Rate of Photosynthesis

The rate of photosynthesis is not constant — it changes depending on the environmental conditions around the plant. Understanding the factors that affect the rate of photosynthesis and how to interpret graphs showing these effects is a key part of the Edexcel GCSE Biology (1BI0) specification.

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Limiting Factors

A limiting factor is the factor that is in shortest supply and therefore restricts the rate of a process. For photosynthesis, there are three main limiting factors:

1. Light intensity
2. Carbon dioxide (CO₂) concentration
3. Temperature

At any given time, only one factor can be the limiting factor. Increasing the supply of the limiting factor will increase the rate of photosynthesis — but only up to a point, when a different factor becomes limiting.

Exam Tip: A common 6-mark question asks you to explain how limiting factors affect the rate of photosynthesis. Structure your answer by discussing each factor separately and explain what happens when it is no longer limiting.

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Light Intensity

How Light Intensity Affects the Rate

• As light intensity increases, the rate of photosynthesis increases proportionally at first. This is because more light energy is available for the light-dependent reactions in the chloroplasts.
• Eventually, the rate levels off (plateaus). Increasing light intensity further has no effect because another factor (such as CO₂ concentration or temperature) has become the limiting factor.

Graph: Rate of Photosynthesis vs Light Intensity

The graph shows a straight-line increase at first, which then curves and flattens into a plateau:

• Rising section: Light intensity is the limiting factor. More light = faster photosynthesis.
• Plateau section: Light is no longer limiting. Another factor (e.g., CO₂ or temperature) is now limiting the rate.

Exam Tip: When describing a graph, always refer to specific regions. Say: "As light intensity increases from 0 to X, the rate increases because light is the limiting factor. Beyond X, the rate plateaus because another factor such as CO₂ concentration becomes limiting."

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The Inverse Square Law

When using a lamp as a light source in experiments, the light intensity reaching the plant depends on the distance between the lamp and the plant. The relationship follows the inverse square law:

Light intensity ∝ 1/d²

Where d = distance from the light source.

This means:

Distance (d)	Relative Light Intensity (1/d²)
10 cm	1/100 = 0.01
20 cm	1/400 = 0.0025
30 cm	1/900 ≈ 0.0011
50 cm	1/2500 = 0.0004

If you double the distance, the light intensity drops to one-quarter (not one-half).

If you halve the distance, the light intensity quadruples (increases by a factor of 4).

Exam Tip: In calculations, always square the distance. If a question asks for relative light intensity at 20 cm, calculate 1/20² = 1/400. You do not need to convert to actual lux values at GCSE unless told otherwise.

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Carbon Dioxide Concentration

How CO₂ Concentration Affects the Rate

• As CO₂ concentration increases, the rate of photosynthesis increases because more CO₂ is available for the light-independent reactions (carbon fixation in the stroma).
• Eventually, the rate levels off. At this point, CO₂ is no longer the limiting factor — either light intensity or temperature is now limiting.

Graph: Rate of Photosynthesis vs CO₂ Concentration

The shape is similar to the light intensity graph:

• Rising section: CO₂ is the limiting factor.
• Plateau: Another factor (light or temperature) is now limiting.

In normal atmospheric conditions, CO₂ concentration is approximately 0.04% (400 ppm). This is often below the optimum for photosynthesis, which is why increasing CO₂ can boost the rate significantly.

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Temperature

How Temperature Affects the Rate

Temperature affects photosynthesis differently from light intensity and CO₂:

• As temperature increases from low values, the rate of photosynthesis increases. This is because the kinetic energy of molecules increases, so enzyme–substrate collisions occur more frequently and with more energy, increasing the rate of enzyme-catalysed reactions.
• The rate reaches an optimum at approximately 25–30°C for most plants.
• Beyond the optimum, the rate drops sharply. At high temperatures, the enzymes involved in photosynthesis denature — their active sites change shape and can no longer bind to substrates. This is an irreversible change.

Graph: Rate of Photosynthesis vs Temperature

This graph has a distinctly different shape:

• Rising section: Temperature is limiting. Higher temperature increases molecular movement and enzyme activity.
• Peak (optimum): The rate is at its maximum. All enzymes are working at peak efficiency.
• Falling section: Enzymes begin to denature. The rate drops rapidly as more and more enzyme molecules lose their functional shape.

Exam Tip: The temperature graph is NOT symmetrical — the drop after the optimum is much steeper than the rise before it. This is because denaturation is rapid and irreversible. Always describe the drop as "sharp" or "rapid" and explain it is due to enzymes denaturing.

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Interpreting Limiting Factor Graphs

Examiners frequently present graphs that combine multiple factors. Here is how to interpret them:

Combined Graph: Two Lines at Different CO₂ Levels

If you see a graph of rate vs light intensity with two curves — one at 0.04% CO₂ and one at 0.1% CO₂:

• Both curves rise initially (light is limiting for both)
• The curve at higher CO₂ reaches a higher plateau — because when light is no longer limiting, the higher CO₂ allows a faster rate
• The curve at lower CO₂ plateaus at a lower rate — CO₂ becomes limiting sooner

How to Identify the Limiting Factor on a Graph

• If the line is rising: the factor on the x-axis IS the limiting factor
• If the line is flat (plateau): the factor on the x-axis is NOT limiting — something ELSE is limiting
• If there are two lines at different levels of a second factor: the difference between the lines tells you that the second factor is having an effect

Exam Tip: When a graph question asks "explain why the rate stops increasing", always name the factor that has become limiting and explain why it prevents further increases. For example: "The rate plateaus because CO₂ concentration has become the limiting factor — there is not enough CO₂ for the light-independent reactions to proceed any faster."

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Commercial Applications: Greenhouses

Farmers and commercial growers use knowledge of limiting factors to maximise crop yields in greenhouses:

Artificial Lighting

• Lamps provide light when natural sunlight is insufficient (e.g., during winter or at night)
• This ensures photosynthesis can continue beyond normal daylight hours
• Some greenhouses use specific wavelengths (red and blue light) that chlorophyll absorbs most efficiently

Heating

• Greenhouses trap heat, raising the temperature closer to the optimum for photosynthesis
• In colder months, heaters maintain the temperature at around 25–30°C
• However, growers must balance heating costs against the increased yield — there is an economic optimum