# 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

> 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.

## 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

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

## 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-063380d8) · Published by Druglandscape Ltd.