Explain how auxins cause a seedling root to grow downwards in response to gravity.

Cambridge International IGCSE Biology (0610) — 14.5 Tropic responses · Explain · 4 marks · View as Markdown

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

A seedling is germinated in complete darkness so that only the effect of gravity influences its growth. The root grows downwards into the soil.

Model answer (4 marks)

Auxins are redistributed by the root cap when the seedling is turned so that they accumulate on the lower side of the root. The higher auxin concentration on the lower side inhibits cell elongation, whereas cells on the upper side, with lower auxin, elongate more rapidly. The unequal elongation causes the root to bend downwards, producing a positive gravitropic response.

Examiner tips

Use the word ‘redistributed’ to show understanding of auxin movement; mention ‘higher concentration inhibits elongation’ and ‘lower concentration allows elongation’; link unequal elongation to bending; keep answer concise and use correct terminology.
Include the terms ‘positive gravitropism’ and ‘root cap’ to demonstrate depth of knowledge.

Common mistakes

Confusing auxin’s effect on stems (promotes elongation) with its inhibitory effect on root cells; not specifying that the redistribution is due to gravity; omitting the root cap’s role in sensing gravity.

Mark scheme (4 marks)

Auxins move towards the lower side of the root in response to gravity
Higher concentration of auxin on the lower side inhibits / reduces cell elongation in the root
Cells on the upper side elongate more / faster than cells on the lower side
Unequal elongation causes the root to bend / curve downwards, which is a positive gravitropism

Key terms in this question

auxins