All the Calvin cycle reactions and many of the light-dependant reactions of photosynthesis are controlled by enzymes and are therefore responsive to temperature. This means that even when the light and carbon dioxide levels are suitable for a very high rate of photosynthesis, unless the temperature is also satisfactory the plant will not be able photosynthesise at the maximum rate. I will make sure that temperature is not a limiting factor by checking the thermometer regularly to sustain a constant temperature.
Chlorophyll concentration is not normally a limiting factor, but it can become one if chlorophyll levels are unusually low. This can be caused by disease, mineral deficiency, and the age of the plant. Iron, magnesium, nitrogen and sunlight are essential for chlorophyll production, so a lack of any one of these can lead to yellowing of the leaves.
Water is necessary for most cell processes, which makes it difficult to measure the effect that a lack in water would have on the rate of photosynthesis. However, water will not be a limiting factor in my experiment as the plant, which I am using (Elodea) is a hydrophyte so water will be in excess.
Prediction
I predict that the closer the lamp is to pondweed the faster photosynthesis will take place because light is needed for the reaction and there will be more input energy. The rate of photosynthesis to the light intensity is inversely proportional so it will increase rapidly at first but will not increases so rapidly when the lamp gets closer to the pondweed. When the lamp gets close to the pondweed the lamp will be giving all the light (energy) that the pondweed will need to photosynthesise at its optimum speed. It cannot photosynthesise any faster because there is only a certain number of chloroplasts containing a limiting amount of chlorophyll and this can only absorb a certain amount of light which is called the light saturation point.
Materials
- Photosynthometers
- Elodea- freshly cut at each end
- Thermometer
- Water
- Clamp stand
- Metre rule
- Light bulb (100W)
- Syringe
- Sodium Hydrocarbonate
Diagram of apparatus
Method
- Assemble the apparatus as shown in the diagram
- Push in the plunger to the end of the syringe, making sure that there are no air bubbles in the tubing
- Add 0.25g of Sodium Hydrocarbonate to the test tube with the elodea and stir it to dissolve
- Measure the distance between the light bulb and the Elodea
- Switch on the light bulb
- Leave the Elodea to equilibrate
- Then collect the oxygen given off for 5 minutes
- Using the plunger in the syringe, carefully draw the bubbles up into the tube so that you can measure the length of the bubble.
- Record this length
- Repeat this procedure, each time moving the lamp away from the Elodea and measuring the distance
- Throughout the experiment regularly checking the thermometer to make sure the temperature of the water remains constant.
Results
After repeating my experiment 3 times I worked out an average of my results so that I would be able to see the relationship between the light intensity and the rate of photosynthesis more clearly. As you can see from the table above and the graphs, which I have drawn, there weren’t really any anomalies. Although in test 2 the length of the bubble was much smaller when the light intensity was the greatest it still followed the same pattern that as the light intensity decreased so did the length of the bubble produced and therefore the amount of oxygen produced.
Conclusion and evaluation
From the experiment that I have carried out and the results that I have obtained I have been able to prove that light is a limiting factor of photosynthesis. Each time I repeated the experiment the results were reasonably consistent and followed a general rule that as the light intensity decreased so did the length of the bubble, which shows us that the amount of oxygen produced was falling.
The graphs show me that an increase in light intensity will undoubtedly increase the rate of photosynthesis. All my graphs and my results support my hypothesis fully and backed up the prediction that I made previously that as the light intensity decreases so will the rate of photosynthesis. This is because light is needed for photosynthesis. When the light energy from the sun reaches the chloroplasts in the leaves, the chlorophyll traps the light, which then provides the energy for the chemical reactions to take place in the plant. If I had have increased the light intensity to a greater extent then at some point the rate of photosynthesis would stop escalating as the rate of photosynthesis would have been restricted by other limiting factors. Also there is a maximum rate at which a plant can photosynthesise which means that above this point the light intensity will have no effect on the rate of photosynthesis.
I made sure that I used the same amount of sodium hydrocarbonate each time so that my experiment was a fair test. In my preliminary work I did a couple of tests using different amounts of sodium hydrocarbonate each time so that I knew that there would be an excess of carbon dioxide so that it would not be a limiting factor. I left the elodea to equilibrate before I began each test to let the plant adjust to the surroundings and allow it to perform photosynthesis at a constant rate I was also extremely vigilant with the amount of time I left the elodea to equilibrate, as I knew that this would have a rather dramatic effect on my results. I put a thermometer into the water surrounding the test tube so that I was able to keep a constant temperature. It was difficult to use exactly the same amount of Elodea each time so if I did this experiment again I would consider weighing the Elodea so that my results were more accurate.
If I were to perform this experiment again then I would use a larger piece of elodea. This would mean that the amount of oxygen would have been larger and I could possibly decrease the light intensity to a greater extent and still be able to accurately measure the length of the bubble produced.
Overall I think that my experiment worked out fairly well, as my results were similar to those, which I had expected. In my experiment I was able to prove that the rate of photosynthesis is affected by the light intensity.
Bibliography
A level Biology, W.D.Phillips and T.J.Chilton, 1989
Heinemann Advanced Science: Biology, Ann Fullock, 1994
Biology for the IB Diploma, Andrew Allott, 2001
Biology for the IB Diploma, Andrew Allott, 2001
A level Biology, W.D.Phillips and T.J.Chilton, 1989
Heinemann Advanced Science: Biology, Ann Fullock, 1994
A level Biology, W.D.Phillips and T.J.Chilton, 1989
A level Biology, W.D.Phillips and T.J.Chilton, 1989
Biology for the IB Diploma, Andrew Allott, 2001
Biology for the IB Diploma, Andrew Allott, 2001
