Investigation into the effect of Light intensity on the rate of photosynthesis

To investigate the effect of light intensity on the rate of photosynthesis.
Elodea (pondweed) is used because as it undergoes photosynthesis it will produce gas bubble which can be seen underwater. The bubbles being produced can be counted per minute to find rate.
A graduated cylinder is taken and ¾ filled with water, a set amount of sodium hydrogen carbonate is added (to maintain carbon dioxide levels) and placed in a water bath to maintain temperature. A stalk of elodea is removed from the bunch and cut at an angle along the stalk using a scalpel to cut through an air sac (cut away from the body for safety). The elodea is then weighed down with a paper clip and placed in the graduated cylinder.
• The graduated cylinder is placed 1m from the lamp and the plant is given two minutes to adjust to the light intensity (needs time to adjust to changing environment).
• The number of bubbles produced per min is then counted and recorded. The graduated cylinder is then moved 10cm closer to the lamp and given two mins to adjust to the light intensity.
• The number of bubbles produced per min is then counted and recorded, this is method is then repeated until the plant reaches 10cm from the lamp.
Result: As the light intensity increases (with a shortened distance) the rate of photosynthesis increases. The rate does level off when photosynthesis reaches its optimum (due to light saturation).