Factors Affecting the Rate of Photosynthesis

What is a Limiting Factor?

Have you ever tried to bake a cake but ran out of eggs? No matter how much flour and sugar you have, the number of eggs limits how many cakes you can make. Plants face a similar problem when making glucose.

The rate of photosynthesis is restricted by the environmental variable that is in the shortest supply at any given time.
This variable is known as the limiting factor.
The four main limiting factors for photosynthesis are temperature, light intensity, carbon dioxide ( $CO_2$ ) concentration, and the amount of chlorophyll.

Interpreting Limiting Factor Graphs

In an exam, you will often need to analyse graphs showing the rate of photosynthesis (the dependent variable on the y-axis) against a changing environmental condition (the independent variable on the x-axis).

The Sloped Line: While the line is sloping upwards, the rate is increasing. This means the variable on the x-axis is the limiting factor.
The Plateau: When the graph levels off horizontally, the rate has reached its maximum for those conditions, known as the saturation point.
A plateau means the factor on the x-axis is no longer limiting. Another environmental factor (like temperature or $CO_2$ ) has now become the limiting factor.

Graphs with Multiple Lines (Interacting Factors) Exams often test your understanding of interacting limiting factors by using graphs with multiple lines.

For example, a graph may show light intensity on the x-axis with two lines representing different conditions (such as 15°C and 25°C).
If the line for 25°C reaches a higher plateau than the line for 15°C, it proves that temperature was the limiting factor at the lower level.
By comparing the different plateaus, you can identify which additional factors are interacting to limit the overall rate of photosynthesis.

Temperature and Enzyme Action

Photosynthesis is an endothermic reaction controlled by photosynthetic enzymes. Because it relies on enzymes, temperature heavily influences the rate of reaction based on collision theory.

Increasing temperature: As temperature rises, the reacting molecules ( $CO_2$ and $H_2O$ ) and the enzymes gain kinetic energy.
They move faster, leading to a higher frequency of collisions between the substrate and the enzyme's active site.
A higher proportion of these collisions will have enough energy to react (meeting or exceeding the activation energy), so the rate of photosynthesis increases.
Beyond the optimum: If the temperature gets too high (typically above 45°C), the thermal energy breaks the bonds holding the enzyme together.
This causes denaturation. The active site permanently changes shape and is no longer complementary to the substrate, causing photosynthesis to stop entirely.

Carbon Dioxide ( $CO_2$ ) Concentration

Carbon dioxide is a reactant in the photosynthesis equation:

6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2

The normal atmospheric concentration of $CO_2$ is very low (around 0.04%).
Increasing the concentration of $CO_2$ increases the number of reactant particles in a given volume.
This increases the frequency of successful collisions with enzyme active sites, increasing the overall rate of reaction until another factor becomes limiting.

Light Intensity and the Inverse Square Law

Photosynthesis requires an input of light energy from the surroundings. This energy is transferred to chloroplasts and absorbed by the pigment chlorophyll.

Higher light intensity means more photons hit the chlorophyll per second, trapping energy faster.
The Plateau Mechanism: As light intensity continues to rise, the graph eventually levels off (plateaus). At this saturation point, light is no longer the limiting factor. Even though more photons are absorbed, the enzymes cannot process the reactants any faster. The rate is now restricted by another factor, such as a lack of $CO_2$ limiting substrate concentration, or low temperature restricting the kinetic energy and frequency of collisions between the substrate and the enzymes' active site.
However, light intensity is inversely proportional to the square of the distance from the light source. This is called the inverse square law.
The Double/Quarter Rule: If you double the distance from a light source, the light intensity falls by a factor of four ( $2^2 = 4$ ).

Calculating Light Intensity Light intensity is often measured in arbitrary units (a.u.) using this formula:

\text{Light Intensity} = \frac{1}{d^2}

Worked Example:

Compare the light intensity when a lamp is 10 cm and 20 cm away from a piece of pondweed.

Step 1: Calculate intensity at 10 cm.

$\text{Intensity} = 1 \div 10^2 = 1 \div 100 = 0.01 \text{ a.u.}$

Step 2: Calculate intensity at 20 cm.

