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 · View as Markdown

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

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.

Model answer (4 marks)

Sieve tube elements lack a nucleus, so the cell lumen is unobstructed and allows assimilates to flow freely.
They contain very few organelles and a reduced cytoplasm, which reduces resistance to flow.
The sieve plates between adjacent elements have pores that provide a continuous pathway for mass flow.
Companion cells supply ATP and other metabolites to maintain the active loading of sucrose, keeping the concentration gradient that drives phloem transport.

Examiner tips

Use the word ‘lack’ or ‘have no’ for the nucleus; mention ‘unobstructed lumen’.
Show the link between ‘few organelles’ and ‘reduced resistance’.
Explain that sieve plates give continuity and that companion cells provide ATP for loading.

Common mistakes

Confusing sieve tube elements with xylem vessels; forgetting the nucleus is absent.
Failing to mention the role of companion cells or the concentration gradient.

Mark scheme (4 marks)

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

Key terms in this question

sieve tube element · assimilates · mass flow · phloem