Deepak carried out an experiment to see the effect of increasing the amount of carbon dioxide available to a pondweed (a submerged plant) on the rate of photosynthesis carried out by the plant. The readings of the experiment are potted in the graph.

Answer these questions.
  1. How do you think he measured the rate of photosynthesis?
  2. How do you think he increased the amount of carbon dioxide in the water?
  3. Since temperature causes a change in the rate of photosynthesis, he kept the temperature constant throughout the experiment. Suggest one more factor that he should have kept constant.
  4. Why did the graph increase initially?
  5. Why do you think it ultimately levelled off?


1. The rate of photosynthesis can be measured by -
  • by measuring the COuptake or the O2 output or
  • by measuring the gas exchange
  • by measuring the amount of increase in dry weight of the photosynthesis plant or a specific plant part.
  • by using Ganong’s Photosynthometer
He must have used any one of the above mentioned methods.

2. The amount of CO2 can be increased the water by adding the chemical potassium hydrogencarbonate. It acts as a source of CO2. The amount of CO2 can be increased by increasing the amount of this chemical.

3. Another factor which he should keep constant is the light intensity.

4. The graph increased initially because the rate of photosynthesis increases with the increase in the amount of carbon dioxide.

5. The graph levelled off because the increase in the rate of photosynthesis with the increase in carbon dioxide, occurs only upto a point. After reaching that point, increase in the concentration of carbon dioxide has no effect on the rate of photosynthesis.

Living Science (2020, 2021)

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