy can be passed to another pigment molecule. This process of one pigment capturing the photon’s energy and passing that energy onto adjacent pigment molecules is the crucial step in energy transformation that takes place in photosynthesis. This is the step that takes light energy and converts it into chemical energy — one of the only known biological processes that allows this type of energy transformation.

A light photon excites an electron of one pigment molecule in the antenna complex, or light harvesting complex, and by resonance this energy is transferred from pigment molecule to pigment molecule The energy transfer makes its way to the reaction center, where the first major chemical reaction in photosynthesis — splitting water — takes place. This reaction is called the light reaction or light-dependent reaction because it requires light. Water is split when the reaction center grabs electrons from water, which separates water into oxygen gas (O 2 ), hydrogen ions (H + ), and electrons (e – ).

To reiterate, the light is captured by the light harvesting complex (antenna complex) where electrons in the chlorophyll atoms are excited and jump up to a higher orbital. When the electron drops back, the energy is transferred to an adjacent pigment atom. This resonance energy travels down the antenna complex to the reaction center, where the captured energy pulls electrons out of water molecules, and water is split into oxygen gas, hydrogen ions, and electrons. The energy that was present in the photons of light has been transferred to the hydrogen ions and the electrons. We’ll see more of how that energy is used in the next section.

• What wavelength(s) of light does chlorophyll a absorb? Chlorophyll b? What wavelengths do these two molecules reflect?
• What pigments make up the antenna complex?
• How is the energy in light transformed in the Light Reaction?

Recall that the overall equation for photosynthesis is:

This equation is made up of two parts called half-reactions . The first half-reaction is an equation summarizing the Light Reaction, where energy from sunlight is used to split water molecules into oxygen gas, some electrons, and some hydrogen ions. The energy from sunlight is transferred from the pigments to these hydrogen ions and electrons. The half-reaction for the Light Reaction is as follows:

12H 2 O -> 6O 2 + 24e – + 24H +

Light independent reaction

The Light-Independent Reaction (LIR) is the second part of photosynthesis. It takes place in the stroma of the chloroplast. Unlike the Light Reaction, it does not require light. In the LIR, two compounds, NADPH and ATP , carry the energy from light that was originally transformed into hydrogen ions and electrons through the splitting of water. The NADPH and ATP, along with carbon dioxide from the atmosphere, enter a process called the Calvin Cycle , where the energy is used to fix carbon into a molecule abbreviated G3P. This process requires the help of an important protein abbreviated RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) that catalyzes the step in the process where the carbon from atmospheric CO 2 is incorporated into an organic molecule. RuBisCO is the most abundant protein in leaves and, given the number of leaves in the world, likely the most abundant protein on the planet. The G3P produced by the carbon fixation process is called a triose phosphate , meaning it is a 3-carbon sugar (triose) with phosphorus and oxygen atoms (phosphate) attached. Triose phosphate moves out of the chloroplast into the mesophyll cell’s cytoplasm, where two of these three-carbon molecules are combined to produce the 6-carbon molecules glucose and fructose. The glucose and fructose molecules then combine to form sucrose , a 12-carbon organic molecule. Sucrose is important because it is the sugar that is transported by the phloem throughout the plant to provide energy and building blocks for other organic molecules like starch and cellulose .

The half-reaction for the LIR is:

24H + + 24e – + 6CO 2 -> C 6 H 12 O 6 + 6H 2 O

• Does the Light Independent Reaction require darkness?
• What sugar is moved throughout the plant through the phloem?

Photosynthesis summary

When we add the two half-reaction equations for LR and LIR together, we get back to the summary equation for photosynthesis:

12H 2 O + 6CO 2 -> 6O 2 + 6H 2 O + C 6 H 12 O 6

The illustration above is a summary of what happens in a mesophyll cell. The rectangular blue outline represents a palisade mesophyll cell in a leaf. Inside the cell is a green rectangle, representing a chloroplast. Inside the chloroplast is a stack of green ovals with black dots. These ovals are the thylakoids, and the stacks are grana. The black dots in the green thylakoid membrane represent the antenna complexes. Light hits the antenna complex and transfers its energy to pigments, and the energy is funneled to the reaction center where water (H 2 O) is split in the light reaction to form the energy carriers ATP and NADPH. This is the Light Reaction. The waste product formed at this stage is oxygen, which might be waste for the plant, but is quite useful for us.

In the Light Independent Reaction the energy is carried to the Calvin Cycle, represented by the multi-pointed star in the chloroplast, which uses the energy in ATP, the NADPH, and CO 2 from the atmosphere to form the three-carbon G3P triose phosphate with the help of RuBisCO. Triose phosphate leaves the chloroplast and passes into the cytoplasm of the mesophyll cell to be transformed into glucose and fructose, which are combined into sucrose that is exported from the mesophyll cell to the phloem.

