Plant Hormones: Auxins, Gibberellins and Ethene — AQA GCSE Biology (Separate)
Plants produce hormones to coordinate and control growth and respond to their environment.
Tropisms
Plants respond to stimuli by growing in a particular direction — a tropism:
- Phototropism — growth in response to light.
- Gravitropism (geotropism) — growth in response to gravity.
Shoots are positively phototropic (grow towards light) and negatively gravitropic (grow up). Roots are positively gravitropic (grow down) and negatively phototropic.
Auxin
Auxin is a plant hormone that controls growth at the tips of shoots and roots.
- Auxin is produced at the tip and moves to the shaded/lower side.
- In a shoot, auxin accumulates on the shaded side and makes those cells elongate more, so the shoot bends towards the light.
- Auxin has the opposite effect in roots (where more auxin inhibits growth), helping roots grow down.
Other plant hormones
- Gibberellins — important in initiating seed germination.
- Ethene — a gas that controls cell division and the ripening of fruits.
Exam tips
- Learn phototropism (light) and gravitropism (gravity), and the responses of shoots vs roots.
- Explain shoot bending: auxin accumulates on the shaded side, causing those cells to elongate.
- Gibberellins → germination; ethene → fruit ripening.
- Auxin promotes growth in shoots but inhibits it in roots.