A plant is kept in darkness for 48 hours and then moved into bright light. Explain what happens to the rate of photosynthesis in the plant immediately after it is moved into the light, and describe how the products of photosynthesis are used once the rate has increased.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 3.1 Photosynthesis · Explain · 5 marks · View as Markdown

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

Plants require specific conditions to carry out photosynthesis. The rate of photosynthesis can change rapidly when environmental conditions change.

Model answer (5 marks)

The plant is dark‑adapted, so the light reactions are inactive. When it is moved into bright light, photons are absorbed by chlorophyll, exciting electrons and initiating the light reactions. This supplies ATP and NADPH, so the Calvin cycle starts and the rate of photosynthesis rises rapidly.

The glucose produced is immediately used in cellular respiration to release energy for growth and maintenance. Excess glucose is polymerised into starch for storage in chloroplasts and other tissues. Some glucose is also incorporated into cellulose for cell‑wall synthesis or combined with nitrogen (as nitrates) to form amino acids and proteins needed for new tissue and repair.

Examiner tips

  • State that light is required for the light reactions and that the plant is dark‑adapted before light exposure.
  • Show that the rate increases immediately after light is supplied.
  • Mention both respiration and storage (starch) and structural uses (cellulose) or protein synthesis.
  • Use the exact terms: light reactions, Calvin cycle, ATP, NADPH, glucose, starch, cellulose, amino acids.

Common mistakes

  • Claiming that photosynthesis can occur in darkness or that the rate stays low after light exposure.
  • Confusing photosynthesis with respiration or omitting the storage of glucose as starch.
  • Using vague terms like ‘energy’ without specifying ATP/NADPH or the Calvin cycle.

Mark scheme (5 marks)

  1. Photosynthesis requires light as a source of energy, so moving the plant into bright light provides the energy needed to drive the reaction
  2. The rate of photosynthesis increases (after exposure to light)
  3. Glucose produced by photosynthesis is used in respiration to release energy for the plant
  4. Glucose is converted into starch for storage in the plant
  5. Glucose is used to make cellulose for cell walls, or combined with nitrates to make amino acids / proteins for growth and repair

Key terms in this question

photosynthesis · rate of photosynthesis

Related

More Photosynthesis questions

▶ Try answering this question with AI marking (free) →