Cellulose and starch

Within a plant, the regions of photosynthesis and sugar production are called the source . Leaves are typically the main source within the plant, since that is where most photosynthesis takes place. Those regions that do not support photosynthesis (like roots), but that still need organic molecules to survive, are called sinks . Movement of solutes (molecules dissolved in water) like sucrose from source to sink through the phloem is called translocation. Translocation of sucrose through the phloem to the sink provides cells with a source of stored energy, and also building blocks for organic molecules, as noted earlier. Sucrose can be broken down to glucose and fructose, building blocks used to form other extremely useful organic compounds. Two particularly useful compounds result from the production of long glucose chains: starch, a key energy storage compound in plant cells, and cellulose, the main constituent of the cell wall and key to a plant’s structural integrity. Wood, for instance, is primarily made up of the cellulose-rich cell walls of dead xylem. Both starch and cellulose are long chains of glucose, but they differ in the way the glucose molecules are linked together.

Cellulose is the molecule into which carbon extracted from atmospheric CO 2 is sequestered for long-term storage. Starch sequesters carbon for a much shorter period of time because it is either eaten, used by the plant for new growth, or decomposed by bacteria and fungi that can utilize starch for energy.

Review question

• Define translocation — what molecules are being transported?
• In the light reaction, pigments in the thylakoid membrane capture energy from sunlight.
• The energy is used to split water, which releases oxygen to the atmosphere.
• The energy used to split water is transferred into electrons and hydrogen atoms, and eventually to ATP and NADPH.
• In the light independent reaction, the ATP and NADPH power the Calvin cycle that captures carbon from atmospheric CO 2 and incorporates it into simple sugar molecules.
• These simple sugars can be translocated to sinks, where they are used for energy, converted into energy storage compounds, or converted into structural molecules.

Name given to living things, namely plants, that use energy from light to produce organic molecules with which they build their cells and store energy; they are self-nourishing.

Monosaccharides; examples include glucose and fructose.

A simple sugar; it can be produced via photosynthesis.

Simple sugar; it can be produced via photosynthesis.

Green photosynthetic pigment found in plants, algae, and cyanobacteria that captures light for photosynthesis.

Light-absorbing pigments, other than chlorophyll, that are found in chloroplasts.

First half-reaction in photosynthesis and occurs with the presence of light and uses light energy to split water, which transforms the energy from the sun into hydrogen ions and electrons; abbreviated LR.

Second half-reaction in photosynthesis and occurs without the presence of light and uses the energy produced in the Light Reaction to grab the carbon from carbon dioxide and use the carbon to build simple sugars; abbreviated LIR.

Type of chlorophyll; it mainly absorbs violet and red light while reflecting green light.

Type of chlorophyll; it mainly absorbs blue and orange light while reflecting green light.

Process in which light is absorbed and converted to energy.

Length of the wave from one peak to the next; it is measured in nanometers.

Yellow and orange pigments that are present in the leaf all growing season, but during the warm part of the season these colors are hidden by the high concentration of green-colored chlorophyll. They take longer to break down than chlorophyll.

An organelle that contains chlorophyll where light energy is captured and where the first steps are taken in the chemical pathway that converts the energy in light into forms of energy that the plant can transport and store, like sugar and starch.

The densely packed, columnar-shaped, elongated cells full of chloroplasts. It is analogous to cortex parenchyma cells in the stem, but in the leaf are specialized for light energy capture.

Interior of the chloroplast; it is the site of the LIR

Membrane-bound compartments inside chloroplasts and cyanobacteria, and are the site of the light-dependent reactions of photosynthesis.

Stacks of thylakoids.

Membrane that surrounds the thylakoid.

Structure of chlorophyll and accessory pigments that are embedded in the thylakoid membranes; it captures and routes energy from sunlight to a collector called a reaction center.

Complex of pigments, proteins, and other factors that execute the primary energy conversion reactions of photosynthesis, primarily where water is split in the LR to form the energy carriers ATP & NADPH.

Energy that is passed from one molecule to the next.

Energy created in the LR and is used to drive the LIR.

A principle molecule for storing and transferring energy in cells; it is created in the LR.

A 3-carbon sugar (triose) with phosphorus and oxygen atoms (phosphate); G3P is an example.

Sugar that is transported by the phloem throughout the plant to provide energy and building blocks for other organic molecules like starch and cellulose.

Key energy storage compound in plant cells; it is a long glucose chain; it sequesters atmospheric carbon for short-term use.

Main constituent of the cell wall and is key to a plant's structural integrity; it is a long chain of glucose. It sequesters atmospheric CO2 for long-term storage.

