Limiting factors of photosynthesis
CIE A-Level Biology· Unit 14: Photosynthesis, 14.1 Energy and Respiration· 45 min read
1. 1. Defining Limiting Factors★★☆☆☆⏱ 10 min
Photosynthesis depends on multiple interacting external factors: light intensity, carbon dioxide concentration, and temperature. At any given time, the rate of photosynthesis is restricted by the factor that is in shortest supply, even if all other factors are increased to optimal levels.
Limiting factor
Any factor that slows the maximum possible rate of photosynthesis when its value is sub-optimal, regardless of changes to other required factors for the reaction.
Example:
If carbon dioxide concentration is low, increasing light intensity will not increase the rate of photosynthesis, so CO₂ is the limiting factor.
A plant is kept at 25°C, 0.01% atmospheric CO₂, and increasing light intensity from 200 to 400 µmol m⁻² s⁻¹ does not change the rate of photosynthesis. What is the limiting factor here?
- 1
- If increasing a factor does not change the reaction rate, that factor is not the limiting factor. The limiting factor must be another sub-optimal required factor.
- 2
- 25°C is near the optimum temperature for most C3 plant photosynthetic enzymes, so temperature is not limiting here.
- 3
- 0.01% CO₂ is far lower than the ~0.1% CO₂ concentration needed for maximum photosynthesis rate.
- 4
Conclusion: Carbon dioxide concentration is the limiting factor.
2. 2. Interpreting Limiting Factor Graphs★★★☆☆⏱ 15 min
A common exam question asks you to identify limiting factors at different points on a graph that plots photosynthesis rate against one changing variable. The shape of the curve directly indicates which factor is limiting at any point.
A graph plots photosynthesis rate against light intensity, at two different CO₂ concentrations with temperature held constant at 25°C. Identify the limiting factor at: (a) point A on the low CO₂ curve, before the plateau; (b) point B on the low CO₂ curve at the plateau; (c) point C on the high CO₂ curve, after the plateau of the low CO₂ curve.
- 1
(a) At point A, increasing light intensity causes a proportional increase in photosynthesis rate. This means light intensity is the limiting factor here, because more light enables faster production of ATP and reduced NADP for the Calvin cycle.
- 2
(b) At point B, increasing light intensity no longer increases rate. The plateau occurs because carbon dioxide concentration (the factor held low here) is now limiting, so CO₂ is the limiting factor.
- 3
(c) At point C, light intensity and CO₂ are both high enough that they are not limiting. The new plateau forms because temperature is held constant at 25°C, so temperature is the limiting factor here.
3. 3. Mechanisms of Limitation★★★☆☆⏱ 15 min
Each of the three main limiting factors acts on a specific step of photosynthesis:
Light intensity: Light drives the light-dependent reaction to split water and produce ATP and reduced NADP. Low light means insufficient of these products for the Calvin cycle, slowing overall rate.
Carbon dioxide concentration: CO₂ is the raw material for carbon fixation in the Calvin cycle. Low CO₂ means less GP can be produced from RuBP, slowing sugar production.
Temperature: All Calvin cycle reactions are enzyme-catalysed. Below optimum, low kinetic energy reduces reaction rate; above optimum, enzyme denaturation slows or stops the reaction.
Explain why carbon dioxide concentration is usually the main limiting factor for field plants on a hot sunny day.
- 1
- On a hot sunny day, light intensity is very high, so light is not limiting.
- 2
- Temperature is also near or just above optimum for most plant enzymes, so temperature is not the main limiting factor.
- 3
- Plants close their stomata on hot days to reduce water loss, which reduces CO₂ diffusion into the leaf. Natural atmospheric CO₂ concentration is only ~0.04%, so CO₂ becomes the limiting factor.
4. 4. Applications to Commercial Crop Production★★★☆☆⏱ 10 min
Commercial glasshouse growers deliberately manipulate limiting factors to maximise photosynthesis rate and crop yield, while balancing the extra yield against the cost of modifying conditions.
A tomato grower finds yield stops increasing when CO₂ reaches 0.1% in their glasshouse. Why should they not increase CO₂ beyond this level?
- 1
- Once CO₂ reaches 0.1%, another factor (such as light intensity or temperature) becomes limiting, so further CO₂ increases do not raise photosynthesis rate or yield.
- 2
- Adding extra CO₂ beyond this point adds unnecessary cost, reducing the grower's profit with no biological or yield benefit.
5. Common Pitfalls
Wrong move:
Claiming light is the limiting factor at the plateau of a light intensity vs rate graph.
Why:
If increasing light does not change rate, light cannot be limiting. The plateau forms because another factor is restricting maximum rate.
Correct move:
On the plateau, the limiting factor is the factor that is held constant in the experiment, such as CO₂ or temperature.
Wrong move:
Stating high temperature can never be a limiting factor on a hot day.
Why:
Temperatures above the optimum cause enzyme denaturation, which slows photosynthesis even when other factors are optimal.
Correct move:
Check if temperature is above or below optimum: high temperatures can limit rate if enzymes denature.
Wrong move:
Claiming increasing any limiting factor will always increase yield regardless of cost.
Why:
Rate only increases until another factor becomes limiting, and extra inputs add cost with no benefit beyond that point.
Correct move:
In commercial contexts, always explain the balance between increased yield and the cost of adding inputs.
Wrong move:
Misattributing the mechanism of limitation, e.g. claiming low temperature limits photosynthesis because there is less light.
Why:
Each factor acts on a specific step of photosynthesis; mixing up mechanisms loses marks in exams.
Correct move:
Remember: light limits ATP/NADPH production, CO₂ limits carbon fixation, temperature limits enzyme activity.
6. Quick Reference Cheatsheet
Factor | Mechanism of Limitation | Common Context |
|---|---|---|
Light intensity | Limits ATP/reduced NADP production | Dawn/dusk, cloudy days |
Carbon dioxide | Limits carbon fixation in Calvin cycle | Hot sunny days, unmodified glasshouses |
Temperature (below optimum) | Low enzyme kinetic energy | Cool spring days, unheated glasshouses |
Temperature (above optimum) | Enzyme denaturation | Very hot days, overheated glasshouses |
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 · 22
Identify limiting factors on a rate graph
- 2021 · 12
Explain limiting factors in glasshouses
- 2023 · 23
Describe temperature's effect on rate
Going deeper
What's Next
Understanding limiting factors of photosynthesis is a core foundation for learning how plants adapt to different environments, and for understanding crop productivity, a common extended response theme in CIE A-Level Biology exams. This concept links closely to stomatal function for gas exchange, where plants balance water loss and carbon dioxide uptake. It also connects directly to the steps of the Calvin cycle, since each limiting factor acts on a specific stage of photosynthesis. Mastering limiting factor graph interpretation will prepare you for data analysis questions that appear consistently across Paper 2 and Paper 4.