# 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

> 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.

## Mark scheme (4 marks)

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

## Key terms

- [companion cells](https://www.gradenine.co.uk/glossary/companion-cells)
- [sieve tube elements](https://www.gradenine.co.uk/glossary/sieve-tube-elements)
- [apoplast pathway](https://www.gradenine.co.uk/glossary/apoplast-pathway)
- [loading](https://www.gradenine.co.uk/glossary/loading)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-companion-cells-are-essential-12d3df4c) · Published by Druglandscape Ltd.