Plant hormones, phototropism and gravitropismSpec 4.5.4.1
In short
Phototropism is a plant's growth response to light, and gravitropism (geotropism) is its response to gravity. Both are caused by an unequal distribution of auxin. In a shoot lit from one side, auxin builds up on the shaded side, so those cells elongate more and the shoot bends towards the light. In roots, auxin on the lower side slows growth.
Plants produce hormones to coordinate and control growth and responses to light (phototropism) and gravity (gravitropism or geotropism).
Unequal distributions of auxin cause unequal growth rates in plant roots and shoots.
| Response | Where auxin builds up | Effect on growth | Result |
|---|---|---|---|
| Shoot growing towards light | On the shaded side of the shoot | Cells on the shaded side elongate more | The shoot bends towards the light |
| Root growing downwards | On the lower side of the root | Growth of cells on the lower side is slowed down | The root bends downwards, in the direction of gravity |
| Shoot growing upwards | On the lower side of the shoot | Cells on the lower side elongate more | The shoot bends upwards, away from gravity |
(opens full size in a new tab)Higher tier (what this means)
Higher tier: only on the Higher tier papers. Foundation students can skip it. What the labels meanGibberellins and ethene
Higher tier (what this means)
Higher tier: only on the Higher tier papers. Foundation students can skip it. What the labels mean- Gibberellins are important in initiating seed germination.
- Ethene controls cell division and ripening of fruits.
Higher tier (what this means)
Higher tier: only on the Higher tier papers. Foundation students can skip it. What the labels meanYou do not need to know how gibberellins and ethene work.
Always say that the auxin is unequally distributed. Then say which side has more auxin and what that does to growth on that side.
Required practical activity 8: investigate the effect of light or gravity on the growth of newly germinated seedlings. Record results as both length measurements and as careful, labelled biological drawings.
- Germinate seeds (for example cress) on damp cotton wool or filter paper in a dish.
- For light: place identical seedlings in a box with a slit that lets light in from one side only. Keep a control set in even light all round.
- For gravity: fix seedlings in a dish, turn the dish on its side so the roots point sideways and keep it in the dark. Keep a control set upright.
- Measure the length of the shoot (and root) with a ruler at regular intervals, and make a labelled drawing of each seedling to show the direction of growth.
- Compare the treated seedlings with the control seedlings.
- Independent variable: the direction of the light, or the position of the seedling (gravity).
- Dependent variable: the direction and length of growth.
- Control variables: the type and age of seedling, temperature and the amount of water. Keep the gravity test in the dark so light does not affect the result.
- Safety: wash your hands after handling seedlings and compost, and do not eat any seeds or seedlings.
- Typical results: shoots bend towards the light; roots bend down; shoots grow upwards even in the dark.
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026