Describe how auxin causes a plant root to grow towards gravity.

Eduqas GCSE Biology — 4.4 Plant hormones · Describe · 4 marks · View as Markdown

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

When a plant root is placed horizontally, it will bend downwards over time. Auxin is unevenly distributed in the root as a result of gravity.

Model answer (4 marks)

Gravity causes auxin to move to the lower side of the root.
Higher auxin concentration on the lower side inhibits cell elongation there.
Lower auxin concentration on the upper side allows cell elongation there.
The unequal elongation bends the root downwards, so it grows towards gravity.

Examiner tips

Use the exact terms ‘auxin’, ‘gravity’, ‘cell elongation’, ‘bend downwards’.
Show the sequence: gravity → auxin redistribution → differential growth → bending.
Keep answer concise, 4 points, no extra words.

Common mistakes

Confusing auxin with gibberellin or ethylene.
Saying auxin *promotes* growth on the lower side instead of inhibiting it.
Omitting the role of gravity in redistributing auxin.

Mark scheme (4 marks)

Gravity causes auxin to move to / accumulate on the lower side of the root
Higher concentration of auxin on the lower side inhibits / slows cell elongation on that side
Lower concentration of auxin on the upper side allows / promotes cell elongation on that side
The difference in elongation between the two sides causes the root to bend / grow downwards

Key terms in this question

auxin · gravity