Explain how the structure of phloem is adapted to transport sucrose around a plant.

OCR GCSE Biology A: Gateway Science (J247) — B2.1 Supplying the cell · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Plants produce sucrose by photosynthesis in their leaves. This sucrose must be transported to other parts of the plant, such as the roots and growing tips, where it is needed.

Model answer (4 marks)

Sieve tubes have no nuclei, giving more space for sucrose solution to flow.
Sieve tubes are connected by sieve plates, allowing continuous movement of sucrose through the phloem.
Companion cells supply ATP for the active transport of sucrose into sieve tubes.
Translocation therefore moves sucrose from source tissues (leaves) to sink tissues (roots, growing tips).

Examiner tips

Use the exact terms ‘sieve tubes’, ‘sieve plates’, ‘companion cells’ and ‘source/sink’
Show the flow direction from source to sink
Mention that the lack of nuclei increases space for flow

Common mistakes

Confusing xylem with phloem, or describing water transport instead of sucrose
Omitting the role of companion cells or active transport
Using vague phrases like ‘transport’ without specifying source and sink

Mark scheme (4 marks)

Sieve tubes have no nuclei, leaving more space for the flow of sucrose solution
Sieve tubes are connected via their cytoplasms, allowing sucrose to move continuously through the phloem
Companion cells provide energy (ATP) needed for translocation / active transport of sucrose
Translocation moves sucrose from a source (e.g. leaves) to a sink (e.g. roots / growing tips) where it is needed

Key terms in this question

phloem