Explain how the structure of a sieve tube element is adapted for the mass flow of assimilates through the phloem.

OCR A-Level Biology A (H420) — 3.3 Transport in plants · Explain · 4 marks · View as Markdown

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

Phloem tissue is responsible for translocating organic solutes such as sucrose from photosynthetically active leaves to sink regions including roots and developing fruits. Sieve tube elements are the main conducting cells of the phloem.

Model answer (4 marks)

Sieve tube elements are highly specialised for bulk transport:
1. They are devoid of a nucleus and most organelles, giving an unobstructed lumen that minimises resistance to flow.
2. The end walls contain sieve plates with large pores, permitting the rapid passage of phloem sap between adjacent elements.
3. The cells are long and arranged end‑to‑end, forming a continuous tube that reduces hydraulic resistance and allows efficient long‑distance movement.
4. Companion cells are tightly connected to each sieve tube element and provide the metabolic energy required for active loading of sucrose, which generates the pressure gradient that drives mass flow.

Examiner tips

  • Use the word ‘mass flow’ and link structure to function; mention lack of nucleus, sieve plates, long shape and companion cells.
  • Show the causal chain: active loading → pressure gradient → bulk flow.
  • Keep the answer concise – 4 points, one sentence each.
  • Use UK spelling and correct terminology (sieve tube element, sieve plate, companion cell).

Common mistakes

  • Confusing sieve tube elements with phloem fibres; they are the conducting cells, not the supportive fibres.
  • Omitting the role of companion cells or the pressure‑gradient mechanism.
  • Using vague terms like ‘good shape’ instead of specific features such as ‘elongated, end‑to‑end arrangement’.

Mark scheme (4 marks)

  1. Sieve tube elements lack a nucleus (and most organelles), providing an unobstructed lumen for flow
  2. Sieve plates have pores that allow mass flow of solution between adjacent sieve tube elements
  3. Companion cells maintain the sieve tube element metabolically, sustaining the living state needed for active loading that drives mass flow
  4. The elongated shape / longitudinal arrangement of sieve tube elements end-to-end forms a continuous tube, reducing resistance and allowing efficient mass flow over long distances

Key terms in this question

sieve tube element · assimilates · phloem · mass flow

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