$\text{Intensity} = 1 \div 20^2 = 1 \div 400 = 0.0025 \text{ a.u.}$

Step 3: Compare the results.

As the distance doubled from 10 cm to 20 cm, the light intensity decreased by four times ( $0.01 \div 0.0025 = 4$ ).

Chlorophyll Levels

Chlorophyll is essential for absorbing light energy. If a plant has less chlorophyll, it will produce less glucose, leaving it with less energy for protein synthesis, which leads to stunted growth.

Factors that reduce chlorophyll include:

Nutrition: A lack of magnesium in the soil prevents the plant from making chlorophyll, causing a yellowing of the leaves called chlorosis.
Disease: Pathogens like the Tobacco Mosaic Virus (TMV) cause a mosaic discoloration, destroying chlorophyll.
Genetics: Some plants have naturally variegated leaves with white patches containing no chlorophyll.

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Get full access to revision notes, key terms, and exam tips for every subtopic.

Exam Tips

Common Mistake
Students often say that enzymes are 'killed' by high temperatures. Enzymes are proteins, not living things — you must always use the term 'denatured'.
Common Mistake
When calculating the inverse square law, students frequently forget to square the distance before dividing. Always do $d^2$ first!
3
In 6-mark 'Explain' questions about temperature, AQA mark schemes demand the phrase 'higher frequency of collisions' rather than just 'more collisions'.
4
When interpreting a graph that levels off, always explicitly state that the factor on the x-axis is 'no longer limiting' and suggest another specific factor (like temperature or CO2) that has become limiting.

Key Terms(20)

Limiting factor: An environmental variable that, when in short supply, restricts the overall rate of a biological process like photosynthesis.
Saturation point: The point where increasing a specific factor no longer increases the rate of reaction, causing a graph to plateau as another factor becomes limiting.
Chlorophyll: The green pigment found in chloroplasts that absorbs light energy for photosynthesis.
Endothermic: A chemical reaction that requires a net input of energy from its surroundings.
Collision theory: The theory that chemical reactions only occur when particles collide with sufficient energy (activation energy).
Kinetic energy: The energy of motion; increased thermal energy gives reacting particles more kinetic energy, causing them to move faster.
Frequency of collisions: How often reacting particles bump into each other in a given amount of time.
Substrate: The specific molecule or reactant that binds to an enzyme's active site and undergoes a chemical reaction.
Active site: The specific region of an enzyme where the substrate binds and the reaction takes place.
Activation energy: The minimum amount of energy required for a collision between particles to result in a chemical reaction.
Optimum: The specific condition, such as temperature, at which an enzyme works at its fastest rate due to the maximum frequency of successful collisions.
Denaturation: The irreversible process where an enzyme's active site permanently changes shape due to high temperatures or extreme pH, stopping it from binding to its substrate.
Chloroplasts: The sub-cellular structures in plant cells where photosynthesis takes place.
Photons: Particles of light energy that are absorbed by chlorophyll during photosynthesis.
Inverse square law: A mathematical relationship where the intensity of light is inversely proportional to the square of the distance from the source.
Arbitrary units (a.u.): A relative unit of measurement used to express a ratio or proportion, often used for light intensity when exact units like lux are not needed.
Protein synthesis: The cellular process of building proteins from amino acids, which requires energy derived from the glucose produced during photosynthesis.
Chlorosis: The yellowing of plant leaves due to a lack of chlorophyll, commonly caused by a magnesium deficiency.
Pathogens: Microorganisms, such as viruses or bacteria, that cause disease in plants or animals.
Variegated: Leaves that have patches of different colours, typically green areas containing chlorophyll and white areas lacking chlorophyll.

