# Explain how the structure of a sieve tube element is adapted for its function in the phloem.

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

> Phloem tissue transports assimilates such as sucrose through the plant. Sieve tube elements are the main conducting cells of the phloem and are closely associated with companion cells.

## Mark scheme (4 marks)

1. Sieve tube elements have no nucleus, allowing more space for mass flow of assimilates
2. Sieve plates have pores that allow mass flow of assimilates between adjacent sieve tube elements
3. Sieve tube elements have little or no cytoplasm / few organelles, reducing resistance to flow
4. Companion cells provide ATP / carry out metabolic functions to support active loading of assimilates into the sieve tube element

## Key terms

- [sieve tube element](https://www.gradenine.co.uk/glossary/sieve-tube-element)
- [phloem](https://www.gradenine.co.uk/glossary/phloem)

## 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-how-the-structure-of-a-09733a59) · Published by Druglandscape Ltd.