Explain why companion cells are essential for the loading of sucrose into sieve tube elements in the apoplast pathway.

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).

In many plant species, sucrose produced in mesophyll cells during photosynthesis must be actively loaded into the phloem for transport to sink tissues. This loading process involves the movement of sucrose from the apoplast (cell wall space) into the phloem.

Model answer (4 marks)

Companion cells are required because sieve tube elements lack the cellular machinery for active transport; they cannot generate ATP or express the sucrose transporters needed to load sucrose from the apoplast. Companion cells, which are metabolically active, use ATP to drive sucrose symporters that import sucrose against a concentration gradient from the apoplast into the companion cell. Once inside, sucrose is transferred into the sieve tube element through plasmodesmata, ensuring a continuous supply for long‑distance transport.

Examiner tips

Mention the lack of ATP and transporters in sieve tube elements
Explain the active transport mechanism in companion cells
State that sucrose moves via plasmodesmata into sieve tube elements
Keep the answer concise and use the exact terminology

Common mistakes

Confusing the direction of transport or saying sucrose moves directly into sieve tube elements without plasmodesmata
Forgetting to mention the ATP‑driven active transport in companion cells
Using vague terms like ‘help’ instead of ‘actively load’

Mark scheme (4 marks)

Sieve tube elements lack the cellular machinery needed to actively load sucrose, so companion cells must carry out this process on their behalf
Companion cells use ATP to actively transport sucrose from the apoplast into the phloem against a concentration gradient
Companion cells are metabolically active and can generate the ATP needed for active loading
Sucrose passes from the companion cell into the sieve tube element via plasmodesmata, maintaining the supply for long-distance transport

Key terms in this question

companion cells · sieve tube elements · apoplast pathway · loading