Previous Subtopic: Photosynthesis ReactionPhotosynthesis Reaction Next NoteLimiting Factors, Inverse Square Law & Greenhouse Economics (HT)

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Key Terms(20)

Limiting factor: An environmental variable that, when in short supply, restricts the overall rate of a biological process like photosynthesis.
Saturation point: The point where increasing a specific factor no longer increases the rate of reaction, causing a graph to plateau as another factor becomes limiting.
Chlorophyll: The green pigment found in chloroplasts that absorbs light energy for photosynthesis.
Endothermic: A chemical reaction that requires a net input of energy from its surroundings.
Collision theory: The theory that chemical reactions only occur when particles collide with sufficient energy (activation energy).
Kinetic energy: The energy of motion; increased thermal energy gives reacting particles more kinetic energy, causing them to move faster.
Frequency of collisions: How often reacting particles bump into each other in a given amount of time.
Substrate: The specific molecule or reactant that binds to an enzyme's active site and undergoes a chemical reaction.
Active site: The specific region of an enzyme where the substrate binds and the reaction takes place.
Activation energy: The minimum amount of energy required for a collision between particles to result in a chemical reaction.
Optimum: The specific condition, such as temperature, at which an enzyme works at its fastest rate due to the maximum frequency of successful collisions.
Denaturation: The irreversible process where an enzyme's active site permanently changes shape due to high temperatures or extreme pH, stopping it from binding to its substrate.
Chloroplasts: The sub-cellular structures in plant cells where photosynthesis takes place.
Photons: Particles of light energy that are absorbed by chlorophyll during photosynthesis.
Inverse square law: A mathematical relationship where the intensity of light is inversely proportional to the square of the distance from the source.
Arbitrary units (a.u.): A relative unit of measurement used to express a ratio or proportion, often used for light intensity when exact units like lux are not needed.
Protein synthesis: The cellular process of building proteins from amino acids, which requires energy derived from the glucose produced during photosynthesis.
Chlorosis: The yellowing of plant leaves due to a lack of chlorophyll, commonly caused by a magnesium deficiency.
Pathogens: Microorganisms, such as viruses or bacteria, that cause disease in plants or animals.
Variegated: Leaves that have patches of different colours, typically green areas containing chlorophyll and white areas lacking chlorophyll.

Factors Affecting the Rate of Photosynthesis

What is a Limiting Factor?

The rate of photosynthesis is restricted by the environmental variable that is in the shortest supply at any given time.
This variable is known as the limiting factor.
The four main limiting factors for photosynthesis are temperature, light intensity, carbon dioxide ( $CO_2$ ) concentration, and the amount of chlorophyll.

Interpreting Limiting Factor Graphs

The Sloped Line: While the line is sloping upwards, the rate is increasing. This means the variable on the x-axis is the limiting factor.
The Plateau: When the graph levels off horizontally, the rate has reached its maximum for those conditions, known as the saturation point.
A plateau means the factor on the x-axis is no longer limiting. Another environmental factor (like temperature or $CO_2$ ) has now become the limiting factor.

Graphs with Multiple Lines (Interacting Factors) Exams often test your understanding of interacting limiting factors by using graphs with multiple lines.

For example, a graph may show light intensity on the x-axis with two lines representing different conditions (such as 15°C and 25°C).
If the line for 25°C reaches a higher plateau than the line for 15°C, it proves that temperature was the limiting factor at the lower level.
By comparing the different plateaus, you can identify which additional factors are interacting to limit the overall rate of photosynthesis.

Temperature and Enzyme Action

Photosynthesis is an endothermic reaction controlled by photosynthetic enzymes. Because it relies on enzymes, temperature heavily influences the rate of reaction based on collision theory.

Increasing temperature: As temperature rises, the reacting molecules ( $CO_2$ and $H_2O$ ) and the enzymes gain kinetic energy.
They move faster, leading to a higher frequency of collisions between the substrate and the enzyme's active site.
A higher proportion of these collisions will have enough energy to react (meeting or exceeding the activation energy), so the rate of photosynthesis increases.
Beyond the optimum: If the temperature gets too high (typically above 45°C), the thermal energy breaks the bonds holding the enzyme together.
This causes denaturation. The active site permanently changes shape and is no longer complementary to the substrate, causing photosynthesis to stop entirely.

Carbon Dioxide ( $CO_2$ ) Concentration

Carbon dioxide is a reactant in the photosynthesis equation:

6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2

The normal atmospheric concentration of $CO_2$ is very low (around 0.04%).
Increasing the concentration of $CO_2$ increases the number of reactant particles in a given volume.
This increases the frequency of successful collisions with enzyme active sites, increasing the overall rate of reaction until another factor becomes limiting.