The Science of Plants Copyright © 2022 by The Authors is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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) and water into sugar. Oxygen is a waste product. It doesn’t necessarily mean more though. When we think of photosynthesis as a process, we can see that there are at least three things that can limit the process: More light won’t help if we don’t have enough water and carbon dioxide. Actually, most places on Earth have the same amount of carbon dioxide in the atmosphere, but a plant can only get it by opening holes in its leaves. These holes are too small for you to see without a strong microscope, but they are big enough to let water vapor out of the plant. So water is an important limit on a plant. More light is actually a problem if water is scarce, because even more water will evaporate from the plant. This is an example of how increasing one factor (sunlight) can lead to another factor (water) being limiting. How can you look at a landscape and tell whether a lot of photosynthesis usually happens there? Thanks for asking,

So by level of light you probably mean light intensity which is something that can be measured. So in the case of a plant, a higher light intensity means more packets of light called “photons” are hitting the leaves. As you rise from low light intensity to higher light intensity, the rate of photosynthesis will increase because there is more light available to drive the reactions of photosynthesis. However, once the light intensity gets high enough, the rate won’t increase anymore because there will other factors that are limiting the rate of photosynthesis. A limiting factor could be the amount of chlorophyll molecules that are absorbing the light. At a very high intensity of light, the rate of photosynthesis would drop quickly as the light starts to damage the plant.

This is a very important aspect of photosynthesis. As you are probably aware, These sugars are then used by the plant as energy for any number of things. The process of photosynthesis requires three things: Light, Carbon dioxide and water. If any one of these things is in short supply, then photosynthesis cannot happen. When you increase the level of light, plants will photosynthesize more. But, if you have too much light, than the other 2 ingredients become limiting and photosynthesis can no longer increase with the level of light. When this occurs, leaves can experience sunburn damage. If you've ever seen a leaf with large dry brown sections on a living leaf, it is because that leaf experienced sunburn. With too little light, photosynthesis cannot occur either and the plant suffers without the production of sugars. There are many complicating interactions between plants and light. I hope that you continue to investigate this as the story gets more interesting and exciting the deeper you go. Cheers,

Photosynthesis needs light, but it also needs other things, and too much light can create heat and dryness that are bad for photosynthesis. For this reason plants in different environments have different structures to help them get the right amount of light.

