Effect of Light Wavelength on Rate of Photosynthesis

This essay is more of a lab write-up than an essay, but that's okay. It was our required writing assignment for the term, so I put it with my essays. (Written 02/02/04).


Introduction:

It is known that plants need light to undergo photosynthesis, which they need to function and make energy. They also produce oxygen, which can be used to measure rate of photosynthesis. But there are factors that effect this rate, such as weather, temperature, moisture, and light wavelength.

Plants tend to function better under certain wavelengths of light, such as those in the blue and red spectrum. They generally reflect wavelengths in the green spectrum (that is why they appear green to us). So, the question is what is the exact effect of different wavelengths of light on photosynthesis (particularly in the plant Elodea)?

Since photosynthesis rates are generally known to be higher under red and blue light wavelengths, then they might be higher still by absorbing the entire spectrum of light. Thus, I hypothesize that the rate of photosynthesis will be highest with clear paper because this is most like the natural setting for Elodea. If Elodea’s photosynthesis rate is highest with the clear paper, then it (the plant) will create the most oxygen under these conditions (absorbing the full spectrum of light) and have the fastest rate of photosynthesis.

Methods:

First we set up two manometers, one as a control (which was covered in foil to avoid any wavelength of light reaching it) and one covered with blue paper (so the plant receives only blue light). The other test colors (clear, red, and green) were performed by different groups. The two manometers were next to the heat sink, while the light source was positioned 15 cm away from and opposite of the manometers.

The dependent variable we were measuring was the rate of photosynthesis as calculated by oxygen production under blue light wavelength conditions. Other groups performed the experiments for clear, red, and green wavelengths effects on rate of photosynthesis. The experiment for green was replicated two times; for red, blue, and clear it was replicated three times. Our independent variables were the distance the light source was from the heat sink and the duration of exposure.

Results:

By taking the readings of the manometers (the initial and final measures of the control and the experiment), and correcting the volume of oxygen produced, we found the average volume of oxygen produced. For green, the volume was 0.02 mL. Red had an average volume of 0.03 mL. The result for blue was 0.04 mL. And lastly, the clear paper had an average of 0.05 mL. This can be correlated to the rate of photosynthesis.

The above data can also be seen in Figure 1.1, and in Table 1.1, which are shown below.

Table 1.1: Average Time of Starch Disappearance in Different pH Environments
Tube Number pH Time of Starch Disappearance (in seconds)
1 5.0 303.3
2 6.0 220.0
3 7.0 140.0
4 8.0 96.7

Discussion:

The question for this experiment asked what the exact effect of different wavelengths of light was on photosynthesis (particularly in the plant Elodea). The hypothesis stated that the rate of photosynthesis would be highest with clear paper because this would be most like the natural setting for the plant. If Elodea’s photosynthesis rate was highest with the clear paper, then it (the plant) would create the most oxygen under these conditions and have the fastest rate of photosynthesis.

Observing the collected data, it would appear that the hypothesis is supported by the experiments. The clear paper had the greatest volume of oxygen produced (0.05 mL), while the green paper had the lowest (0.02 mL). This can be correlated to a faster photosynthesis rate for the clear paper than for the green, because the Elodea was able to absorb the wavelengths of light it needed to undergo photosynthesis. The wavelengths for red and blue were in between these two values, as was to be expected, with oxygen production at 0.03 and 0.04 mL respectively. These values indicate photosynthesis occurring at a moderate rate.

I conclude that our hypothesis was correct and was supported by the data observed.

Problem that could be involved in this experiment include the following:

- The tests weren’t completely standardized. Each group may have varied with the measurements for distance and time.

- To have a more complete experiment, photosynthesis rate for each color should also have been measured with the light source various distances from the heat sink.


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