A student leaves a healthy green plant in a dark cupboard for 48 hours. After this time, the student tests a leaf from the plant for starch and finds no starch is present. The student then moves the plant to a sunny windowsill for 6 hours. A leaf tested at the end of this period gives a positive result for starch. Explain why starch is absent after the dark period and why starch is present after the plant has been placed in sunlight.

Pearson Edexcel A-Level Biology A (9BI0) — 3.1 Photosynthesis · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

In the dark photosynthesis cannot occur because light is a raw material for the light‑dependent reactions. Consequently no glucose is produced and the glucose that was already present in the leaf is consumed by respiration, so no starch remains.

When the plant is moved into sunlight, light energy drives the light‑dependent reactions, producing ATP and NADPH. These energy carriers are used in the Calvin cycle to fix CO₂ into glucose. The glucose produced is then stored as starch in the leaf.

Examiner tips

  • Use the word ‘photosynthesis’ and link light to the light‑dependent reactions
  • Explain that glucose is consumed by respiration in the dark
  • Show that light allows glucose synthesis and its storage as starch

Common mistakes

  • Confusing respiration with photosynthesis
  • Saying starch is produced in the dark
  • Omitting that glucose is the intermediate that becomes starch

Mark scheme (5 marks)

  1. In the dark, photosynthesis cannot occur / stops
  2. Because light is a raw material / required for photosynthesis (so no glucose is produced)
  3. Any existing starch / glucose is used up by respiration during the dark period
  4. In sunlight, photosynthesis occurs / the plant uses light energy, carbon dioxide and water to produce glucose
  5. Glucose is converted to / stored as starch in the leaf

Key terms in this question

starch

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