I am not sure what you mean by "level" of light, but I will answer your question in to ways - in terms of the intensity of light and wavelength of light. Photosynthesis needs water, carbon dioxide, chlorophyll, light, and the right temperature. Light is an extremely important factor for the process. If there is enough water, carbon dioxide, and the temperature is right, light becomes the factor which will affect photosynthesis. Most of the time, But, this rate has a limit, and once that limit is hit you can't increase the rate past that limit. Chlorophyll is a green pigment in the chloroplast of the plant cell which absorbs the light. This mean it will absorb any wavelength of light which is not in the green spectrum of light. If you look at a spectrum from 400nm-700nm. The amount of light absorbed will increase until it reaches a peak at about 450nm ​(blue light). Then it will start decreasing and be very low (almost 0) through the 500-550nm (green light) and then it will increase again peaking at about 700nm (red and yellow light). to return to the search form. Turn That Light Up: Examining the Effect of Light Intensity on Photosynthesis as Measured by Oxygen Production in Elodea canadensis Minkyong Song Christina Thompson Photosynthesis is an essential reaction occurring in all plants as it provides their source of energy. As light is one of the required reactants, we chose to investigate the effects of differing light intensities on the rate of photosynthesis, hypothesizing that greater light intensity will result in greater photosynthetic rate. The rate was measured by determining the change in oxygen concentration in the medium by the aquatic plant, Elodea canadensis . The plant was immersed in the medium and subjected to different light intensities over three independent trials. Although we were unable to find any statistical differences between change in oxygen concentration and the light intensity, a slight trend suggested light intensity did have an effect on the rate of photosynthesis. The effect of light intensity followed a Michealis-Menten curve where increases in light intensity corresponded to increases in oxygen production until a peak in oxygen production occurred at a light intensity of 6000 lux. From this, we suspect that E. canadensis has an optimal photosynthetic rate at light intensities of 6000 lux. At greater intensities oxygen inhibition begins to occur due to the ability of the enzyme ribulose phosphatase to act in both photosynthesis and cellular respiration. However, as our results were not statistically significant, further studies would have to be performed to determine the effect of light intensity on photosynthesis in E. canadensis . Developed By Français (Canada) Information For Readers For Authors For Librarians Investigating the Rate of Photosynthesis ( AQA A Level Biology ) Revision note. Biology & Environmental Systems and Societies Apparatus & Techniques: Investigating the Rate of Photosynthesis Investigations to determine the effects of light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis can be carried out using aquatic plants , such as Elodea or Cabomba (types of pondweed ) Light intensity – change the distance ( d ) of a light source from the plant (light intensity is proportional to 1/ d 2 ) Carbon dioxide concentration – add different quantities of sodium hydrogencarbonate (NaHCO 3 ) to the water surrounding the plant, this dissolves to produce CO 2 Temperature (of the solution surrounding the plant) – place the boiling tube containing the submerged plant in water baths of different temperatures For example, when investigating the effect of light intensity on the rate of photosynthesis, a glass tank should be placed in between the lamp and the boiling tube containing the pondweed to absorb heat from the lamp – this prevents the solution surrounding the plant from changing temperature Distilled water Aquatic plant, algae or algal beads Sodium hydrogen carbonate solution Thermometer Test tube plug This will ensure oxygen gas given off by the plant during the investigation form bubbles and do not dissolve in the water This will ensure that the plant contains all the enzymes required for photosynthesis and that any changes of rate are due to the independent variable Ensure the pondweed is submerged in sodium hydrogen carbonate solution (1%) – this ensures the pondweed has a controlled supply of carbon dioxide (a reactant in photosynthesis) Cut the stem of the pondweed cleanly just before placing into the boiling tube Measure the volume of gas collected in the gas-syringe in a set period of time (eg. 5 minutes) Change the independent variable (ie. change the light intensity, carbon dioxide concentration or temperature depending on which limiting factor you are investigating) and repeat step 5 Record the results in a table and plot a graph of volume of oxygen produced per minute against the distance from the lamp (if investigating light intensity), carbon dioxide concentration, or temperature The effect of light intensity on an aquatic plant is measured by the volume of oxygen produced Results - Light Intensity The closer the lamp, the higher the light intensity (intensity ∝ 1/ d 2 ) Therefore, the volume of oxygen produced should increase as the light intensity is increased This is when the light stops being the limiting factor and the temperature or concentration of carbon dioxide is limiting the rate of photosynthesis The effect of these variables could then be measured by increasing the temperature of water (by using a water bath) or increasing the concentration of sodium hydrogen carbonate respectively Rate of photosynthesis = volume of oxygen produced ÷ time elapsed Limitations Immobilised algae beads are beads of jelly with a known surface area and volume that contain algae, therefore it is easier to ensure a standard quantity Immobilised algae beads are easy and cheap to grow, they are also easy to keep alive for several weeks and can be reused in different experiments The method is the same for algae beads though it is important to ensure sufficient light coverage for all beads Light intensity – the distance of the light source from the plant (intensity ∝ 1/ d 2 ) Temperature - changing the temperature of the water bath the test tube sits in Carbon dioxide - the amount of NaHCO 3 dissolved in the water the pondweed is in Also remember that the variables not being tested (the control variables) must be kept constant. Required Practical: Affecting the Rate of Dehydrogenase Activity The light-dependent reactions of photosynthesis take place in the thylakoid membrane and involve the release of high-energy electrons from chlorophyll a molecules These electrons are picked up by the electron