Rate of Photosynthesis
Revise Rate of Photosynthesis for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Light intensity, carbon dioxide and temperature are the limiting factors; the required practical uses pondweed and the inverse square law for light.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
The rate of photosynthesis is affected by four factors: light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll.
Any one of these can be the limiting factor — the factor in shortest supply that is holding the rate back. On a graph, the rate rises as a factor increases and then levels off when something else becomes limiting.
Temperature behaves differently from the others. Beyond the optimum the rate falls sharply rather than levelling off, because the enzymes controlling photosynthesis are denatured. Light intensity follows the inverse square law, so doubling the distance from a lamp quarters the intensity.
Revision notes
Limiting factors
The limiting factor is whichever factor is in shortest supply and therefore restricting the rate.
On a graph, the rate increases while a factor is limiting and levels off once it is no longer the constraint. Identifying which factor is limiting at a given point is a common exam question.
Temperature
Increasing temperature increases the rate up to an optimum, because reactions happen faster.
Beyond the optimum the rate falls sharply because the enzymes controlling photosynthesis are denatured. This produces a peaked curve, unlike the levelling-off seen for light and carbon dioxide.
The inverse square law
Light intensity is proportional to 1 ÷ distance². Doubling the distance from the lamp reduces the intensity to a quarter.
In the required practical, pondweed is placed at measured distances from a lamp and the oxygen bubbles produced per minute are counted. A glass tank of water absorbs heat so temperature stays constant.
Key points
- Light intensity, carbon dioxide, temperature and chlorophyll affect the rate.
- The limiting factor is the one in shortest supply.
- The rate levels off when a factor is no longer limiting.
- Beyond the optimum temperature the rate falls as enzymes denature.
- Light intensity ∝ 1 ÷ distance².
- A glass tank of water keeps the temperature constant.
Worked examples
Example 1
Explain what is meant by a limiting factor. [2 marks]
Model answer
A limiting factor is the factor that is in shortest supply (1 mark) and is therefore restricting the rate of photosynthesis, so increasing it would increase the rate (1 mark).
Example 2
A lamp is moved from 10 cm to 20 cm from a plant. Explain what happens to the light intensity. [2 marks]
Model answer
Light intensity is proportional to 1 ÷ distance² (1 mark), so doubling the distance reduces the intensity to one quarter of its original value (1 mark).
Example 3
Explain why the rate of photosynthesis falls at very high temperatures. [2 marks]
Model answer
The enzymes controlling photosynthesis are denatured (1 mark), so their active sites change shape and can no longer catalyse the reactions (1 mark).
Common mistakes
Saying the rate always levels off.
It does for light and carbon dioxide, but temperature produces a peak followed by a fall.
Forgetting the inverse square relationship.
Doubling the distance quarters the intensity, not halves it.
Saying enzymes are killed at high temperature.
They are denatured — the active site changes shape.
Not controlling temperature in the practical.
A glass tank of water between lamp and plant absorbs heat so only light intensity varies.
Exam tips
- Identify which factor is limiting at each part of a graph.
- Use the inverse square law when distances change.
- Explain the temperature fall through enzyme denaturing.
- Mention the heat shield when describing the practical.
Key terms
- Limiting factor
- The factor in shortest supply that restricts the rate.
- Inverse square law
- The rule that intensity is proportional to 1 ÷ distance².
- Optimum
- The condition giving the fastest rate.
- Denatured
- Having an active site that has changed shape.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.