Light Spectrum: Photosynthesis Matt, Mike, Julia

How do different wavelengths of light (visible spectrum) affect photosynthesis, and its efficiency in living spinach leaf disks in a baking soda detergent solution? Question

Under the “natural” light (CFL), it is predicted that all 10 leaf disks will float in the solution because photosynthesis occurs in plants under natural light. The 10 leaf disks will float under the green light because the plants are green. It is also predicted that the 10 disks will rise rapidly under the yellow light, for it is the closest wavelength to a natural one. Finally, the red and blue wavelengths will also allow disks to float, but only about 4 or 5. This is predicted because the red and blue wavelengths are not that close in visible color to the leaf or natural light. Hypothesis

Experimental Control In order to focus the experiment on how varying wavelengths affect the leaf disks’ ability to create oxygen from photosynthesis, several controlled factors needed to be put in place. For instance, the light bulbs were placed exactly 7 cm from the tabletop. This ensured that the wavelengths would be focused solely on the leaf disks in the cups, and all the cups would receive the same amount of light. The amount of water, soap and baking soda used for each cup that the disks were placed in was also controlled to make sure that all of the disks had to float the same distance to the surface of the solution, and that each cup’s solution had the same density. Another control factor was the double-layer cellophane placed over the CFLs. This was done to make sure that only the desired wavelength(s) of light went to their corresponding cups, and no unwanted light wavelength(s) did. Each of the leaf disks under the different wavelengths of light had the same amount of time to produce oxygen through photosynthesis, being 15 minutes for each cup.

Add a pinch of baking soda and two drops of dish soap to 30 mL of water in a clear plastic cup. Stir the solution and make sure not to create suds from the the dish soap while stirring. Repeat this process to make enough of the solution for the number of different light wavelengths being tested in the experiment. 5 cups will be used, so 150 mL of water is used total, along with 10 drops of soap and 5 pinches of baking soda. The different wavelengths being tested are red, yellow, green, and blue through cellophane. This solution can be mixed all at once. Punch full holes in spinach leaves. The small circles punched out of the leaves will be the leaf disks. There should be 10 leaf disks for each of the wavelengths of light. Distribute the leaf disks into the clear cups containing the solution so there will be 10 in each cup. Label these cups to avoid confusion. Going cup by cup, use the process of leaf disk filtration to remove currently present oxygen from the leaf disks, return the disks to the cups, and store the cups in a dark place to avoid uncontrolled photosynthesis. Procedure

Procedure Continued Make sure no leaf disks are floating after their filtration. Set up each CFL so that the light from each does not overlap. Cover the CFLs in two layers of the same color cellophane, to distribute only the desired wavelength(s) of light. Make sure to leave one CFL without any cellophane to use as the control of the experiment. Using a ruler, set all CFLs 7 cm above the table. Place each cup directly under the corresponding CFL, and turn the lights on. It is helpful to run multiple wavelength tests at the same time to increase efficiency. Run the CFLs for 15 minutes for each cup. Record the amount of leaves that float in the solution at 30 second intervals in a data table. Once all the data for each cup is recorded, empty the containers of fluid and leaves, and clean up the lab station.

Data This graph represents the number of floating spinach disks throughout a span of 15 minutes. The legend represents the different color lights that were used to cover the lamp. As seen in the graph, blue was the most unsuccessful while green was the most successful.

Data Light Spectrum Data Table This data table consists of the number of disks floating under certain colors, for up to 15 minutes. The green light had all 10 disks floating by 8 minutes while on the other hand the blue light did not make any disks float.

Analysis After completing the experiment, many statements can be made. The green light was the most effective, with the natural light being slightly less effective. After the natural light, the pink was the next most effective, followed by yellow. Blue light appears to have no effect on the leaf disks at all in the sense of photosynthesis. A trend that can be extracted from the data is that except for green, all the other colored lights were efficient in the same order as the light spectrum is from left to right. This means that the longer wavelengths, like red were more efficient that the shorter wavelengths, except for green. This could mean that because red is the longest wavelength in the visible spectrum, it had a better effect on the leaf disks than the other colors such as yellow or pink because it moves the fastest.

Analysis Continued There are biological reasons for why the data turned out the way it did, however. There are four types of pigments found in higher plants (trees, bushes etc.). These pigments are chlorophyll a, chlorophyll b, xanthophyll, and carotene. While all four of these pigments are in spinach leaves, only two are active when the leaf is green. Chlorophylls a and b make a green color when they are functioning, while carotene will produce an orange color and xanthophyll will make a yellow and brown color (these are the colors of leaves in the fall). Since the spinach leaves that were used are green, it can be assumed that only chlorophylls a and b were functioning at the time of the experiment. Given this information, it is also known that chlorophyll a absorbs light in the blue and violet section of the light spectrum, a.k.a. the high frequency or short wavelengths. Chlorophyll b absorbs all light ranging from red, on the low frequency or long wavelength side, to the beginning of the blue-violet side.

Analysis Continued 2 Due to these known facts it can be claimed that chlorophyll b was the sole pigment at work over the course of the experiment. This also explains why green was the highest functioning color, even though of the light being absorbed it was the slowest (excluding blue because it was not absorbed). Green is in the middle of the visible spectrum of light wavelengths. This means that it has a good amount of energy, without sacrificing the speed of the wavelength, whereas red has a lower energy but travels faster. Since there was a color similarity between red, pink, and yellow, and a gap between the latter colors and green, the results showed that the middle color is the highest functioning. The green light has enough speed to travel fast enough, but also excite the electrons efficiently, what causes photosynthesis. Since red, pink and yellow are closer together on the light spectrum, red was the most efficient of the three because of its speed. Pink and yellow are only slightly faster and don’t have much more energy, making them less efficient.

Possible Experimental Errors A possible problem with the experiment, to which the answer is unknown could have occurred. The light bulbs in the experiment were CFLs, a type of light bulb which excites gasses inside to create light. The only problem is that these bulbs may not have used all the gasses necessary to simulate white light. Since white light is a combination of all wavelengths in the visible spectrum, this means that the CFLs may have been missing some types of light. Then, the cellophane would not be able to produce the designated color’s light, and similar light may have been able to pass through, throwing off the results of the experiment.

After the experiment, it can be concluded that the hypothesis was not correct. This was partially because the hypothesis was more focused on quantitative data predictions than qualitative. While green light did work well, it was only partially because the plants were green. While the pigment’s color was green, that didn’t mean that green was the most efficient color because of the pigment color. The pigment’s color only designates which cpigment is present and active. Chlorophyll b just happened to absorb green light, and its properties made it the most efficient. The hypothesis is also wrong in the fact that it states yellow would work better than red, and red would work the same as blue. Red and blue will most likely have different results if any plant was used, because they are on opposite sides of the visible spectrum and most plant pigments do not absorb the entire spectrum. Yellow was also less effective because red is faster, and yellow barely has more energy than it while being enough slower than red to make red the second most efficient color tested. Conclusion