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The Effects of Different Light Colors on Photosynthetic Rate

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Emily S

on 4 March 2015

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Transcript of The Effects of Different Light Colors on Photosynthetic Rate

How does the color of light affect the rate of photosynthesis in plants?
Autotrophs such as plants capture free energy from the environment through photosynthesis and chemosynthesis. The process of photosynthesis occurs in a series of enzyme-mediated steps that capture light energy to build energy-rich carbohydrates. The process is summarized by the following reaction: 2H2O + CO2 + light → carbohydrate (CH2O) + O2 + H2O. Photosynthesis takes place in chloroplasts that have chlorophyll in them. Chlorophyll absorbs the light from the sun and combines carbon dioxide and water to make sugar and oxygen, which leaves through the stomata. To determine the net rate of photosynthesis, one could measure the production of O2 or the depletion of CO2. In this lab, students first removed all the O2 in a plant disk through a syringe vacuum, then placed the plant in a solution of CO2. When placed in sufficient light, the photosynthetic processes then produce oxygen bubbles that change the buoyancy of the disk, eventually causing them to rise. Therefor, the faster the rate of photosynthesis, the faster the leaves rises.
There are five matters that affect the rate of photosynthesis: light intensity, temperature, CO2, water, and the color of light in general. In this experiment, the team aims to manipulate the color of light and control the following variables: CO2, water, and temperature. While certain wavelengths are absorbed, others are reflected is produced, this is essentially the reason why plants are green, because the main pigment, chlorophyll a, absorbs all colors in the visible light specturm but green. Other pigments known as accessory pigments (chlorophyll b and carotenoids) are present at well. These different pigments absorb different wavelengths due to structural differences and reflect on the production of oxygen, which causes the leaves to float.
If white light is a mixture of several wavelengths of colors and the chlorophyll in green leaves absorb energies from all visible light except green, then exposing white light to a green plants will result in the fastest rate of photosynthesis, followed by blue or red. Green or yellow light will have the slowest rates of photosynthesis because they are reflected by the pigments in the plant.
In this lab experiment, the team examines how the rate of photosynthesis is affected by different light colors in the leafs. Five different variables were tested; green light, red light, yellow light, blue light, and regular white light at a controlled distance of approximately 10cm. The plant responded to the different colored light as follows: The rate of photosynthesis was the most successful in white light, and more successful in red and blue lighting, than in green and yellow lighting systems. This outcome occurred because different pigments in chloroplasts only absorb certain wavelengths of light to continue photosynthesis. Our results show that different color lights definitely does have a strong direct correlation with the rate of photosynthesis/growth of a plant.
Absorption spectra showing how the different side chains in chlorophyll a and chlorophyll b result in slightly different absorptions of visible light. Light with a wavelength of 460 nm is not significantly absorbed by chlorophyll a, but will instead be captured by chlorophyll b, which absorbs strongly at that wavelength. The two kinds of chlorophyll in plants complement each other in absorbing sunlight. Plants are able to satisfy their energy requirements by absorbing light from the blue and red parts of the spectrum. However, there is still a large spectral region between 500 and 600 nm where chlorophyll absorbs very little light, and plants appear green because this light is reflected.
5 test tubes
Hole puncher
4 lamps
Transparent color papers (blue, red, green, yellow)
Light intensity measuring device
Bicarbonate solution
Liquid soap
10 clear plastic cups
2 syringes
1. Prepare 300 mL of 0.2% bicarbonate solution for each experiment. The bicarbonate will serve as a source of carbon dioxide for the leaf disks while they are in the solution.
2. Pour bicarbonate solution into 4 plastic cups to 3 cm. Label cup with the color of light that will be tested. Fill a fifth cup with only water to be used as a control group. Label this cup “Without CO2 .”
3. Using a pipette, add one drop of a dilute liquid soap solution to the solution in each cup.
4. Using a hole punch, cut 10 or more uniform leaf disks for each cup. Avoid major leaf veins. (The choice of plant material is perhaps the most critical aspect of this procedure. The leaf surface should be smooth and not too thick.)
5. Draw the gases out of the spongy mesophyll tissue and infiltrate the leaves with the sodium bicarbonate solution by performing the following steps:
a. Remove the piston or plunger from both syringes. Place the 10 leaf disks into each syringe barrel.
b. Replace the plunger, but be careful not to crush the leaf disks. Push in the plunger until only a small volume of air and leaf disk remain in the barrel (<10%).
c. Tap each syringe to suspend the leaf disks in the solution. Make sure that, with the plunger inverted, the disks are suspended in the solution. Make sure no air remains.
d. Create the vacuum by holding a finger over the narrow syringe opening while drawing back the plunger. Hold this vacuum for about 10 seconds. While holding the vacuum, swirl the leaf disks to suspend them in the solution. Now release the vacuum by letting the plunger spring back. The solution will infiltrate the air spaces in the leaf disk, causing the leaf disks to sink in the syringe.
6. Pour the disks and the solution from the syringe into the appropriate clear plastic cup. Disks infiltrated with the bicarbonate solution go in the “With CO2 ” cup, and disks infiltrated with the water go in the “Without CO2 ” cup.
7. Cover the several lights with red, blue, green, and yellow films. Place the cups under the light source and start the timer. At the end of each minute, record the number of floating disks.
8. To make comparisons between experiments, a standard point of reference is needed. Repeated testing of this procedure has shown that the point at which 50% of the leaf disks are floating (the median or ET50, the Estimated Time it takes 50% of the disks to float) is a reliable and repeatable point of reference for this procedure.
9. Record or report findings.
Rate of photosynthesis (measured by ET50, time it took for 50% of the leaf disks to float up)
Color of light (wavelength of light)
Light intensity, temperature, bicarbonate concentration (0.2%), depth of bicarbonate solution, direction of incoming light, pH, amount of soap, size of leaf disk, type of plant
Ten trials for each color, however for each trial, a few disks did not float up. We did not count those disks into our data.
White light

