Limiting Factors of Photosynthesis

Building on the previous lessons, this lesson takes a deeper look at how light intensity, carbon dioxide concentration and temperature interact as limiting factors. You will also explore how commercial growers use this knowledge to maximise crop production. This is a frequently tested area in the AQA GCSE Combined Science Trilogy (8464) exam.

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Recap: What Is a Limiting Factor?

A limiting factor is the factor that is in the shortest supply at a given time, and so directly controls the rate of photosynthesis. Increasing this factor will increase the rate — until a different factor becomes limiting instead.

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Light Intensity as a Limiting Factor

At low light intensities, light is the limiting factor. Increasing light intensity increases the rate of photosynthesis proportionally.

At the plateau on a graph of rate vs light intensity, light is no longer limiting. Either CO₂ concentration or temperature is now the factor preventing a further increase.

Graph Description

graph LR
    A["Low light"] -->|"Rate rises linearly"| B["Moderate light"]
    B -->|"Rate increase slows"| C["High light — plateau"]
    C --> D["Another factor is now limiting"]

On the graph:

• The gradient of the rising portion tells you how strongly light intensity affects the rate.
• The plateau level is determined by whichever other factor is limiting.

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CO₂ Concentration as a Limiting Factor

Carbon dioxide is a reactant in photosynthesis. In the atmosphere, CO₂ is present at about 0.04%, which is often the limiting factor in bright light.

• Increasing CO₂ concentration increases the rate — up to a new plateau.
• At the new plateau, either light intensity or temperature has become limiting.

Interpreting a Graph with Two CO₂ Levels

If you plot rate vs light intensity at two CO₂ concentrations (e.g. 0.04% and 0.1%):

Feature	Explanation
Both lines rise at low light	Light is limiting for both
The 0.04% CO₂ line plateaus first	CO₂ becomes limiting at a lower rate
The 0.1% CO₂ line reaches a higher plateau	More CO₂ available, so the rate can continue increasing until temperature or light becomes limiting
Both lines start from the same point at zero light	No photosynthesis in the dark regardless of CO₂

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Temperature as a Limiting Factor

Temperature affects enzyme activity. The enzymes involved in photosynthesis (such as RuBisCO) have an optimum temperature.

$$\text{As temperature increases} \rightarrow \text{kinetic energy increases} \rightarrow \text{more enzyme–substrate collisions} \rightarrow \text{rate increases}$$As temperature increases→kinetic energy increases→more enzyme–substrate collisions→rate increases $$\text{Above the optimum} \rightarrow \text{enzymes denature} \rightarrow \text{active site changes shape} \rightarrow \text{rate drops sharply}$$Above the optimum→enzymes denature→active site changes shape→rate drops sharply

Temperature	Effect on Rate
0–10 °C	Very slow — low kinetic energy
10–25 °C	Rate increases as kinetic energy rises
~25–30 °C	Optimum — maximum rate
>30–40 °C	Rate falls — enzymes begin to denature
>45 °C	Photosynthesis stops — enzymes fully denatured

Exam Tip: The temperature graph is not symmetrical. The rise is gradual, but the drop above the optimum is steep. This is because denaturation is a rapid, often irreversible process.

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How the Three Factors Interact

In reality, all three factors act simultaneously. At any given moment, one factor is limiting the rate. Change that factor and the rate increases — until a different factor becomes the new bottleneck.

flowchart TD
    A["Start: Low light, normal CO₂, 20 °C"] --> B{"Is light the limiting factor?"}
    B -->|"Yes"| C["Increase light intensity → rate increases"]
    C --> D{"Is CO₂ now limiting?"}
    D -->|"Yes"| E["Increase CO₂ → rate increases"]
    E --> F{"Is temperature now limiting?"}
    F -->|"Yes"| G["Increase temperature to optimum → rate increases"]
    G --> H["Maximum rate achieved under these conditions"]
    B -->|"No"| D
    D -->|"No"| F

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Application: Greenhouses

Commercial growers use their understanding of limiting factors to create optimal conditions for plant growth inside greenhouses:

Factor	How It Is Controlled	Effect
Light	Artificial lighting (especially in winter or at night)	Ensures light is not limiting
CO₂	CO₂ enrichment systems (e.g. burning paraffin heaters, piping in CO₂)	Removes CO₂ as a limiting factor
Temperature	Heating in winter; ventilation in summer	Keeps temperature near the optimum (~25 °C)
Water	Automatic irrigation systems	Ensures water supply is not limiting
Minerals	Fertilisers in the soil or hydroponic solution	Provides nitrates, phosphates and other minerals for growth

Economic Considerations

There is a cost–benefit balance. Adding artificial lighting, heating and CO₂ costs money. The grower must ensure the increase in crop yield (and revenue) outweighs the extra costs. Beyond a certain point, adding more of a factor gives diminishing returns, making it uneconomical.

Exam Tip (AQA 8464): You may be asked to explain why a grower would not increase CO₂ to extremely high levels. The answer is economic: the additional yield does not justify the additional cost. Always link your answer to the idea of diminishing returns and cost–benefit analysis.

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Worked Example: Interpreting a Multi-Factor Graph

A graph shows rate of photosynthesis vs light intensity for a plant at:

• 0.04% CO₂ and 20 °C (Curve A)
• 0.1% CO₂ and 20 °C (Curve B)
• 0.1% CO₂ and 30 °C (Curve C)

Observations:

1. All three curves start at the origin (no photosynthesis in the dark).
2. Curve A plateaus at the lowest rate — CO₂ is limiting at 0.04%.
3. Curve B plateaus at a higher rate — more CO₂ removes it as a limiting factor, but now temperature is limiting.
4. Curve C has the highest plateau — both CO₂ and temperature have been increased, so only light limits the rate at low light, and at high light there is an even higher plateau.

Conclusion: Each factor limits the rate at different stages. To maximise the rate, you must optimise all three factors.

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Common Mistakes