A student places a green plant in a windowsill on a sunny day. After several hours, small bubbles are seen rising from the leaves when the plant is submerged in water. Explain what these bubbles are and why they are being produced.

WJEC GCSE Biology (Wales) — 1.5.2 Photosynthesis · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The bubbles are oxygen.
They are produced because the plant is carrying out photosynthesis.
Light from the sun supplies the energy needed for the light‑dependent reactions.
During photosynthesis carbon dioxide and water are converted into glucose and oxygen, the latter being released as bubbles.

Examiner tips

  • Use the exact term "oxygen" for the bubbles. Show the link between sunlight and the light‑dependent reactions. Mention the overall photosynthesis equation. Keep the answer short and to the point.

Common mistakes

  • Calling the bubbles water or carbon dioxide. Forgetting to mention that light is required. Using vague phrases like "the plant is making something" instead of the specific products of photosynthesis.

Mark scheme (4 marks)

  1. The bubbles are oxygen
  2. Oxygen is a product of photosynthesis
  3. Light is required / the plant is photosynthesising because it is in sunlight
  4. Carbon dioxide and water are converted into glucose (and oxygen) during photosynthesis

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