These results tell me that an increase in light intensity does increase the rate of photosynthesis. The gradual decrease in the rate of photosynthesis (the straightening of the curve) can be linked to the other factors limiting the rate of photosynthesis. As light intensity increases, the rate is being limited by certain factors, such as carbon dioxide and temperature. These factors do not immediately limit the rate of photosynthesis, but gradually. As light intensity increases further, so the rate of photosynthesis is being limited by other factors more and more, until the rate of photosynthesis is constant, and so is almost certainly limited by another factor.
The rate increases with the light intensity because a higher light intensity involves a greater level of light energy, which can then be transferred to a special protein environment designed to convert the energy (cholorophyll). When enough energy has been gathered at a reaction centre, ATP can be made from ADP. The oxygen given off in the experiment is the by-product of this reaction, and so it is clear to see that the more light energy, the more ADP is being converted into ATP and more oxygen is produced as a result.
This area of the graph has a straight line. This shows that the rate of photosynthesis is increasing at a constant rate. This is because the light is increasing constantly. However the rate of photosynthesis decreases at the end because the chlorophyll has got enough energy but needs more carbon dioxide to combine with water to increase the rate. This part of the graph shows me that the increase in rate is proportional to the increase in light intensity (i.e. a straight line) and I can show this by taking some readings from the graph (See other Sheet)
Evaluation
I have identified two anomalous results which are highlighted on the table in red. There were errors in the experiment which could have caused this. Some human errors and some errors we can’t help. Measuring the distance from the filament to the plant each time, measuring the length of the bubble and timing are human errors
Also when the lamp is close to the plant it gives it heat energy as well. This will increase the rate of photosynthesis so our bubble will become bigger. This only really affects the plant when is close to it. This explains why my anomalies are when the lamp is close to the plant. Background light could have also increased the rate which would have caused errors.
To improve the accuracy and reliability of the results, I would use a variable wattage lamp so I could increase or decrease the light intensity without moving the lamp and causing errors. I would also place a thick piece of glass between the plant and lamp so no additional heat can get to the plant.
To provide additional evidence to support my conclusion I would measure the temperature of the pond water and try to keep it constant by adding cold or hot water. This will make it a fair test.
All these errors reduce the reliability of my results. However it is clear that all of my results on the graphs (the points) support my conlusion.
A different technique could be used to improve this investigation:
Method
- Set up apparatus as above
- Add sodium hydrogen carbonate to the water. This is put in as it acts as carbon dioxide. This makes sure that the only variable is the light.
- Starting at 50cm place the lamp in front of the beaker
- Give it two minutes to settle and adjust to the new light intensity.
- Start the stopwatch and count the number of bubbles produced in one minute.
- Repeat 5 times
- Repeat steps 4, 5 and 6 with distances 30cm, 15cm, 10cm and 5cm.
A glass wall is kept in front of the beaker so no heat from the lamp can affect the rate of Photosynthesis.
After getting all the data I would plot some of the data on a graph. I would plot the amount of bubbles with the light intensity. Then I could find out how light intensity affects Elodea with greater accuracy.
To get extra information about the rate of photosynthesis, I could try to relate some of the other limiting factors to the same experiment, as well as investigating them on their own. It could also be worthwhile to explore the effects of coloured lights on the rate of photosynthesis, which could lead to other experiments such as whether or not other types of light, such as fluorescent lights or halogen lights, would have a different effect on the rate of photosynthesis.
