A farmer removes a complete ring of bark (including the phloem) from around the trunk of an apple tree. Explain what happens to the sugars produced in the leaves after this ring of bark is removed.

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).

When a ring of bark is removed from a tree trunk, all the phloem tissue at that point is destroyed. The xylem tissue beneath remains intact.

Model answer (4 marks)

The removal of the bark ring destroys the phloem, so sucrose produced in the leaves can no longer be loaded into the phloem and transported downwards. Consequently the sugars accumulate in the leaves and the stem above the cut. Roots and other tissues below the cut are cut off from the sugar supply; they cannot carry out respiration or growth and will eventually die.

Examiner tips

  • Use the phrase ‘cannot be transported downwards’ to show understanding of phloem function.
  • Show the sequence: loss of phloem → sugar build‑up above cut → loss of supply to roots → root death.
  • Mention that the sugars are produced in the leaves and normally move via phloem.
  • Use UK spelling (e.g., ‘sucrose’).

Common mistakes

  • Saying the sugars are ‘destroyed’ instead of ‘accumulate’ above the cut.
  • Confusing xylem with phloem – noting that xylem remains intact but does not transport sugars.
  • Failing to mention the eventual death of the roots due to lack of sugars.

Mark scheme (4 marks)

  1. Sugars (sucrose) cannot be transported downwards past the ring because the phloem has been removed
  2. Sugars accumulate / build up above the ring (in the leaves or stem above the cut)
  3. Roots (and other sinks below the ring) are no longer supplied with sugars
  4. The roots eventually die because they cannot respire or grow without a supply of sugars

Key terms in this question

phloem

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