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Light Quality on the Rate of Photosynthesis Measure

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This study was undertaken to determine the relationship of different wavelengths of light and the rate of photosynthesis in spinach leafs. The rate of photosynthesis was measured every five min under light colors of white, green, red, blue and yellow under a light intensity of 2000 lux. The rate of photosynthesis was measured by the spinach disk method in which we replaced the air from the disks with sodium bicarbonate using a vacuum. Under photosynthesis, oxygen, a product of photosynthesis, replaced the bicarbonate solution, made the disks less dense and rise to the top. The rate of photosynthesis was the greatest under white light, followed by red, blue, yellow and green, which produced no detectable photosynthesis signifying that chlorophyll reflects, rather than absorbs, green light. White light was also expected to have the highest rate of photosynthesis as it gives the chlorophyll all the colors of light to absorb. In general, filtered light reduced the rate of photosynthesis because the chlorophyll didn’t receive all the different colors of the spectrum it desired as it did in white light to absorb.


Photosynthesis is the ability to convert light energy into chemical energy in the form of sugar (Freeman 2002). The photosynthetic reactions are divided into two sections: light dependent and light independent reactions. In this experiment, we will be dealing with the light dependent reactions. The actual chemistry of the light dependent reaction is with the addition of light: H2O + ADP + inorganic phosphate + NADP+ ¨ 1/2O2 + ATP + NADPH + H.

Light is a necessary input for photosynthesis to take place. Light’s electromagnetic radiation carried in photons are absorbed by the photosynthetic pigments in plants. These photons can cause electron excitation to a higher energy level, which can be added to NADP+ form NADPH or the excited electrons can also be passed down an electron transport chain producing ATP. These processes occur in photosystem I and photosystem II respectively. The splitting of water into electrons and oxygen occurs in photosystem II. We can measure the rate of photosynthesis by measuring the amount of oxygen, because oxygen is a product of photosynthesis. The reduced molecules produced from the light dependent reactions are used in the light independent reactions to produce one of the final products of photosynthesis, glucose.

Photosynthesis occurs in the leaf of plants in their chlorophyll. The pigment chlorophyll absorbs red and blue light and transmits green light, it is responsible for the green color in plants (Freeman 2002). There are also other pigments in plants such as carotenes and xanthophylls that help photosynthesis by increasing the range of wavelengths or amount of light absorbed. This is beneficial for the plant because the more absorption of photons, the more electrons can be excited, and thus photosynthesis can occur at a faster rate. Green light is reflected by plant leafs and not absorbed and so it is my hypothesis that the spinach disks receiving green light will have no photosynthetic reactions take place, thus no spinach disks rising to the top. This is because the chlorophyll cannot use the green light to excite electrons because there is no step difference–no difference in possible energy states–that correspond to the amount of energy in a green photon (Freeman 2002).

It is also my hypothesis that the blue and red light will stimulate photosynthesis because chlorophyll absorbs those colors and uses those photons to activate photosynthetic reactions by electron excitation. Because the pigments of chlorophyll, chlorophyll a and chlorophyll b, absorb more blue light than red light, it is also my hypothesis that the rate of photosynthesis will be higher with the blue light than the red light. As for the yellow light, because chlorophyll doesn’t absorb much yellow light, my hypothesis is that with the yellow filter only a small amount of spinach disks will float to the top due to photosynthesis. The white light should have the most photosynthesis because all of the colors of the spectrum are available for the plant to run photosynthesis, not just one color. In the presence of white light, the chlorophyll will be receiving both blue and red which will increase the rate of photosynthesis.

My hypothesis for the rates of photosynthesis due to variable color light in increasing order is: green, yellow, red, blue and white.

In the following experiment, we will measure the photosynthetic rates of spinach disks under different wavelengths (blue, yellow, green, red and white) of light.

Materials and Methods

To measure the relative rates of photosynthesis, we measured the production of a product of the light dependent reactions, oxygen. In order to do this, we used spinach leaf disks that we cut from spinach leaves, which were all the same size. We took approximately 50-60 spinach leaf disks and placed them in a flask containing 100 ml of 0.2% bicarbonate solution, NaHCO3. Using a water aspirator, we replaced the gas in the leaf disks with the 0.2% bicarbonate solution. This caused the spinach leaf disks to sink to the bottom of the solution because the 0.2% bicarbonate solution is denser than the air spaces it replaced in the spinach leaf disks.

With five Petri dishes filled 2/3 to the top with 0.2% bicarbonate solution we placed 10 spinach disks that sunk to the bottom in each of them. We used 0.2% bicarbonate solution again in the Petri dishes because the disks remained sunk on the bottom.

After, we set up our variable light treatment. Using 3 flood lamps clamped to a metal stand, we placed them about a foot above the table. We then turned all of them on illuminated towards the table. Because in our experiment we are just testing the variable of light quality, we needed to keep other variables controlled. The light intensity needed to be kept controlled because if one Petri dish received a higher intensity of light than the other, the results of the rate of photosynthesis may be due to the intensity rather than the quality of the light. We used a lux meter and found five spots under the flood lamps in which the light intensity was 2000 lux with their respective filters.

The variable treatments we used were exposing the sunken spinach disks to different colors of light. Using filters, our variable treatments were green light, red light, blue light and yellow light. Our control treatment was using white light, which didn’t have a filter. The control of just white light is used to compare the rate of photosynthesis that occurs naturally in the environment when all colors of light are available to the chlorophyll. At the spots where the light intensity was 2000 lux with their respective filters, we placed a Petri dish under the color filter and under a clear bowl containing water. The bowl containing water was used as a heat filter, absorbing the heat from the bulbs, because we didn’t want the heat affecting the spinach leaf disks and the rate of photosynthesis.

