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

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

> Phloem tissue is responsible for the translocation of organic solutes, such as sucrose, from sources to sinks throughout the plant. Sieve tube elements are the main conducting cells of the phloem and have several structural features that reflect their function.

## Mark scheme (4 marks)

1. Sieve tube elements lack a nucleus, allowing unobstructed flow of assimilates through the cell lumen.
2. Sieve tube elements have few organelles / reduced cytoplasm, reducing resistance to flow.
3. Sieve plates with pores connect adjacent sieve tube elements, allowing continuity of flow between cells.
4. Companion cells provide metabolic support / ATP to maintain active loading and the solute concentration gradient that drives mass flow.

## Key terms

- [sieve tube element](https://www.gradenine.co.uk/glossary/sieve-tube-element)
- [assimilates](https://www.gradenine.co.uk/glossary/assimilates)
- [mass flow](https://www.gradenine.co.uk/glossary/mass-flow)
- [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-00bc134c) · Published by Druglandscape Ltd.