acceptor NADP in a reaction catalysed by dehydrogenase However, if a redox indicator (such as DCPIP or methylene blue ) is present, the indicator takes up the electrons instead of NADP DCPIP: oxidised ( blue ) → accepts electrons → reduced ( colourless ) Methylene blue: oxidised ( blue ) → accepts electrons → reduced ( colourless ) The colour of the reduced solution may appear green because chlorophyll produces a green colour When light is at a higher intensity, or at more preferable light wavelengths, the rate of photoactivation of electrons is faster, therefore the rate of reduction of the indicator is faster Light activates electrons from chlorophyll molecules during the light-dependent reaction. Redox indicators accept the excited electrons from the photosystem, becoming reduced and therefore changing colour. Isolation medium Pestel and mortar Aluminium Foil Method - Measuring light as a limiting factor This produces a concentrated leaf extract that contains a suspension of intact and functional chloroplasts The medium must have the same water potential as the leaf cells so the chloroplasts don’t shrivel or burst and contain a buffer to keep the pH constant The medium should also be ice-cold (to avoid damaging the chloroplasts and to maintain membrane structure) The room should be at an adequate temperate for photosynthesis and maintained throughout, as should carbon dioxide concentration If different intensities of light are used, they must all be of the same wavelength (same colour of light) - light intensity is altered by changing the distance between the lamp and the test tube If different wavelengths of light are used, they must all be of the same light intensity - the lamp should be the same distance in all experiments DCPIP of methylene blue indicator is added to each tube, as well as a small volume of the leaf extract A control that is not exposed to light (wrapped in aluminium foil) should also be set up to ensure the affect on colour is due to the light This is a measure of the rate of photosynthesis A graph should be plotted of absorbance against time for each distance from the light This is because the lowered light intensity will slow the rate of photoionisation of the chlorophyll pigment, so the overall rate of the light dependent reaction will be slower This means that less electrons are released by the chlorophyll, hence the DCPIP accepts less electrons. This means that it will take longer to turn from blue to colourless A higher rate of decrease, shown by a steep gradient on the graph, indicates that the dehydrogenase is highly active. This experiment is not measuring the rate of dehydrogenase activity directly (through measuring the rate of substrate use or product made) but is instead predicting what the rate would be by measuring the rate of electron transfer from the photosystems It is therefore important to control the amount of leaf used to produce the chloroplast sample and also how much time is spent crushing the leaf to release the chloroplast It is also a good idea to measure a specific wavelength absorption by each sample on the colorimeter before and after the experiment so you can get a more accurate change in oxidised DCPIP concentration Results should also be repeated and the mean value calculated The time taken to go colourless is subjective to each person observing and therefore one person should be assigned the task of deciding when this is In chemistry the acronym ‘OILRIG’ is used to remember if something is being oxidised or reduced. Oxidation Is Loss (of electrons) and Reduction Is Gain (of electrons). Therefore the oxidised state is when it hasn’t accepted electrons and the reduced state has accepted electrons. You've read 0 of your 10 free revision notes Get unlimited access. to absolutely everything: Downloadable PDFs Unlimited Revision Notes Topic Questions Past Papers Model Answers Videos (Maths and Science) Join the 100,000 + Students that ❤️ Save My Exams the (exam) results speak for themselves: Did this page help you? Author: Alistair Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University. Science & Math Sociology & Philosophy Law & Politics Lab Answers: How Does Light Intensity Affect the Rate of Photosynthesis Lab Answers: How Does Light… Photosynthesis represented by the equation 6H 2 O + 6CO 2 → C 6 H 12 O 6 + 6O 2 is one of the most crucial chemical reactions on earth as it produces O 2 molecules– a gas vital to the survival of many organisms (Vidyasagar, 2018) This process takes place during the light dependent phase of photosynthesis and is indicated by the production of bubbles and can be used to analyse the rate of photosynthesis (Benckiser, 2016).  Light intensity refers to the strength or amount of light produced and is the measure of the wavelength-weighted power emitted by a light source (Maximum Yield, 2016). The rate of photosynthesis is a function of light intensity. The aquatic plant elodea, also known as anacharis, provides a habitat for small aquatic animals and is used frequently by fish to protect recently hatched fish (Aquatic Biologists, 2007). Elodea is very common throughout the world, particularly Oceania, and has a rapid growth rate (Asta, n.d.) It also is an excellent oxygen producer. The rate of photosynthesis in Elodea depends on the intensity of light. With increasing, water pollution levels light that reaches aquatic plants such as Elodea significantly decreases thus leading to lesser light intensity underwater (Denchak, 2018). Light is an important factor in photosynthesis, “how does a decreased light intensity impact photosynthetic rate of Elodea” is an important contemporary ecological question to be considered.  Research Question How does decreasing light intensity affect the rate of photosynthesis in an elodea plant under a fixed period of 2 minutes? Original Experiment  The research methodology used to study the above question was adapted from: BBC Bitesize- Investigating the rate of photosynthesis, https://www.bbc.com/bitesize/guides/zpwmxnb/revision/4 The original experiment shined a light onto a large beaker filled with tap water with an elodea plant placed in it. The experiment started with placing the light 10cm away from the beaker and increased the distance by 10cm until 50cm, to decrease the light intensity at the elodea plant. Modifications to the Methodology To ensure that sufficient and relevant data was collected the original experiment was modified to increase the number of trials, as the original experiment only had one trial. The reliability of the data collected was improved by modifying the original methodology (see refinements). To minimize error and increase reliability, all other variables were controlled as per the original experiment. Refined by: Three trials from each sample will be taken to ensure sufficient data is available statistical analysis. Leaves per elodea plant will be limited to 10 to ensure fairness in the data. A vacuum will be created using a flask and a test tube (Appendix 1) on top of the water to ensure bubbles produced directly moved upwards in the test tube increasing the accuracy and confidence in the data.  