Light Intensity and the Inverse Square Law

Photosynthesis requires an input of light energy from the surroundings. This energy is transferred to chloroplasts and absorbed by the pigment chlorophyll.

Higher light intensity means more photons hit the chlorophyll per second, trapping energy faster.
The Plateau Mechanism: As light intensity continues to rise, the graph eventually levels off (plateaus). At this saturation point, light is no longer the limiting factor. Even though more photons are absorbed, the enzymes cannot process the reactants any faster. The rate is now restricted by another factor, such as a lack of $CO_2$ limiting substrate concentration, or low temperature restricting the kinetic energy and frequency of collisions between the substrate and the enzymes' active site.
However, light intensity is inversely proportional to the square of the distance from the light source. This is called the inverse square law.
The Double/Quarter Rule: If you double the distance from a light source, the light intensity falls by a factor of four ( $2^2 = 4$ ).

Calculating Light Intensity Light intensity is often measured in arbitrary units (a.u.) using this formula:

\text{Light Intensity} = \frac{1}{d^2}

Worked Example:

Compare the light intensity when a lamp is 10 cm and 20 cm away from a piece of pondweed.

Step 1: Calculate intensity at 10 cm.

$\text{Intensity} = 1 \div 10^2 = 1 \div 100 = 0.01 \text{ a.u.}$

Step 2: Calculate intensity at 20 cm.

$\text{Intensity} = 1 \div 20^2 = 1 \div 400 = 0.0025 \text{ a.u.}$

Step 3: Compare the results.

As the distance doubled from 10 cm to 20 cm, the light intensity decreased by four times ( $0.01 \div 0.0025 = 4$ ).

Chlorophyll Levels

Factors that reduce chlorophyll include:

Nutrition: A lack of magnesium in the soil prevents the plant from making chlorophyll, causing a yellowing of the leaves called chlorosis.
Disease: Pathogens like the Tobacco Mosaic Virus (TMV) cause a mosaic discoloration, destroying chlorophyll.
Genetics: Some plants have naturally variegated leaves with white patches containing no chlorophyll.

Sign up to continue reading

Get full access to revision notes, key terms, and exam tips for every subtopic.

Exam Tips

Common Mistake
Students often say that enzymes are 'killed' by high temperatures. Enzymes are proteins, not living things — you must always use the term 'denatured'.
Common Mistake
When calculating the inverse square law, students frequently forget to square the distance before dividing. Always do $d^2$ first!
3
In 6-mark 'Explain' questions about temperature, AQA mark schemes demand the phrase 'higher frequency of collisions' rather than just 'more collisions'.
4
When interpreting a graph that levels off, always explicitly state that the factor on the x-axis is 'no longer limiting' and suggest another specific factor (like temperature or CO2) that has become limiting.

Key Terms(20)

Limiting factor: An environmental variable that, when in short supply, restricts the overall rate of a biological process like photosynthesis.
Saturation point: The point where increasing a specific factor no longer increases the rate of reaction, causing a graph to plateau as another factor becomes limiting.
Chlorophyll: The green pigment found in chloroplasts that absorbs light energy for photosynthesis.
Endothermic: A chemical reaction that requires a net input of energy from its surroundings.
Collision theory: The theory that chemical reactions only occur when particles collide with sufficient energy (activation energy).
Kinetic energy: The energy of motion; increased thermal energy gives reacting particles more kinetic energy, causing them to move faster.
Frequency of collisions: How often reacting particles bump into each other in a given amount of time.
Substrate: The specific molecule or reactant that binds to an enzyme's active site and undergoes a chemical reaction.
Active site: The specific region of an enzyme where the substrate binds and the reaction takes place.
Activation energy: The minimum amount of energy required for a collision between particles to result in a chemical reaction.
Optimum: The specific condition, such as temperature, at which an enzyme works at its fastest rate due to the maximum frequency of successful collisions.
Denaturation: The irreversible process where an enzyme's active site permanently changes shape due to high temperatures or extreme pH, stopping it from binding to its substrate.
Chloroplasts: The sub-cellular structures in plant cells where photosynthesis takes place.
Photons: Particles of light energy that are absorbed by chlorophyll during photosynthesis.
Inverse square law: A mathematical relationship where the intensity of light is inversely proportional to the square of the distance from the source.
Arbitrary units (a.u.): A relative unit of measurement used to express a ratio or proportion, often used for light intensity when exact units like lux are not needed.
Protein synthesis: The cellular process of building proteins from amino acids, which requires energy derived from the glucose produced during photosynthesis.
Chlorosis: The yellowing of plant leaves due to a lack of chlorophyll, commonly caused by a magnesium deficiency.
Pathogens: Microorganisms, such as viruses or bacteria, that cause disease in plants or animals.
Variegated: Leaves that have patches of different colours, typically green areas containing chlorophyll and white areas lacking chlorophyll.

