IGCSE Biology glossary
Auxin
Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Definition
A plant growth hormone (specifically indole-3-acetic acid, IAA) that controls cell elongation and is responsible for phototropism and gravitropism. Auxin accumulates on the shaded side of a shoot, causing cells there to elongate more, bending the shoot towards light.
What to include in an exam answer
In phototropism of shoots: auxin moves to the shaded side, causes more cell elongation on that side, so the shoot bends towards the light. In roots, high auxin concentration inhibits growth, so roots bend downwards (positive gravitropism).
Example in context
Explain how auxin causes a plant shoot to bend towards light (3 marks): Light causes auxin to move from the illuminated side to the shaded side of the shoot tip. The higher concentration of auxin on the shaded side stimulates more cell elongation there. The unequal growth causes the shoot to bend towards the light source.
Related syllabus topic
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