Photosynthesis (Cambridge O Level Biology)

A student investigated the effect of light intensity on the rate of photosynthesis.

1. They placed a 10 cm piece of Elodea pondweed in a beaker of water placed 10 cm away from a lamp.
2. A glass tank with water was placed between the lamp and the beaker containing the pondweed.
3. The pondweed was left for 5 minutes before the number of bubbles were counted and recorded over a period of 2 minutes.
4. A thermometer was used to monitor the temperature throughout the experiment.
5. Step 3 was repeated when the lamp was placed at different distances from the pondweed.
6. The experiment was repeated once more.

Fig. 1 shows the experimental setup.

Fig. 1

Fig. 2 shows the results for the two repeats that the student wrote in their notebook.

Fig. 2

Prepare a suitable results table to present the data.

Your table should include:

• Distance from the lamp
• The rate of photosynthesis in bubbles min^-1 

[4]

State a conclusion for the results.

[1]

Calculate the difference in the mean rate of photosynthesis when the lamp was moved from a distance of 10 cm to 50 cm away from the pondweed.

State the independent variable for this investigation.

[1]

State two variables that should be kept constant in this investigation.

[2]

Suggest an improvement for determining the rate of photosynthesis in this investigation.

[1]

Describe how this experiment could be adapted to investigate the effect of carbon dioxide on the rate of photosynthesis.