The distances from the lamp to the experiment that I plan to use are 5cm, 10cm, 15cm, 20cm, 25cm and 30cm. I will use these because it is a fairly wide range, but the gaps between the distances aren’t so big that I won’t be able to get any really concise and conclusive results.
Only one variable is allowed to be altered, in the experiment. Light intensity is the variable being tested, and by moving the lamp closer or further away I will be changing the intensity of the light. The variables, or factors that must be kept the same are as follows: -
- The carbon dioxide concentration must be kept the same, because the more CO2 that is in the air, or water, then the more that can diffuse into the leaf, and if the plant had more CO2 then it would be able to photosynthesise faster. The concentration can be kept the same by adding a fixed amount of sodium bicarbonate to the beaker and elodea. The experiment should also be carried out in one session, less than two hours, so that the plant does not use up a significant percentage of the carbon dioxide.
- The amount of water must be kept the same because water is needed for the reactions to occur in a plant, and so if there is a lack of water, then reactions will be slower, if they take place at all. Another reason that the amount of water must be kept the same is because when plants do not have enough water, they close the stomata, to prevent further water loss. However, when the stomata are closed, carbon dioxide cannot diffuse into the leaf, which would also slow down the rate of photosynthesis.
- Temperature must remain constant because enzymes are used in photosynthesis and the respiration of the plant. Therefore, as the temperature increases so will the rate of reaction of the enzymes, up to a certain point, where the enzymes will be denatured. The temperature could be kept constant by performing the experiment in one session, so that the air temperature should not change enough to affect the water temperature.
- The type of plant should be kept the same because different plants have different rates of photosynthesis due to different leaf structures. Even plants of the same species can have slightly different rates of photosynthesis, because there may be more or less chlorophyll in the leaves to absorb light. The size of the plant must also stay the same, because a bigger plant would have a bigger surface area for gas exchange. The only way to control this variable is to use the same plant throughout the experiment.
- Light energy is absorbed by chlorophyll. Chlorophyll can easily absorb blue light, in the 400-450 nm range, and also easily absorbs red light, in the 650-700 nm range. Chlorophyll does not absorb green or yellow light effectively, but tends to reflect them, decreasing the amount of light absorbed and the rate of photosynthesis. Therefore, the light wavelength must stay the same. This can be controlled by using the same bulb throughout the experiment.
Results
Conclusion
From the results that I have collected I can maintain that increasing the light intensity results in an increase in the rate of photosynthesis. This is because plants need energy to join together water and carbon molecules, to form glucose, and light provides the energy that they need for this process. Therefore, if the light intensity increases, the plant is provided with more energy, and so can photosynthesise at a faster rate.
My results do agree with my prediction to a certain extent, because I predicted that the rate of photosynthesis would increase as the light intensity did because the light provides energy for the reaction, and the more energy the plant has, the faster it will be able to photosynthesise. This is what happened in the experiment, and so my prediction was correct. However, I had expected the rate of photosynthesis to stop increasing after a certain point and level off. I thought this because as the light intensity is becoming higher and higher, I expected something to take over as the limiting factor (the factor in the shortest supply, such as temperature or carbon dioxide concentration), and prevent the plant from photosynthesising any faster. However, I think that the reason that this didn’t happen is because the light intensity did not go high enough. This wasn’t possible using the equipment that I did. I could have moved the lamp closer, but not by very much, so it wouldn’t have been that useful because the results would have been close to the results that I gathered for 5cm anyway, and so it would be unlikely that I would be able to conclude anything from them.
Evaluation
The results I got were fairly reliable, but not completely. Even though I got the results that I expected, there were things that could be improved to make my results more accurate. The main problem was that all of the bubbles were different sizes, which made it difficult to count. Because of this I did collect some anomalous results, were I there were no bubbles, or only one big one. I repeated these results until I was satisfied that the sizes of the bubbles were the same, or very similar to the rest of the results that I had collected. In a future experiment, a more accurate way of measuring the amount of gas given off would be to measure the volume of gas produced, rather than just counting the bubbles. I would do this by having a scale on the test tube, where the gas collects. Then I could measure how much the water level drops by, or how much more gas there is at the end of the minute than there was at the beginning of the experiment.
Another problem was that there were air bubbles trapped in the funnel and on the test tube, from when I had set the equipment up. I tried to release all of the bubbles before the experiment started, but this was not very easy and so some remained, mainly on the edge of the test tube. Although I am almost certain that the bubbles that I counted were from the plant as a result of photosynthesis, it is possible that one or two were the air bubbles that had been there since the beginning. A way to get rid of the bubbles on the test tube could be to use a stirring rod, or something similar to knock them off the edge, so that they rose to the top.