A student grows some algae in a sealed, transparent container filled with water. The algae carry out photosynthesis. Explain how the algae use the raw materials available to them to produce glucose during photosynthesis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Algae are aquatic producers that photosynthesise using dissolved carbon dioxide and water. A student places algae in a sealed, transparent container and exposes it to sunlight. Over several hours, the student notices that oxygen bubbles form inside the container.
Model answer (5 marks)
1. The algae take in dissolved CO₂ and H₂O from the surrounding water.
2. Light energy from the sun is absorbed by chlorophyll in the chloroplasts.
3. In the light‑dependent reactions, the absorbed energy splits water, releasing electrons, protons and O₂.
4. The energy and reducing power are used in the Calvin cycle to fix CO₂ into glucose.
5. Glucose is produced and oxygen is released as a by‑product, forming the observed bubbles.
2. Light energy from the sun is absorbed by chlorophyll in the chloroplasts.
3. In the light‑dependent reactions, the absorbed energy splits water, releasing electrons, protons and O₂.
4. The energy and reducing power are used in the Calvin cycle to fix CO₂ into glucose.
5. Glucose is produced and oxygen is released as a by‑product, forming the observed bubbles.
Examiner tips
- Use the exact terms: CO₂, H₂O, chloroplasts, light‑dependent reactions, Calvin cycle, glucose, O₂.
- Show the sequence: raw materials → light absorption → water splitting → CO₂ fixation → glucose + O₂.
- Mention the bubble formation as evidence of O₂ release.
- Keep the answer concise, 5 points, no extra words.
Common mistakes
- Confusing photosynthesis with respiration (e.g., writing O₂ as a reactant).
- Omitting the role of chloroplasts or light energy.
- Using vague terms like ‘energy’ without specifying light or water splitting.
Mark scheme (5 marks)
- Photosynthesis uses carbon dioxide and water as raw materials
- Light energy is required / absorbed for photosynthesis to occur
- The reaction takes place in the chloroplasts
- Glucose is produced as a product of photosynthesis
- Oxygen is also released as a product, which explains the bubble formation
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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