Plant hormones — AQA GCSE Biology
How auxin controls phototropism and gravitropism, plus gibberellins, ethene and the uses of plant hormones by growers.
Revision notes
2 short notes, in the order of the specification. Each one 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.
Uses of plant hormones
Higher tierPlant hormones are used in agriculture and horticulture to control plant growth. Auxins are used as weed killers, as rooting powders and to promote growth in tissue culture. Ethene is used to control the ripening of fruit during storage and transport. Gibberellins are used to end seed dormancy, promote flowering and increase fruit size.
Exam questions
2 exam-style questions (6 marks), each with its mark scheme.
Answer the questionsFlashcards
8 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of homeostasis and response on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheet
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
Quick check questions
Which hormone causes unequal growth in roots and shoots?
Show answer
Auxin.
Why does a shoot bend towards light?
Show answer
Auxin builds up on the shaded side, so cells there grow more and the shoot bends towards the light.
What do the terms phototropism and gravitropism mean?
Show answer
Phototropism is a growth response to light. Gravitropism (geotropism) is a growth response to gravity.
Higher tier Name one use of gibberellins.
Show answer
Any one of: ending seed dormancy, promoting flowering, increasing fruit size.
Frequently asked questions
Why do plants grow towards light?
Plants grow towards light because of an unequal distribution of the hormone auxin. When light comes from one side, auxin builds up on the shaded side of the shoot. The cells on the shaded side elongate more than the cells on the lit side, so the shoot bends towards the light. This response is called phototropism.
How does auxin make roots grow downwards?
Auxin makes roots grow downwards because it builds up on the lower side of the root, where it slows down the growth of cells. The upper side grows faster than the lower side, so the root bends downwards, in the direction of gravity. This is gravitropism (geotropism). In shoots, auxin on the lower side makes the shoot bend upwards.
What are gibberellins used for?
Higher tier Gibberellins are plant hormones that are important in initiating seed germination. Growers use them to end seed dormancy, to promote flowering and to increase fruit size. Other plant hormones have different uses: auxins are used as weed killers and rooting powders, and ethene is used to control the ripening of fruit during storage and transport.
How do you investigate the effect of light on seedling growth?
Germinate seedlings such as cress, then put some in a box with a slit so light comes from one side only, keeping a control set in even light all round. Measure the shoot length at regular intervals and make labelled drawings. Keep the seedling type, temperature and water the same. The shoots bend towards the light.
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026