The Rate of Photosynthesis

The rate of photosynthesis is not constant — it changes depending on environmental conditions. Understanding the factors that affect the rate and the concept of limiting factors is a key part of the AQA GCSE Combined Science Trilogy specification (8464). This lesson explores each factor, explains how to interpret graphs, and introduces the inverse square law for light intensity.

---

What Is the Rate of Photosynthesis?

The rate of photosynthesis measures how quickly a plant converts carbon dioxide and water into glucose and oxygen. It can be quantified by:

• Volume of oxygen produced per unit time (e.g. cm³/min)
• Volume of carbon dioxide absorbed per unit time
• Increase in biomass (dry mass) over a period

A higher rate of photosynthesis means the plant is producing more glucose (and therefore growing faster) in a given time.

---

Limiting Factors

A limiting factor is the factor that is in the shortest supply at any given moment and therefore directly controls the rate of a reaction. Even if other conditions are ideal, the rate cannot increase beyond the point set by the limiting factor.

The three main limiting factors for photosynthesis are:

1. Light intensity
2. Carbon dioxide concentration
3. Temperature

graph TD
    A["Rate of Photosynthesis"] --> B["Light Intensity"]
    A --> C["CO₂ Concentration"]
    A --> D["Temperature"]
    B --> B1["More light → faster rate, up to a plateau"]
    C --> C1["More CO₂ → faster rate, up to a plateau"]
    D --> D1["Higher temp → faster enzymes, up to an optimum"]
    D --> D2["Beyond optimum → enzymes denature → rate drops"]

Exam Tip: Limiting factors is one of the most frequently examined concepts in AQA 8464 Bioenergetics. You must be able to define a limiting factor, identify which factor is limiting from a graph, and explain why the rate plateaus.

---

Factor 1: Light Intensity

Light provides the energy that drives photosynthesis. As light intensity increases, the rate of photosynthesis increases proportionally — up to a point.

Light Intensity	Effect on Rate	Explanation
Very low / dark	No or very slow photosynthesis	Insufficient energy to drive the reaction
Increasing	Rate increases proportionally	More light energy available for chlorophyll to absorb
High (plateau)	Rate levels off	Light is no longer limiting — CO₂ or temperature is now the limiting factor

The Inverse Square Law

When you use a lamp in an experiment, light intensity is inversely proportional to the square of the distance from the light source:

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

where $d$d is the distance from the lamp.

Worked Example:

A student places a lamp 10 cm from a piece of pondweed, then moves it to 20 cm.

$$\text{At 10 cm: relative light intensity} = \frac{1}{10^2} = \frac{1}{100} = 0.01$$At 10 cm: relative light intensity=1021​=1001​=0.01 $$\text{At 20 cm: relative light intensity} = \frac{1}{20^2} = \frac{1}{400} = 0.0025$$At 20 cm: relative light intensity=2021​=4001​=0.0025

The light intensity at 20 cm is one-quarter of the intensity at 10 cm (because 20 = 2 × 10, and $2^2 = 4$22=4).

Exam Tip: In calculations, always show your working. Write the formula, substitute values, and state the conclusion. Marks are awarded for the method even if your final answer is wrong.

---

Factor 2: Carbon Dioxide Concentration

Carbon dioxide is one of the raw materials for photosynthesis. As CO₂ concentration increases, the rate of photosynthesis increases — until it plateaus.

At the plateau, another factor (often light intensity or temperature) has become limiting.

In the atmosphere, CO₂ is present at roughly 0.04% — this is often the limiting factor for plants growing outdoors in bright light.

Application: Greenhouses

Commercial growers increase CO₂ concentration inside greenhouses (sometimes to 0.1% or higher) to remove it as a limiting factor and maximise crop yields.

---

Factor 3: Temperature

Temperature affects the rate because photosynthesis involves enzymes. As temperature increases:

• Enzyme activity increases (molecules have more kinetic energy, so collisions are more frequent and energetic).
• The rate rises until the optimum temperature (typically around 25–30 °C for many plants).
• Beyond the optimum, enzymes begin to denature — their active sites change shape and can no longer bind to substrates.
• The rate drops sharply.

Temperature Range	Effect	Reason
Below optimum	Rate increases with temperature	More kinetic energy → more successful enzyme–substrate collisions
At optimum (~25–30 °C)	Maximum rate	Enzymes working at peak efficiency
Above optimum	Rate decreases rapidly	Enzymes denature — active sites change shape permanently

---

Interpreting Graphs

You may be given a graph showing rate of photosynthesis against one factor (e.g. light intensity) at different levels of another factor (e.g. two different CO₂ concentrations).

Key things to look for:

1. Rising portion — the factor on the x-axis is the limiting factor.
2. Plateau — another factor has become limiting.
3. Higher plateau on a second curve — the second factor was increased, showing it was previously limiting.

graph LR
    A["Graph: Rate vs Light Intensity"] --> B["Line rises — light is limiting"]
    B --> C["Line plateaus — CO₂ or temp is now limiting"]
    C --> D["Second curve with higher CO₂ has higher plateau"]
    D --> E["Proves CO₂ was the factor that became limiting"]

---

Common Mistakes

Mistake	Correction
Saying the rate "stops" at the plateau	The rate levels off (stays constant) — photosynthesis does not stop
Saying enzymes are "killed" at high temperatures	Enzymes are denatured — they are proteins, not living things
Forgetting to square the distance in inverse square law	Light intensity $\propto 1/d^2$∝1/d2, not $1/d$1/d
Confusing limiting factor with "most important factor"	A limiting factor is the one in shortest supply at that moment, not necessarily the most important overall

---

Summary