Photosynthesis

• The aim of the photosynthesis practical is to investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed, measured by counting the number of O2 bubbles formed per minute.
• The equipment for the photosynthesis practical includes a boiling tube, freshly cut 10 cm piece of pondweed, a light source, a ruler, a test tube rack, a stopwatch, 0.2% solution sodium hydrogen carbonate, and a glass rod.
• In the first step of the photosynthesis practical, a test tube rack containing a boiling tube is placed 10 cm away from the light source, measured using the ruler.
• The boiling tube is filled with a fixed volume of sodium hydrogen carbonate solution.
• The cut pondweed is placed into the boiling tube with the cut end at the top.
• The boiling tube is gently pushed down with a glass rod.
• The boiling tube is left to rest for 5 minutes.
• The stopwatch is started and the number of bubbles produced in one minute is counted.
• The count is repeated for each light intensity/distance, and a mean is taken.
• The photosynthesis practical involves repeating the previous steps for 3 more distances (20, 30, 40 cm) of the boiling tube from the light source.
• A graph of the rate of photosynthesis (given by the number of bubbles) against light intensity (using the inverse square law, light intensity = 1/distance 2 between pondweed and light source) is plotted.
• Temperature may also be a factor affecting the rate of photosynthesis and this is not taken account of in the photosynthesis practical.
• Bubbles may form too quickly to be counted in the photosynthesis practical.