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The rate of photosynthesis is not constant — it varies depending on environmental conditions. Understanding the factors that affect the rate of photosynthesis and the concept of limiting factors is a core part of the AQA GCSE Biology Bioenergetics topic. This lesson explores each factor in detail and explains how they interact.
The rate of photosynthesis is a measure of how quickly a plant converts carbon dioxide and water into glucose and oxygen. It can be measured in several ways:
A faster rate of photosynthesis means the plant is producing more glucose and oxygen in a given time period.
A limiting factor is the factor that is in shortest supply at any given time, and therefore directly controls the rate of a reaction. Even if other factors are plentiful, the rate cannot increase beyond the point set by the limiting factor.
The three main limiting factors for photosynthesis are:
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 point]
B --> B2[Then another factor becomes limiting]
C --> C1[More CO2 = faster rate up to a point]
C --> C2[Then another factor becomes limiting]
D --> D1[Increasing temp speeds up enzymes]
D --> D2[Too hot = enzymes denature and rate drops]
Exam Tip: Limiting factors is one of the most commonly examined concepts in Bioenergetics. Make sure you can explain what a limiting factor is, identify which factor is limiting from a graph, and explain why the rate plateaus (levels off) when another factor becomes limiting.
Light provides the energy needed for photosynthesis. As light intensity increases, the rate of photosynthesis increases — up to a point.
| Light Intensity | Effect on Rate | Explanation |
|---|---|---|
| Very low / dark | No photosynthesis (or very slow) | Not enough energy to drive the reaction |
| Increasing | Rate increases proportionally | More light energy is available to be absorbed by chlorophyll |
| High (plateau) | Rate levels off | Light is no longer the limiting factor — CO2 or temperature is now limiting |
A typical graph of light intensity vs rate of photosynthesis shows:
At the plateau, another factor (usually CO2 concentration or temperature) has become the limiting factor.
Light intensity is inversely proportional to the square of the distance from the light source. This means:
This relationship is expressed as: light intensity is proportional to 1 / d squared (where d = distance from the light source).
Exam Tip: In the required practical, you use the inverse square law to calculate relative light intensity at different distances. The formula is: light intensity = 1 / d squared. Make sure you can use this formula and explain why moving the lamp closer increases light intensity.
Carbon dioxide is one of the raw materials for photosynthesis. The atmosphere contains approximately 0.04% CO2.
| CO2 Concentration | Effect on Rate | Explanation |
|---|---|---|
| Very low | Very slow rate | Not enough CO2 molecules available for the reaction |
| Increasing | Rate increases | More CO2 molecules are available to react with water |
| High (plateau) | Rate levels off | CO2 is no longer limiting — light intensity or temperature is now limiting |
Commercial growers often increase CO2 concentration inside greenhouses (using gas burners or CO2 generators) to increase the rate of photosynthesis and therefore crop yield. The optimal concentration is typically around 0.1% — about 2.5 times the atmospheric level.
Temperature affects the enzymes that catalyse the reactions of photosynthesis.
| Temperature | Effect on Rate | Explanation |
|---|---|---|
| Very low (below 5 degrees C) | Very slow | Molecules have little kinetic energy; few successful collisions between enzymes and substrates |
| Increasing (up to about 25-35 degrees C) | Rate increases | Molecules have more kinetic energy; more frequent successful collisions |
| Optimum (about 25-35 degrees C) | Maximum rate | Enzymes work at their fastest |
| Too high (above about 40-45 degrees C) | Rate drops sharply | Enzymes denature — their active site changes shape and can no longer bind to the substrate |
graph LR
A[Low temperature] --> B[Molecules move slowly]
B --> C[Few enzyme-substrate collisions]
C --> D[Slow rate of photosynthesis]
E[Optimum temperature] --> F[Molecules move at ideal speed]
F --> G[Maximum enzyme-substrate collisions]
G --> H[Maximum rate of photosynthesis]
I[High temperature] --> J[Enzymes denature]
J --> K[Active site changes shape]
K --> L[Rate drops sharply to zero]
Exam Tip: When describing the effect of high temperature, use the word denature — not "die" or "killed." Enzymes are proteins, not living things. Denaturing means the active site permanently changes shape so the substrate can no longer fit.
In reality, all three factors interact simultaneously. At any given moment, only one factor is the limiting factor.
A plant is in bright light (high light intensity) on a warm day (25 degrees C) but CO2 concentration is low:
If you then increased CO2 but the temperature dropped to 5 degrees C:
When a graph line is rising, the factor on the x-axis is limiting. When the graph line is flat (a plateau), something else is limiting.
| Graph Feature | Meaning |
|---|---|
| Line going up | The x-axis factor is limiting — increasing it increases the rate |
| Line flat (plateau) | The x-axis factor is no longer limiting — another factor is now limiting |
| Shift upwards when a second factor is increased | Confirms the second factor was previously limiting |
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