Effects of Light Intensity on the Rate of Photosynthesis

Aim
To work out how light intensity can affect the rate of photosynthesis.

Hypothesis
I think that the further away the light source is the smaller the rate of photosynthesis is. This is because there is a further distance for the light to each the plant in order for it to photosynthesise. Although the closer the light source is, the more photosynthesis, the light source could also kill or damage the Elodea (type of pondweed) because it could become too hot and denature its cells. But because light intensity is also a limiting factor of photosynthesis, the rate of photosynthesis could go up and up and reach its maximum level. So, if it would be shown in a graph, the graph would show a line going up and up and eventually continuing as a straight line. The straight line is due to the limiting factors of light intensity (Carbon dioxide and concentration of Chlorophyll).
Glucose + Oxygen (+ light) Carbon dioxide + Water
Safety
• Don't place hands on bulb- damage to skin may occur.

Variables
Independent: distance of light bulb
Dependant: Amount of oxygen bubbles, distance of bulb away from pondweed and the amount of water.
Control: amount of water
Method
• Place some Elodea in a boiling tube.
• Place 7cm³ of oxidised water in the boiling tube.
• Place boiling tube in test tube rack.
• Place the light source (bulb) 0cm away from the boiling tube and use a stop watch to time it for 1 minute.
• Count how many bubbles you see appearing in the water.
• Repeat experiment for other distances (10-50cm).

Conclusion
On my graph, I drew a best fit curve. The curve clearly shows that the further away the light bulb is from the Elodea, the less (average) oxygen bubbles there is and the less photosynthesis going on. This doesn't agree with my hypothesis. I was right in saying that the further away the light bulb is from the pondweed the smaller the rate of photosynthesis is. But I mentioned that the Elodea's cells could become denatured because there might be too much heat if it was placed really close to the light bulb. This shows that the light source didn't damage the pondweed as much as I thought. It did help it in way as it made the process of photosynthesis faster.

Evaluation
I think that the experiment was fair. This is because the oxygen bubbles were counted accurately and the light intensity was timed. I think that my experiment is fairly reliable because I repeated it. But to make it more reliable, I think I should do the experiment another time so the results are more reliable. Also, I would time the rate of photosynthesis for two minutes instead of one to make it even more reliable. I don't think that my experiment was accurate. This is because the timing isn't accurate by the mille-second. Also, there could have been oxygen bubbles that I never saw and never counted.