All elodea samples were randomly chosen from a fish tank to reduce the sample bias.   Safety and ethical considerations Table A – Potential risks and their solutions Risk Solution Use of fragile glassware Keep all glassware away from ledges and breakages were cleaned up immediately  Use of water and electrical appliances Wipe hands before handling electrical appliances Risk of allergy to elodea plants Use gloves when handling Elodea Processed data Data obtained was analysed using the following statistical methods to allow appropriate interpretation of the data of the data: The mean was chosen as the most appropriate measure of central tendency. Standard deviation was calculated as a measure of central tendency and used to calculate standard error. Standard error was chosen as a measure of uncertainty. A confidence interval was chosen as a measure of validity. Table B – Statistical Calculations for 10cm system Table C – Processed data table for the effect light intensity on rate of photosynthesis. Trial Number Calculations (Below represents distance from the light source) 10cm 20cm 30cm 40cm 50cm Trial1 (no. of bubbles produced) 18 15 10 6 3 Trial 2 (no. of bubbles produced) 19 13 10 7 2 Trial 3 (no. of bubbles produced) 18 14 9 5 2 Sample Size 3 3 3 3 3 Average number of bubbles produced 18 14 10 6 2 Standard Deviation 0.68 1 0.68 1 0.68 Standard Error (SE) 0.33 0.68 0.33 0.68 0.33 Confidence Interval 1.4 2.5 1.4 2.5 1.4 Interpretation : The data shows that the average number of bubbles produced are in between the ranges of 18±0.33, 14±0.68, 10±0.33, 6±0.68 and 2±0.33 respectively from 10cm to 50cm. The standard error has been used as a measure of the uncertainty associated with these averages (±SE). The low standard error suggests that the results collected are close to the population mean and indicates the reliability of the mean. The Standard deviation values obtained (0.68, 1, 0.68, 1 and 0.68 respectively) are close to zero indicating that the data is close to the expected mean of the dataset (John, 2009). The low standard error and standard deviation of the data may suggest higher precision during data collection and therefore the sample represents the accepted values (McHugh, n.d.). Analysis – – The standard deviation of 0.68 obtained from the data for 10cm, 30cm and 50cm systems show that data is closely clustered around the mean suggesting that the data is closer to the true mean. The standard deviation of 1 for 20cm and 40cm systems suggest that the data is more dispersed away from the mean relative to that of 10cm, 30cm and 50cm systems. This may be attributed to experimental error as well as natural variations in the samples such as size of the leaves (John, 2009). Interpretation –The graph of average number of bubbles produced vs distance (Figure A) demonstrates a linear pattern with a very strong negative correlation as demonstrated by the R 2 value of 0.9981. Figure A also shows that from 10cm to 50cm, the mean number of bubbles produced (y-variable) appears to decrease in exact decrements of 4. As light intensity decreases with the distance, these observations suggest that decreasing light intensity had a negative impact on the number of bubbles produced. According to Reckitt Benckiser (2016), as light intensity decreases the rate of photosynthesis also decreases in an almost linear pattern. As the rate of photosynthesis is indicated by the number of bubbles produced the data collected fits this literature. Analysis – As shown in figure A, the error bars are less spread out and they do not overlap at all. This suggests that results collected are all statistically significant and do not fall in the same range as each other indicating that there was a negative impact on rate of photosynthesis since the start of the experiment. According to Figure A, the error bars for 20cm and 50cm systems slightly larger (0.68) relative to that of 10cm, 30cm and 50cm systems (0.33) which may suggest the variability of the plotted data may be less precise then the other measurements. Analysis – The data indicates, with 95% confidence, that the sample means falls within the ranges of 18±1.4, 14±2.5, 10±1.4, 6±2.5, 2±1.4 from 10cm to 50cm respectively. Although very close, the error bars (confidence intervals) do not overlap until 30cm. However, as the overlaps in 30cm, 40cm and 50cm error bars are not extensive and therefore indicates that there is a statistical difference between all the means. Limitations of Evidence Standard error, error bars and confidence intervals are all examples of the uncertainty and limitations observed from analysis of the evidence. This can be explained by a lack of reliability and validity in the experimental process as well as the statistical sampling error.  The standard error (Table C) indicates that how far the sample mean of the data is likely from the true population mean. The natural variation within the population and the sample size cause this error. Moreover, the bubble counting error (not all bubbles in every sample were counted as some were trapped beneath leaves and some bubbles were too small to be observed by the naked eye), and the resultant errors associate with the average number of bubbles produced and statistical parameters based on that data increases the standard error. This, in conjunction with the high standard deviation (Table C) suggests that not all variables were fully controlled and indicates low precision in the measuring devices or high random biological variation in the samples.  The small sample size of this experiment is a major factor in determining the length of the confidence intervals (Figure 2). Consequently, the evidence is limited in its ability to be used to extrapolate the findings of the experiment to the population of elodea.  