Cells and organisation · GCSE Biology
Plant hormones
Define auxin, phototropism and gravitropism for GCSE Biology: unequal distribution of auxin, shoot and root responses, gibberellins in germination and stem elongation, and commercial uses in agriculture.
Auxin in shoots: more auxin on the shaded side, cells elongate, the shoot bends to light. Auxin in roots: more auxin on the lower side inhibits growth, so the root bends down. Gibberellins trigger germination and make stems grow.
The important bits
What you need to know
- 1
Plant hormones (plant growth regulators) are chemicals made in small amounts that control growth and responses. They move by diffusion and transport in phloem.
- 2
Auxin is produced mainly in the tips of shoots and roots. It controls cell elongation in shoots. Unequal auxin distribution causes tropisms.
- 3
Phototropism is growth towards light. Light from one side causes auxin to move to the shaded side of the shoot tip. More auxin there makes cells elongate faster, so the shoot bends towards the light.
- 4
Gravitropism (geotropism): shoots grow away from gravity (negative gravitropism); roots grow towards gravity (positive gravitropism). In a horizontal root, auxin accumulates on the lower side and inhibits cell elongation there, so the root bends down.
- 5
Auxin has opposite effects in shoots and roots at the same concentration: promotes elongation in shoots, inhibits elongation in roots. That is why roots and shoots bend in different directions.
- 6
Gibberellins stimulate seed germination, stem elongation and flowering in some plants. They are used commercially to break seed dormancy and to grow larger fruit.
- 7
Ethene (ethylene) is a gas that triggers ripening of fruit. It is used to ripen bananas during transport. Farmers may use auxin as a selective weedkiller (broad-leaved weeds) and gibberellins to increase fruit size.
- 8
Plant responses are slower than animal nervous responses but allow plants to compete for light and anchor in soil. Do not confuse tropisms with turgor or nastic movements (e.g. stomata opening).
Quotations worth analysing
Short evidence. Real method.
“Unequal distribution of auxin causes unequal growth, so the plant bends.”
Name the hormone, the side where it accumulates, and whether elongation increases or decreases. “It grows towards light” without auxin loses the mechanism mark.
“In shoots, auxin promotes cell elongation; in roots, the same concentration inhibits elongation.”
This explains why phototropism and gravitropism work differently in roots and shoots. One sentence saves a confused diagram.
“Gibberellins can break seed dormancy and stimulate stem growth.”
Link gibberellin to a commercial use: larger grapes, uniform germination, or earlier flowering — not just “helps plants grow”.
Go deeper
Phototropism step by step
Start with a shoot tip receiving light from one side. Auxin is produced at the tip and moves away from the bright side (or is destroyed on the lit side in some explanations — either way, the shaded side ends up with more auxin). In the shoot, more auxin means more cell elongation on that side. The shaded side grows longer, the shoot curves towards the light. That is positive phototropism in shoots. Leaves intercept more light for photosynthesis; the plant outcompetes neighbours. In an exam, draw the tip, label the shaded side with higher auxin, and say cells elongate more there. Do not say the plant “wants” light — say unequal auxin causes unequal growth.
Go deeper
Gravitropism: why roots go down and shoots go up
Place a seedling on its side. Auxin settles by gravity to the lower side of both root and shoot. In the shoot, more auxin on the lower side promotes elongation, so the lower side grows faster and the shoot bends up (negative gravitropism). In the root, more auxin on the lower side inhibits elongation, so the upper side grows faster and the root bends down (positive gravitropism). The same hormone, opposite response in root versus shoot, is the mark scheme’s favourite trap. Roots anchor and absorb water; shoots reach light. If a question asks why roots respond differently, give the shoot/root elongation contrast, not a new hormone name.
Go deeper
Gibberellins and ethene in agriculture
Gibberellins break dormancy in seeds that would otherwise wait for winter (barley malting, uniform crop emergence). They can lengthen stems and enlarge fruit (grapes, apples) — useful for yield, but stems may need support. Ethene is a ripening hormone: bananas are picked green and ripened in ethylene-rich stores so they arrive yellow but not overripe on the shelf. Auxin-based weedkillers (synthetic auxins such as 2,4-D) disrupt broad-leaved plant growth at doses that grasses tolerate — selective weed control. Evaluation questions may ask about cost, effect on biodiversity, or organic farming objections. Name the hormone, the crop problem, and the intended effect.
See the idea in action
Explain phototropism in a shoot (4 marks). Auxin is made in the tip. Light from one side causes unequal auxin distribution — more auxin on the shaded side. In shoots, auxin promotes cell elongation. Cells on the shaded side elongate more, so the shoot bends towards the light. This increases light capture for photosynthesis.
Exam technique
Turn knowledge into marks
For any tropism: hormone → uneven distribution → elongation on each side → direction of bend. Separate shoot and root auxin effects before you draw.
Common mistakes
Do not give these marks away
- 01
Saying auxin always promotes growth — in roots it inhibits at the same concentration.
- 02
Confusing phototropism with photosynthesis, or gravitropism with geotropism spelling only without mechanism.
- 03
Naming nastic movements (e.g. stomata) as tropisms — tropisms are directional growth responses.
A shoot is placed horizontally. Auxin accumulates on the lower side. What happens?
AThe lower side grows less and the shoot bends down
BThe lower side elongates more and the shoot bends upwards away from gravity
CThe root tip produces ethene and the shoot stops growing
DAuxin is destroyed and the shoot stays horizontal
Show the answer
The lower side elongates more and the shoot bends upwards away from gravity. In shoots, auxin promotes elongation. More growth on the lower side lifts the tip — negative gravitropism.
Quick questions
If this is the bit you searched
What is auxin?
A plant hormone that controls cell elongation, mainly made in shoot and root tips. Uneven distribution causes phototropism and gravitropism.
Why do roots and shoots respond differently to auxin?
The same auxin concentration promotes elongation in shoots but inhibits elongation in roots, so bends go in opposite directions.
What are gibberellins used for?
Seed germination, stem elongation and larger fruit. Commercially used to break dormancy and increase crop yield.
How is ethene used in food production?
It triggers fruit ripening. It is used to ripen bananas and other fruit after harvest during storage and transport.