A potato plant stores starch in its underground tubers during summer. Explain how sucrose produced in the leaves reaches the tubers, naming the tissue involved and the direction of movement.

Cambridge International IGCSE Biology (0610) — 8.4 Translocation · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Sucrose is produced in the leaves, which act as the source.
It is loaded into the phloem sieve‑elements.
The phloem transports the sucrose from the source (leaves) to the sink (tubers) – i.e. downwards.
The tubers are the sink because the sucrose is stored there as starch.

Examiner tips

  • Use the term ‘source’ for leaves and ‘sink’ for tubers. Mention the phloem sieve‑elements as the transport tissue. State the direction as ‘from leaves to tubers’ or ‘downwards’. Show the conversion of sucrose to starch in the sink.

Common mistakes

  • Confusing xylem with phloem. Saying the movement is ‘upwards’ or ‘from tubers to leaves’. Omitting that the tubers are a sink or that sucrose is converted to starch.

Mark scheme (4 marks)

  1. Sucrose is produced / made in the leaves, which act as the source
  2. Sucrose is transported through the phloem
  3. Movement is from source (leaves) to sink (tubers / roots), i.e. downwards
  4. The tubers are the sink because sucrose is stored / used there (converted to starch for storage)

Key terms in this question

sucrose

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