With this apparatus we measured the relative rates of photosynthesis among the variable treatments. As the spinach disks under went photosynthesis, now that light was available, oxygen was produced. The oxygen replaced the 0.2% bicarbonate solution in the spaces of the spinach disks, causing the spinach disks to become less dense. If enough oxygen was produced, the spinach disks rose to the top of the Petri dish because the spinach disks became less dense than the 0.2% bicarbonate solution. We measured the rate of photosynthesis by measuring the number of disks out the 10 sunken disks that rose to the top in a given amount of time. If more disks rose to the top, more oxygen was produced in the disks, signifying a higher photosynthetic rate.

Although it may seem that we only indirectly measured photosynthesis because oxygen is a product from photosystem II, not of photosystem I. So technically, the rate of photosystem I could have been increasing, but we wouldn’t have detected it with our apparatus because oxygen isn’t produced in photosystem I. However, because the photosystems overlap a lot in the absorption and their rates, measuring oxygen was still a good measure of the rate of photosynthesis that took place.

We measured the amount of disks that floated to the top in each treatment every five min for thirty min. Originally, the time intervals for reading were ten min, but to obtain twice as many points and produce a better graph, we took readings every five min.


Figure 1.

Figure 1. % spinach disks floating vs. time due to variable light color. In the experiment, as time increased the percentage of spinach disks that floated increased in all of the colors of light except green light which remained at zero. Within the first five min, forty percent or more of the spinach disks had already risen to the top in four of the five treatments. The treatment in which that didn’t occur was in green light, which didn’t have any spinach disks rise throughout the 30 min. At 15 min, 100% of spinach disks rose in white and red light treatments and 90% of blue’s disks and 80% of yellow’s disks rose. After the 30 min, 100% of the spinach disks rose to the top in all treatments except under the green light. Under the treatment of white light, 100% of spinach disks rose to the top first followed by red, blue and yellow lights. This is also the order of the rates of photosynthesis under the different light treatments from highest to lowest rates.


Some of the results that we obtained are in agreement with my hypothesis. Referring to figure 1, the light treatment to have all the spinach disks rise to the top was in the white light, followed by red light, blue light, yellow light and green light in which no spinach disks rose over the 30 min. Because all of the white light’s spinach disks rose in a less amount of time than the other treatments, its photosynthetic rate must have been faster. Therefore the rates of photosynthesis due to variable color light in increasing order are: green, yellow, blue, red and white. These results didn’t exactly support my hypothesis. In my hypothesis, I thought blue light would have a higher rate of photosynthesis than the red light. The results of yellow light producing a significant rate of photosynthesis rejected my hypothesis. However, my hypothesis of green light producing no photosynthesis and white light producing the most did support my hypothesis.

One explanation for these results that didn’t support my hypothesis is attributing it to experimental error. Maybe when we measured the number of disks that did float, we over counted as it was difficult to determine the depth of each disk given only a few centimeter of difference from floating to remaining on the bottom. Another error that could have occurred experimentally is that the light intensity at each Petri dish wasn’t exactly 2000 lux. It was easy to error to have a higher light intensity at one area than the other because just small placement movements in the lux meter caused large changes in the intensity of light from the bulbs. Although we found points that were 2000 lux, when we removed our markers and placed the Petri dishes, we were a little off.

This could easily have caused the error of red light producing a higher photosynthetic rate than blue light, not because of the different wave lengths but because the Petri dish receiving red light accidentally had a higher intensity of light. Another step where experimental error may have occurred was when we aspirated the spinach disks. We may have done it improperly and damaged the spinach disks so the rate of photosynthesis was thrown off from the damaged disks. Another experimental error that could have occurred is that maybe our results are from a small sample size and that there were irregular spinach disks. So a follow up of replicating this experiment but using twice as many spinach disks could take out chance error, the error due to an unusual sample size not representative of the entire spinach disk population.

However, our results could be true and that my hypothesis needs to be reevaluated. There could be other pigments in the spinach leaf other than chlorophyll. Other pigments could increase the amount of absorption due to certain wavelengths and also increase the range of wavelengths absorbed. Assuming no experimental error occurred and only the chlorophyll pigment was present, the data of the photosynthetic rate from yellow light shouldn’t be significant, because the chlorophyll pigments don’t absorb much yellow light. However, there could be other pigments that do absorb yellow light that we don’t know about in the spinach leaf that is absorbing the yellow light photons, exciting electrons and increasing the rate of photosynthesis. Likewise, this explanation of other pigments could also explain that red light produced a higher photosynthetic rate than blue light. Maybe there are other pigments that increase the amount of red light absorbed and thus red light producing a higher rate of photosynthesis than blue light.

The next step to determine what pigments are in spinach leafs would be to do a paper chromatography of the spinach leaves. By doing this we could separate the pigments of the spinach leaves and using a spectrophotometer to measure the light wavelengths is absorbs, we could distinctly determine what wavelengths the spinach leaf absorbs. This way we could determine if only chlorophyll pigments are present, and thus can attribute our data to experimental error, or if there are other pigments acting in the system, which caused unexpected rates of photosynthesis in red and yellow light, which rejected my hypothesis of those two light colors.

Another investigation to consider would be interesting to replicate this experiment over again, however taking readings in shorter intervals, for example every two min. This would produce a better graph than what we obtained because in our experiment taking readings every five min for 20 min almost all the disks had all risen to the top by 15 min in all but the green treatment. For example, within the first five min, eighty percent of the spinach disks under the white light had already risen. It would be interesting to see the photosynthetic rate when the disks are actively floating to the top which occurred mainly in the first ten min. By taking more readings we could develop a better graph.

Literature Cited

Freeman, S. 2002. Biological Science. New Jersey: Prentice Hall.

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