Sources of Error Most appropriate equipment to collect data were not used which may contribute to the data being inaccurate i.e. the ruler used to measure the distance between the beaker and lamp was imprecise (±0.5mm). Human and parallax errors contribute to imprecision in the experiment I.e. the distance had approximated to a certain extent when marking it on paper and when assessing distance from above (Appendix 1) and only bubbles visible to the naked eye were counted which may affect both accuracy and precision of the experiment. Elodea samples were not genetically screened. Therefore, random biological variation might exist within the sample. This could explain some of the remaining imprecision in the data. Diminishing carbon dioxide, temperature variation and the time elodea plant takes to acclimatise to changed intensity of light can potentially affect the rate of photosynthesis. The experiment was conducted in a standard classroom with opened doors and windows while students often walked past the experiment. These could have impacted the intensity of light for the experiment. Affecting validity The amount of CO 2 in water was not pre-determined. With photosynthesis continuing the supply of carbon dioxide is rapidly used up. Hence there is the possibility that the decreased number of bubbles is a function of this. Limiting leaves to 10 does not directly determine the number of elodea cells which may affect the rate of photosynthesis. Suggested improvements and extensions Suggested improvements . Reducing the random error in the experimental process would improve its reliability. In this experiment, the reliability of the data could be improved by increasing the number of trials more than once to decrease standard error due to ‘regression to the mean’ (Schnell, 2006). To maintain the supply of carbon dioxide a compound such as sodium hydrogen carbonate can be added to the water (Benckiser, 2016). Measure distance with a ruler equipped with a precise digital LED digital display to increase accuracy. Light sources of different intensities should be used as this would reduce random errors. Also, the accuracy of data could be improved by using a hemocytometer to directly quantify the number of Elodea cells. (Fuentes, n.d.) Conduct the experiment in a dark room with minimal exposure to outside factors such as open door and windows to ensure the only source of light is the lamp.  Suggested extensions  Redirect the experiment by using varying light intensities and determine the optimum light intensity for the maximum rate of photosynthesis. Extend the experiment by investigating the effect of light intensity of various species of Elodea or different plants all together. In conclusion, the evidence suggests that decreasing light intensity reduces the rate of photosynthesis in a 2-minute fixed growth period. Therefore, more pollution levels mean aquatic plants (such as Elodea) are not able to photosynthesis as well impacting the level of oxygen underwater. However, there are noted limitations in the experimental procedure such as small sample size and further statistical analysis would be required to support this conclusion.   Bibliography Aquatic Biologists. (2007, 09 24). Elodea (Canadian Water Weed) . Retrieved from Aquatic Biologists: https://www.aquaticbiologists.com/elodea-canadian-water-weed/ Asta, J. (n.d.). Rate of Growth of Elodea . Retrieved from eHow: https://www.ehow.com/info_10061234_rate-growth-elodea.html BBC Bitesize. (n.d.). Photosynthesis. Retrieved from BBC Bitesize: https://www.bbc.com/bitesize/guides/zpwmxnb/revision/4 Benckiser, R. (2016). Rate of photosynthesis: limiting factors. Rate of photosynthesis: limiting factors , 2. Denchak, M. (2018, 05 14). Water Pollution: Everything You Need to Know. Retrieved from Natural Resources Defense Council: https://www.nrdc.org/stories/water-pollution-everything-you-need-know Fuentes, M. (n.d.). Cell counting equipment essentials . Retrieved from hemocytometer.org: https://www.hemocytometer.org/starter-kit/ John, T. S. (Director). (2009). Interpreting the standard deviation [Motion Picture]. Lumen. (n.d.). Describing Variability . Retrieved from Lumen: https://courses.lumenlearning.com/boundless-statistics/chapter/describing-variability/ Maximum Yield. (2016, 04 15). Light Intensity . Retrieved from Maximum Yield: https://www.maximumyield.com/definition/2036/light-intensity McHugh, M. L. (n.d.). Standard error: meaning and interpretation . Retrieved from Biochemia Medica: https://www.biochemia-medica.com/en/journal/18/1/10.11613/BM.2008.002 Schnell, A. (2006, April 23). What Is Regression to the Mean? Retrieved from Analysis Factor: https://www.theanalysisfactor.com/what-is-regression-to-the-mean/ Vidyasagar, A. (2018, October 15). What Is Photosynthesis? Retrieved from Live Science : https://www.livescience.com/51720-photosynthesis.html Related Posts Temperature of Hot Water and Cooling Rate Lab Effect of Color of Light on the Rate of Photosynthesis: Lab Explained Lab Exercise 6.1.1: Determining the Rate of Reaction Refraction of Light Lab Answers Lab: Refraction of Light- Air into Glass Answers Leave a Reply Cancel reply Your email address will not be published. Required fields are marked * Save my name, email, and website in this browser for the next time I comment. Post comment 120 IMAGES Light Intensity Photosynthesis Simulation Investigate the effect of light intensity on the rate of photosynthesis CORE PRACTICAL Effect of light intensity on photosynthesis, using hydrogencarbonate How the light intensity affects the rate of photosynthesis Light response curves for photosynthesis. The light compensation point COMMENTS How Light Intensity and Distance Affect Photosynthesis In this virtual photosynthesis lab, students can manipulate the light intensity, light color, and distance from the light source. A plant is shown in a beaker and test tube which bubbles to indicate the rate of photosynthesis. Students can measure the rate over time. There is an included data table for students to type into the simulator, but I ... Effect of light intensities on the photosynthesis, growth and Introduction. Photoinhibition often occurs when light energy is excessive, which reduces photochemical efficiency and even causes photooxidative system damage (Ma et al., 2015; Dias et al., 2018).Furthermore, low light intensity influences photosynthesis, which is central to plant productivity, and can therefore severely restrict plant growth (Zhu et al., 2014), and even death (Wang et al., 2021). Photosynthetic Physiology of Blue, Green, and Red Light: Light In this paper, we present a comprehensive study to explore potential interactive effect of light intensity and light quality on C 3 photosynthesis and underlying processes. We quantified the photosynthetic response of plants to blue, green, and red light over a wide PPFD range to better describe how light intensity and waveband interact. In ... Practical: Investigating Factors Affecting the Rate of Photosynthesis The effect of light intensity on an aquatic plant is measured by the volume of oxygen produced. Results - Light Intensity. The closer the lamp, ... Algae is often used in experiments on photosynthesis and respiration rates but it can be very hard to maintain