The white light displays the effect colors have on the plant

In an ideal investigation, all the leaves should rise at the same time. However, in order to make an accurate conclusion and experiment, the team decided to take the ET50 of the trials, which is the time at which 50% of the leaves have floated up. The time vs. the inverse of ET50 was used to produce the graph so that the results would display a positive correlation. Looking at the graph, the plant under white light had the greatest 1/ET50 value, followed by blue, red, yellow, and green.

The team’s hypothesis was supported because the average time it took for the leaves under green light to float was greatest, and white light the least. The energy that plants receive to generate ATP and NADPH essentially comes from absorbing sunlight energy.These results are favorable because plants have light absorbing pigments within Photosystem II and I on thylakoid membrane. Pigments containing chlorophyll a and b are able to use light energy to boost the electrons onto the primary electron acceptor. The electron now has high potential energy and as it travels down the electron transport chain it’s able to transform potential energy into ATP and NADPH. The ATP and NADPH are ultimately used to produce G3P (a sugar). Note that the electron are donated by the splitting of water and produces oxygen as byproduct, and the reason why leaves float is because the production of oxygen, a very light molecule, indicating that the chlorophylls have successfully absorbed energy from light. Realizing the fact that light intensity is another variable which may affect results, the team measured the intensity of the different color lights.
During the experiment, some of the leaf disks would not rise from the bottom of the beaker. These leaves were outliers on the far right of the graphs of the data, skewing the data. This situation caused the number of trials to decrease, leading the team to obtain a less accurate ET50 value. In addition, since the team used different lights for the different colors, the light intensity may have been unequal, also resulting in inaccurate data.
If we were to do the lab again, we would include a trials with more colors of the visible light spectrum such as orange and violet to get a more comprehensive and seamless set of data. We could compare our tested data with the absorption spectrum of the chlorophyll in the plant. In addition, a trial with no light would show the effect of light versus no light in the plant’s production of photosynthesis.
Other variables that we could tweak are concentration of bicarbonate and type of plant.
Comparing experimental results
By: Emily, David, Michael
Mr. Minnich
AP Biology
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