Identify photosynthesis limiting factors
Name light intensity, carbon dioxide concentration and temperature as examples of limiting factors.
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- Chloroplasts contain photosynthetic pigments, and enzymes catalyse metabolic reactions.
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Quick readiness check
Which gas concentration can limit the rate of photosynthesis?
What you will learn
- Identify the three syllabus examples of limiting factors
- Relate limiting factors to the rate of photosynthesis
Build the explanation
A limiting factor is a condition that restricts the rate of a process when other requirements are available. For photosynthesis, light intensity, carbon dioxide concentration and temperature are examples. Which factor limits rate depends on the conditions. A graph that plateaus as light increases does not by itself identify the new limit; carbon dioxide or temperature may then constrain the rate.
Apply it carefully
Distinguish a limiting factor from a raw material or product. Carbon dioxide is a substrate and can be limiting by concentration; light supplies energy; temperature affects enzyme-controlled reactions. Do not name oxygen as one of the three examples requested in this learning outcome.
Pause and explain
Does a plateau in a light-response graph prove that temperature is the limiting factor?
Worked example
Name three examples of limiting factors of photosynthesis.
Show the worked solution
- Use the environmental conditions specified in the syllabus.
- Light intensity, carbon dioxide concentration and temperature.
Answer Light intensity, carbon dioxide concentration and temperature.
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Name the three syllabus examples of limiting factors and explain why the limiting one can change during an investigation.
Give me a hint
List all three and consider a rising light-response curve.
Compare my reasoning
- The examples are light intensity, carbon dioxide concentration and temperature.
- As one factor is increased, another may become the factor restricting the rate.
Light intensity, carbon dioxide concentration and temperature are examples of limiting factors. If light is increased, carbon dioxide concentration or temperature may then restrict rate, so the limiting factor can change.
Look for these in your work
- All three named
- Changing limit explained
- No fixed factor assumed
At low light, photosynthesis rate rises as light increases. It then plateaus. What does the plateau show, and what does it not identify?
Give me a hint
Use the response pattern without guessing the new factor.
Compare my reasoning
- Under the tested conditions, more light no longer increases the measured rate.
- Another factor may be limiting, but the plateau alone does not identify which one.
The plateau shows that further light increase did not raise the measured rate under those conditions. Another factor may now be limiting, but the curve alone does not tell whether it is carbon dioxide, temperature or something else.
Look for these in your work
- Observed plateau stated
- Alternative limiting factor inferred
- Specific cause not overclaimed
A greenhouse trial increases carbon dioxide and sees no rate change at a cool temperature. Explain why carbon dioxide may not be the limiting factor in that trial.
Give me a hint
Consider the other supplied condition.
Compare my reasoning
- Temperature may be the current limiting factor.
- Adding carbon dioxide will have little effect if another required condition restricts the rate.
At the cool temperature, temperature may be limiting, so extra carbon dioxide may not increase rate. A controlled temperature change could test that possibility; the unchanged result alone should not be treated as proof without adequate controls.
Look for these in your work
- Temperature considered
- Interaction explained
- Further test suggested
Common mistakes
- Treating a plateau as proof of a particular limit
- Confusing a limiting factor with a photosynthetic product
- Ignoring that more than one factor can constrain rate
What can you explain now?
Which gas concentration can limit the rate of photosynthesis?
Compare with the explanation
Carbon dioxide concentration
Carbon dioxide is fixed during the Calvin cycle.
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Make it stick
What three factors can limit photosynthesis, and how can the limiting factor change?
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