Previous Subtopic: Photosynthesis ReactionPhotosynthesis Reaction Next NoteLimiting Factors, Inverse Square Law & Greenhouse Economics (HT)

Back to Rate of Photosynthesis

Put your knowledge into practice — try past paper questions for Biology

Key Terms(20)

Limiting factor: An environmental variable that, when in short supply, restricts the overall rate of a biological process like photosynthesis.
Saturation point: The point where increasing a specific factor no longer increases the rate of reaction, causing a graph to plateau as another factor becomes limiting.
Chlorophyll: The green pigment found in chloroplasts that absorbs light energy for photosynthesis.
Endothermic: A chemical reaction that requires a net input of energy from its surroundings.
Collision theory: The theory that chemical reactions only occur when particles collide with sufficient energy (activation energy).
Kinetic energy: The energy of motion; increased thermal energy gives reacting particles more kinetic energy, causing them to move faster.
Frequency of collisions: How often reacting particles bump into each other in a given amount of time.
Substrate: The specific molecule or reactant that binds to an enzyme's active site and undergoes a chemical reaction.
Active site: The specific region of an enzyme where the substrate binds and the reaction takes place.
Activation energy: The minimum amount of energy required for a collision between particles to result in a chemical reaction.
Optimum: The specific condition, such as temperature, at which an enzyme works at its fastest rate due to the maximum frequency of successful collisions.
Denaturation: The irreversible process where an enzyme's active site permanently changes shape due to high temperatures or extreme pH, stopping it from binding to its substrate.
Chloroplasts: The sub-cellular structures in plant cells where photosynthesis takes place.
Photons: Particles of light energy that are absorbed by chlorophyll during photosynthesis.
Inverse square law: A mathematical relationship where the intensity of light is inversely proportional to the square of the distance from the source.
Arbitrary units (a.u.): A relative unit of measurement used to express a ratio or proportion, often used for light intensity when exact units like lux are not needed.
Protein synthesis: The cellular process of building proteins from amino acids, which requires energy derived from the glucose produced during photosynthesis.
Chlorosis: The yellowing of plant leaves due to a lack of chlorophyll, commonly caused by a magnesium deficiency.
Pathogens: Microorganisms, such as viruses or bacteria, that cause disease in plants or animals.
Variegated: Leaves that have patches of different colours, typically green areas containing chlorophyll and white areas lacking chlorophyll.

Factors Affecting the Rate of Photosynthesis

What is a Limiting Factor?

Interpreting Limiting Factor Graphs

Temperature and Enzyme Action

Carbon Dioxide (CO2CO_2CO2​) Concentration

Light Intensity and the Inverse Square Law

Chlorophyll Levels

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Exam Tips

Key Terms(20)

Key Terms(20)

Factors Affecting the Rate of Photosynthesis

What is a Limiting Factor?

Interpreting Limiting Factor Graphs

Temperature and Enzyme Action

Carbon Dioxide (CO2CO_2CO2​) Concentration

Light Intensity and the Inverse Square Law

Chlorophyll Levels

Sign up to continue reading

Exam Tips

Key Terms(20)

Key Terms(20)

Carbon Dioxide ( $CO_2$ ) Concentration

Carbon Dioxide ( $CO_2$ ) Concentration