By the end of this lesson
- Explain effects of light, carbon dioxide and temperature on photosynthesis rate.
- Identify a limiting factor from graphs and comparisons.
- Predict interactions among factors.
BIG QUESTION
Why can adding more light stop increasing photosynthesis even when the lamp becomes much brighter?
The rate responds to the factor in shortest effective supply; once that constraint is relieved, another factor limits the pathway.
A limiting factor controls the current rate
A limiting factorThe factor in shortest supply, such as light intensity, carbon dioxide concentration or temperature, that stops the rate of a process from increasing. is a condition in shortest effective supply, restricting the rate. Increasing it raises rate until another factor becomes limiting.
A plateau does not mean light or carbon dioxide has stopped being required; it means extra supply cannot be used while another constraint remains.
Light and carbon dioxide usually produce saturation curves
At low light intensity, more photons absorbed increases the light-dependent reactions. At high intensity, the curve plateaus when carbon dioxide, temperature or biochemical capacity limits rate.
Increasing carbon dioxide provides more substrate for carbon fixation until another factor limits. Plants can also close stomata under water stress, indirectly restricting carbon dioxide entry.
Temperature produces an optimum curve
Rising temperature increases kinetic energy and enzyme-controlled reaction rates up to an optimum. Above it, enzymes lose functional shape and stomatal closure may reduce carbon dioxide entry.
Temperature therefore differs from light and carbon dioxide: a sufficiently high value can reduce rate rather than merely produce a plateau.
Factors interact
The limiting factor can change with conditions. At low light, raising carbon dioxide may have little effect; at high light, the same carbon-dioxide increase may raise rate substantially.
Commercial growers weigh benefits against costs when controlling light, carbon dioxide and temperature. Maximum biological rate is not automatically maximum economic return.
Graph skills: SLAPUK and choosing a scaleOxygen volume against light intensity: choose an even scale, not the raw data.
Expected graph patterns
Interpret the shape before naming the limiting factor.
| Factor increased | Typical pattern | Reason for plateau or fall |
|---|---|---|
| Light intensity | Rate rises then plateaus. | Another factor becomes limiting. |
| Carbon dioxide concentration | Rate rises then plateaus. | Another factor or pathway capacity becomes limiting. |
| Temperature | Rate rises to an optimum then falls. | Enzymes denature and other stress responses occur. |
| Two factors together | One curve may plateau higher than another. | Relieving a second limitation permits a higher rate. |
Language that earns precision
- limiting factor
- A factor whose shortage currently restricts the rate of a process.
- light intensity
- Light power reaching a unit area.
- saturation
- A region where further increase in one factor gives little or no rate increase.
- optimum
- The condition producing the highest measured rate.
Common misconceptions
Claim: Water can never limit photosynthesis.
Correction: Water stress can restrict photosynthesis, although the named specification factors are light, carbon dioxide and temperature.
Claim: The plateau proves light is no longer needed.
Correction: Light remains necessary; another factor now limits the rate.
Claim: Higher temperature always increases photosynthesis.
Correction: Rate falls above the optimum as enzymes and cells are disrupted.
Claim: Only one factor can limit a plant forever.
Correction: The limiting factor changes as conditions change.
Match the answer to the command word
Quote axes, trend, plateau or optimum and relevant data.
State why the factor raises rate and why another factor takes over.
Use matched light or carbon-dioxide values when comparing curves.
Recall, apply, then justify
Explain why a light-intensity curve plateaus.
Hint
The added light is no longer the shortest supply.
Suggested marking points
- Light initially limits and increasing it raises rate.
- At the plateau light is no longer limiting.
- Carbon dioxide, temperature or another capacity now limits the rate.
Explain why photosynthesis rate falls above the optimum temperature.
Hint
Use enzymes and active sites.
Suggested marking points
- High temperature disrupts interactions maintaining enzyme shape.
- Active sites change or enzymes denature.
- Fewer enzyme–substrate complexes form, reducing reaction rate.
At low light, two carbon-dioxide concentrations give the same rate. Suggest why.
Hint
Identify the factor in shortest supply.
Suggested marking points
- Light intensity is limiting.
- Additional carbon dioxide cannot be used until more light energy is available.
A greenhouse manager can add heat, light or carbon dioxide. Design an evidence-based order for testing investments.
Planning hint
Use controlled trials and marginal gains rather than assuming one universal answer.
What a strong design includes
- Measure baseline rate or yield with environmental conditions recorded.
- Alter one factor at a time over realistic ranges.
- Test interactions where single-factor curves plateau.
- Include energy and carbon-dioxide costs.
- Repeat across time and crop stages.
- Choose the intervention with the greatest reliable gain per cost, not merely the highest rate.