consistency in the number of algae and it can be hard to handle directly in the water. Light and photosynthetic pigments Plants, on the other hand, are experts at capturing light energy and using it to make sugars through a process called photosynthesis. This process begins with the absorption of light by specialized organic molecules, called pigments, that are found in the chloroplasts of plant cells.Here, we'll consider light as a form of energy, and we'll also see how pigments - such as the chlorophylls ... Investigating factors affecting the rate of photosynthesis The students can be allocated to investigate a particular factor that affects the rate of photosynthesis, or they can choose from this list, or they can develop their own ideas. Light intensity or distance of the Elodea from the lamp. (Light intensity is proportional to 1/distance 2. Temperature of the water. Carbon dioxide concentration. Measuring photosynthesis experiment Try out this experiment in Atomic Labs. Go to the Biology lab and try the light intensity and photosynthesis experiment. Leave for five minutes for the pondweed to acclimatise to the new light ... Explore How Light Affects Photosynthesis They do however, contain chlorophyll which give them the ability to perform photosynthesis; they use sunlight to convert carbon dioxide and water into energy and oxygen. The limiting factors that affect the rate of photosynthesis are carbon dioxide concentration, temperature, and light intensity. For this experiment students will explore how to ... Effect of light intensity on photosynthesis The effect of light intensity on photosynthesis close photosynthesis A chemical process used by plants to make glucose and oxygen from carbon dioxide and water, using light energy. Oxygen is ... Investigate the effect of light intensity on the rate of photosynthesis Video summary. A demonstration of the key points of the required practical to investigate the effect of light intensity on the rate of photosynthesis for GCSE biology and combined science. This ... Turn That Light Up: Examining the Effect of Light Intensity on oxygen (Jordan and Ogren 1984). We will examine the change in O2 concentration as a measure of the rate of. photosynthesis and determine the effects of light intensity on this rate in Elodea canadensis. HO: Increasing light intensity will decrease or will have no effect on the rate of photosynthesis in E. canadensis. Effect of Light Intensity on Photosynthesis PAR intensity is an important factor that determines the rate of photosynthesis. Too high or too low PAR intensities adversely affect the photosynthetic machinery. At low light intensities above the light compensation point (LCP), photosynthetic rate increases proportionally to the light intensity and reaches a maximum. Use Floating Leaf Disks to Study Photosynthesis Temperature also plays a significant role, as photosynthesis is an enzyme-mediated reaction. This is because at high temperatures, enzymes can get damaged and thus become inactivated. Other factors that affect the rate of photosynthesis are the light intensity, the amount of chlorophyll and other color pigments in a plant, and the color of light. 11.2 Light and Photosynthesis Recall that the overall equation for photosynthesis is: water + carbon dioxide -> oxygen, water, and simple sugars. 12H 2 0 + 6CO 2-> 6O 2 + 6H 2 O + C 6 H 12 O 6. This equation is made up of two parts called half-reactions.The first half-reaction is an equation summarizing the Light Reaction, where energy from sunlight is used to split water molecules into oxygen gas, some electrons, and some ... How does the level of light affect the rate of photosynthesis? In photosynthesis, the energy from the sun is used to turn carbon dioxide (CO2) and water into sugar. Oxygen is a waste product. More light can mean more photosynthesis. It doesn't necessarily mean more though. When we think of photosynthesis as a process, we can see that there are at least three things that can limit the process: light ... Turn That Light Up: Examining the Effect of Light Intensity on Photosynthesis is an essential reaction occurring in all plants as it provides their source of energy. As light is one of the required reactants, we chose to investigate the effects of differing light intensities on the rate of photosynthesis, hypothesizing that greater light intensity will result in greater photosynthetic rate. The rate was measured by determining the change in oxygen ... Practical The effect of light intensity on photosynthesis can be investigated in water plants. Use Cabomba or Elodea, which are sold in aquarium shops. The plants will release bubbles of oxygen - a ... PDF Lab Photosynthesis Learn about photosynthesis. Design an experiment to test how light affects photosynthetic rates. Plot and interpret the data you obtain from your experiment. ... The distance of the algae from the light source will affect the intensity of light that the algae receive. Using the inverse square law, you can calculate the relative light intensity each Effect of Light Intensity Light intensity is one of the factors affecting the rate of photosynthesis. Other factors are concentration of carbon dioxide, temperature and to a lesser degree, water. Light intensity directly affects the light-dependent reaction in photosynthesis and indirectly affects the light-independent reaction. Light is a limiting factor when the light ... Photosynthesis practical-Leaving Cert-Light Intensity A summary of the photosynthesis practical for leaving cert biology-examining the effect of varying light intensity on the rate of photosynthesis. This video ... 5.1.7 Investigating the Rate of Photosynthesis The effect of light intensity on an aquatic plant is measured by the volume of oxygen produced. Results - Light Intensity. The closer the lamp, ... Algae is often used in experiments on photosynthesis and respiration rates but it can be very hard to maintain consistency in the number of algae and it can be hard to handle directly in the water. Lab Answers: How Does Light Intensity Affect the Rate of Photosynthesis Redirect the experiment by using varying light intensities and determine the optimum light intensity for the maximum rate of photosynthesis. Extend the experiment by investigating the effect of light intensity of various species of Elodea or different plants all together. Conclusion . In conclusion, the evidence suggests that decreasing light ... Experiments to investigate photosynthesis The effect of light intensity on photosynthesis can be investigated in water plants. Use Cabomba or Elodea, which are sold in aquarium shops. The plants will release bubbles of oxygen - a ... essay homework paper phd project resume review speech study thesis