Investigating the rate of photosynthesis

This is an example of a common experiment used to investigate light intensity and the rate of photosynthesis. This should help you understand how to work scientifically.

Aim of the experiment

To use pondweed to see how light intensity affects the rate of photosynthesis.

Method

  1. Set up the apparatus as in the diagram.
  2. Leave for five minutes for the pondweed to acclimatise to the new light intensity.
  3. Count the number of bubbles given off in one minute.
  4. Move the light 10 cm further back.
  5. Leave for five minutes for the pondweed to acclimatise again.
  6. Count the number of bubbles given off in one minute.
  7. Repeat by moving the lamp away by 10 cm intervals until 50 cm is reached.
A lamp is moved away from a beaker containing pondweed, by 10 cm intervals up to 50 cm along a ruler, in an experiment to investigate light intensity on the rate of photosynthesis.A light intensity experiment

Variables

  • Independent variable - the light intensity (how close the light is).
  • Dependant variable - the number of oxygen bubbles given off (the rate of photosynthesis).
  • Controlled variables - the size of the pondweed, the volume of water used and its temperature.

Risks

Care must be taken with glassware.

Expected results

Distance from lamp to beaker (cm)Number of bubbles given off per minute
1015
207
303
401
500

What the results mean

More oxygen bubbles are produced when the light is closer to the beaker. So a greater light intensity gives a greater rate of photosynthesis.

Evaluation

  • Your measurements are accurate if they are close to their true value.
  • Your measurements are precise if they are similar when completed again.
  • Your experiment is repeatable if you get precise measurements when it is repeated.
  • Your experiment is reproducible if others get precise measurements when they repeat it.
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