A student places a green plant on a windowsill in sunlight. After several hours, small bubbles are seen rising from the leaves when the plant is submerged in water. Explain what is happening inside the plant's cells to produce these bubbles.

Eduqas GCSE Biology — 1.5.2 Photosynthesis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A student places a green plant on a windowsill in sunlight. After several hours, small bubbles are seen rising from the leaves when the plant is submerged in water.

Model answer (4 marks)

The plant is carrying out photosynthesis.
Light energy from the sunlight is absorbed by chlorophyll and used to split water molecules.
The water and carbon dioxide are converted into glucose and oxygen.
The bubbles that rise from the leaves are oxygen, a by‑product of photosynthesis.

Examiner tips

  • Use the word ‘photosynthesis’ early to show understanding of the process.
  • Explain that light energy drives the splitting of water and the production of O₂.
  • Mention that the bubbles are oxygen, not carbon dioxide.
  • Keep the answer concise – 4 marks can be earned with four short sentences.

Common mistakes

  • Saying the bubbles are carbon dioxide instead of oxygen.
  • Failing to mention that light energy is required for the reaction.
  • Using vague terms like ‘energy’ without linking it to sunlight or chlorophyll.

Mark scheme (4 marks)

  1. The plant is carrying out photosynthesis
  2. Light energy is used / sunlight provides the energy needed for photosynthesis
  3. Water and carbon dioxide are converted into glucose and oxygen
  4. The bubbles are oxygen, which is a product / by-product of photosynthesis

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