Explain how limiting factors change photosynthesis rate
Explain the effects of light intensity, carbon dioxide concentration and temperature on rate, including plateaus and temperature limits.
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
What happens to the rate when light intensity rises while light is limiting?
What you will learn
- Interpret light and carbon dioxide response curves
- Explain temperature effects through enzyme activity and other limits
Build the explanation
Increasing light intensity can raise photosynthesis rate when light is limiting; the rate then plateaus when another factor limits. Increasing carbon dioxide concentration can also raise rate when carbon dioxide is limiting, until a different condition limits. Temperature affects enzyme-controlled reactions: warming can increase rate up to a range in which photosynthesis is fastest, but excessive heat disrupts enzyme function and can reduce rate. Effects depend on the other factors and the range tested.
Apply it carefully
Do not claim that rate always rises with temperature or that any plateau identifies one specific cause. Describe the tested range and separate the observed response from a biological explanation. Comparisons need other relevant factors controlled.
Pause and explain
Why can very high temperature reduce photosynthesis rate?
Worked example
Why can increasing light intensity stop increasing photosynthesis rate?
Show the worked solution
- A different requirement may become limiting.
- The curve plateaus when another factor restricts the rate.
Answer Another factor, such as carbon dioxide concentration or temperature, may become limiting.
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Explain the expected response when light intensity increases from low levels and then continues to rise beyond the first plateau.
Give me a hint
Describe both phases and the role of another limiting factor.
Compare my reasoning
- At low intensity, more light can raise rate if light is limiting.
- At a plateau, another factor limits; further light alone has little effect.
When light is limiting, increasing its intensity can raise photosynthesis rate. Once the curve plateaus, another factor such as carbon dioxide concentration or temperature may limit rate, so extra light alone produces little further increase.
Look for these in your work
- Low-light increase explained
- Plateau interpreted
- Other factor qualified
A controlled trial raises carbon dioxide and rate increases, then becomes constant. Interpret the pattern.
Give me a hint
Carbon dioxide can be limiting first; a later plateau suggests another limit.
Compare my reasoning
- The initial increase is consistent with carbon dioxide being limiting.
- At the plateau, a different factor may limit; the data do not identify it without another test.
The initial increase is consistent with carbon dioxide concentration limiting the rate. At the plateau, another factor may have become limiting; a separate controlled change is needed to identify it.
Look for these in your work
- Initial factor interpreted
- New limitation inferred
- Evidence scope respected
Rate is higher at 25 °C than 10 °C but lower at 45 °C. Give a temperature-based explanation and one control needed.
Give me a hint
Relate moderate warming to enzyme activity and high temperature to enzyme function.
Compare my reasoning
- Warming from 10 °C can increase enzyme-controlled reaction rates.
- At 45 °C enzyme function may be disrupted; control light and carbon dioxide across trials.
The higher rate at 25 °C is consistent with faster enzyme-controlled reactions than at 10 °C. The lower rate at 45 °C may reflect impaired enzyme function at excessive temperature. Light intensity and carbon dioxide concentration should be controlled so they do not explain the difference.
Look for these in your work
- Both parts of curve explained
- Enzyme role stated
- Controls named
Common mistakes
- Assuming higher temperature always increases rate
- Saying a plateau proves the apparatus is broken
- Comparing trials with uncontrolled light or CO₂
What can you explain now?
What happens to the rate when light intensity rises while light is limiting?
Compare with the explanation
It increases
Afterward, another factor may limit and produce a plateau.
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Make it stick
How do light, carbon dioxide and temperature affect rate when each is limiting?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.