Factors Affecting Photosynthesis

This lesson covers the factors that affect the rate of photosynthesis — light intensity, carbon dioxide concentration and temperature — as required by the Edexcel GCSE Combined Science specification (1SC0). You also need to understand limiting factors and the inverse square law for light intensity.

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What Is a Limiting Factor?

A limiting factor is the factor that is in shortest supply and therefore restricts the rate of a reaction. Even if other conditions are ideal, the rate of photosynthesis cannot increase beyond the limit set by the factor in shortest supply.

At any given moment, one factor will be the limiting factor. Increasing that factor will increase the rate of photosynthesis — but only up to a point, at which a different factor becomes limiting.

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Factor 1: Light Intensity

Light provides the energy for photosynthesis. As light intensity increases:

• The rate of photosynthesis increases (more energy available).
• Eventually the rate plateaus — light is no longer the limiting factor; another factor (such as CO₂ concentration or temperature) takes over.

The Inverse Square Law

Light intensity follows the inverse square law when you move a lamp further from a plant:

$\text{Light intensity} \propto \frac{1}{d^2}$Light intensity∝d21​

Where d is the distance from the light source.

Distance (d)	Relative light intensity (1/d²)
10 cm	1/100 = 0.0100
20 cm	1/400 = 0.0025
30 cm	1/900 = 0.0011
40 cm	1/1600 = 0.000625

Exam Tip: If the distance doubles, the light intensity decreases by a factor of 4 (not 2). This is a favourite calculation question. Always square the distance.

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Factor 2: Carbon Dioxide Concentration

CO₂ is a raw material for photosynthesis.

• Increasing CO₂ concentration increases the rate of photosynthesis — more CO₂ molecules are available for the reaction.
• Eventually the rate plateaus because another factor becomes limiting (e.g. light intensity or temperature).
• In a commercial greenhouse, farmers may add extra CO₂ (for example by using a gas burner) to increase crop yields.

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Factor 3: Temperature

Temperature affects the enzymes that catalyse the reactions of photosynthesis.

• As temperature increases, molecules have more kinetic energy and collide more frequently with the enzyme active site, so the rate increases.
• At the optimum temperature (roughly 25–35 °C for most plants) the rate is at its maximum.
• Above the optimum, the enzyme's active site begins to change shape (denature), and the rate drops sharply.

graph LR
    A["Low temperature"] -->|"Rate increases"| B["Optimum temperature ~25–35 °C"]
    B -->|"Rate decreases rapidly"| C["High temperature — enzymes denature"]

Exam Tip: Do not say enzymes are "killed" at high temperatures — they are proteins, not living things. The correct term is denatured: the active site changes shape so the substrate can no longer fit.

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Interpreting Graphs of Limiting Factors

You may be given a graph showing rate of photosynthesis against light intensity at different CO₂ concentrations or temperatures. The key features are:

1. Rising section — light intensity is the limiting factor; increasing light increases the rate.
2. Plateau — light is no longer limiting; another factor (CO₂ or temperature) is now limiting.
3. Higher CO₂ or temperature shifts the plateau upward — showing the other factor was limiting.

Graph feature	What it tells you
Steep upward slope	Light intensity is the limiting factor
Flat plateau	Another factor is now limiting
Higher plateau at higher CO₂	CO₂ was the limiting factor at the first plateau
Rate drops at very high temperature	Enzymes are denaturing

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Applying Limiting Factors in Greenhouses

Commercial growers control conditions to maximise photosynthesis and crop yield:

Factor controlled	Method	Benefit
Light	Artificial lighting	Extends growing period / increases light intensity
CO₂	Gas burners or CO₂ enrichment systems	Increases raw material for photosynthesis
Temperature	Heaters and ventilation	Keeps enzymes at or near optimum

graph TD
    A["Greenhouse conditions"] --> B["Artificial light — increases light intensity"]
    A --> C["CO₂ enrichment — removes CO₂ as limiting factor"]
    A --> D["Temperature control — enzymes at optimum"]
    B --> E["Maximum rate of photosynthesis"]
    C --> E
    D --> E
    E --> F["Higher crop yield"]

Growers must balance the cost of providing extra light, CO₂ and heat against the extra income from increased yields.

Exam Tip: In extended-answer questions about greenhouses, always mention cost–benefit analysis: "the farmer must weigh the cost of providing the extra factor against the increase in crop yield and profit."

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Worked Example — Inverse Square Law

A lamp is placed 10 cm from a beaker of pondweed. The student counts 60 bubbles per minute. The lamp is then moved to 20 cm.

Calculate the expected light intensity relative to the first position:

$\text{At 10 cm: } \frac{1}{10^2} = \frac{1}{100}$At 10 cm: 1021​=1001​

$\text{At 20 cm: } \frac{1}{20^2} = \frac{1}{400}$At 20 cm: 2021​=4001​

The light intensity at 20 cm is ¼ of the intensity at 10 cm.

If the rate of photosynthesis is directly proportional to light intensity (i.e. light is still the limiting factor), the student would expect approximately 60 ÷ 4 = 15 bubbles per minute.

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Summary

• The three main factors affecting the rate of photosynthesis are light intensity, CO₂ concentration and temperature.
• A limiting factor is the factor in shortest supply that restricts the rate of the reaction.
• Light intensity follows the inverse square law: intensity ∝ 1/d².
• Increasing CO₂ increases the rate until another factor becomes limiting.
• Temperature affects enzyme activity; above the optimum, enzymes denature.
• Greenhouses allow growers to control all three factors to maximise yield.
• Graphs of limiting factors show a rising section (factor is limiting) followed by a plateau (different factor is limiting).

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Worked Example — Inverse Square Law with a Rate Calculation

A class sets up pondweed at 15 cm from a lamp and counts 80 bubbles per minute. They move the lamp to 45 cm and assume CO₂ and temperature remain non-limiting.

Step 1 — Calculate relative light intensities:

• At 15 cm: 1/d² = 1/225 = 0.00444
• At 45 cm: 1/d² = 1/2025 = 0.000494