The photosynthesis required practical GCSE is heavily assessed by AQA, Edexcel and OCR, particularly through limiting factors and graph analysis questions. Many students understand that light, carbon dioxide and temperature affect photosynthesis, but lose marks when describing the method or interpreting data precisely. This guide focuses on what examiners actually reward.

The Limiting Factors Experiment (Method Marks)

In most specifications, students investigate how light intensity affects the rate of photosynthesis using pondweed (for example, Elodea).

A clear method should include:

• Place pondweed in a beaker of water containing sodium hydrogencarbonate (to provide carbon dioxide).

• Position a lamp at a measured distance.

• Count oxygen bubbles produced in one minute (or measure gas volume using a gas syringe).

• Repeat at different distances to change light intensity.

• Keep temperature constant using a water bath.

Examiners expect you to identify:

• Independent variable: distance from lamp (light intensity)

• Dependent variable: rate of oxygen production

• Control variables: temperature, carbon dioxide concentration, length of pondweed

Simply describing “move the lamp” without naming variables limits marks.

For structured required practical revision, see [GCSE Biology Revision Hub].

Graph Analysis: Where Students Drop Marks

A common exam question presents a graph showing rate of photosynthesis against light intensity.

Students often say:
“Photosynthesis increases as light increases.”

That is not enough.

You must describe:

• The initial proportional increase (light is the limiting factor).

• The plateau (another factor becomes limiting).

• The optimum point if temperature is involved.

Use precise language:
“The rate increases rapidly at low light intensities, then levels off, indicating light is no longer the limiting factor.”

Examiners reward linking graph trends to limiting factor theory.

Practise interpreting similar graphs at [GCSE Science Exam Questions].

GCSE-Style Exam Question

Question (6 marks)
Describe how a student could investigate the effect of light intensity on the rate of photosynthesis and explain how they would ensure the results are valid.

Common Exam Mistakes

Across AQA, Edexcel and OCR mark schemes, frequent errors include:

• Forgetting to mention sodium hydrogencarbonate

• Not controlling temperature

• Describing brightness instead of measurable distance

• Failing to explain why the graph plateaus

• Not using the term “limiting factor” correctly

Precision and scientific vocabulary are assessed.

Exam Technique Tip

When analysing a graph, always state:

1. The trend

2. The scientific reason

3. Which factor is limiting

This three-step structure consistently secures higher-band marks.

For guided support with practical questions and extended responses, explore [GCSE Science Tuition].