AQA GCSE Biology (8461)

4.5.4 Plant hormones (Bio only)

Plants cannot run, but they can still respond to the world by growing in the right direction. Topic 4.5.4 of AQA 8461 is about the chemical messengers — plant hormones — that make this happen. You need to know three hormones in detail: auxin, gibberellins and ethene. Auxin makes shoots bend towards light and roots grow down; gibberellins switch on seed germination and trigger flowering; ethene ripens fruit. The same hormones are used commercially: rooting powders, selective weedkillers, seedless fruit, malting barley for brewing, and ripening bananas in transport. This page sets out the biology, the experiments (light and gravity on seedlings), and the commercial uses you can be asked about for up to six marks.

Why this matters

Plants do not have a brain or a nervous system, so all their responses to the environment are slow and chemical. A plant hormone is a chemical messenger made in one part of the plant that travels (usually in the xylem, phloem or by simple diffusion through cells) to influence growth somewhere else. Three hormones are named on the AQA 8461 specification: auxin (which controls tropisms — growth responses to a stimulus), gibberellins (a family of hormones that start seeds germinating, stretch stems and trigger flowering) and ethene (a gas that ripens fruit and controls cell division). The big idea you need to carry into the exam is that the same hormone can have opposite effects in different organs. Auxin stimulates cell elongation in shoots but inhibits it in roots — that one fact explains both phototropism and gravitropism. You also need to know the required practical-style investigation: how seedlings respond to one-sided light or to being laid on their side, usually grown in petri dishes on damp cotton wool or filter paper. Finally, plant hormones are big business: growers and food companies use synthetic auxins as selective weedkillers and rooting powders, gibberellins to make seeds germinate uniformly for brewing or to grow seedless fruit, and ethene gas to ripen fruit that was picked unripe for transport.

How to learn this topic

Build on what you already know

What a hormone is and that it travels in fluid to a target — from topic 4.5.3 (animal hormones).
Cell structure and the idea that cells can elongate by taking up water into the vacuole.
Photosynthesis happens in leaves, so reaching the light matters for survival (topic 4.4.1).
Basic experimental design: control variables, repeats, and what counts as a fair test.

Define plant hormone, tropism, phototropism and gravitropism — clarify positive vs negative tropism.
Build the auxin model in shoots: auxin made at the tip, redistributes to the shaded side, stimulates elongation there, shoot bends towards light.
Switch organ: same auxin in roots, gathers on the lower side, but INHIBITS elongation, so the upper side grows faster and the root bends down.
Introduce gibberellins (seed germination, stem elongation, flowering) and ethene (fruit ripening, cell division).
Run through the required-practical-style experiments: seedlings under one-sided light; seedlings rotated on a clinostat or laid on their side.
Map each hormone to its commercial uses (rooting powder, weedkiller, tissue culture, seedless fruit, malting, fruit transport).
Practise four- and six-mark answers that name the hormone, describe redistribution, and finish with the directional outcome.

Key terms

plant hormone: A chemical messenger made in one part of a plant that travels to another part and controls growth or development. (Be ready to say 'chemical messenger' and name a target effect.)
tropism: A directional growth response of a plant to an external stimulus such as light or gravity.
phototropism: A directional growth response to light. Shoots are positively phototropic — they bend towards the light source. (Use the phrase 'bends towards the light'.)
gravitropism: A directional growth response to gravity. Roots are positively gravitropic (grow down); shoots are negatively gravitropic (grow up).
auxin: A plant hormone made in shoot and root tips that controls growth direction. Stimulates cell elongation in shoots and inhibits cell elongation in roots. (Mark schemes want 'stimulates elongation in shoots' AND 'inhibits elongation in roots'.)
gibberellins: A family of plant hormones that initiate seed germination, promote stem elongation and trigger flowering. (Spelled with an 's' — 'gibberellins' is a family, not a single hormone.)
ethene: A gaseous plant hormone that controls cell division and ripens fruit. Used commercially to ripen fruit during and after transport.
cell elongation: The way young plant cells grow by stretching — taking up water into the vacuole so the cell gets longer behind the tip.
seed dormancy: A resting state in which a viable seed does not germinate, often broken by gibberellins, water and warmth.
selective weedkiller: An auxin-based herbicide that kills broad-leaved weeds by making them grow uncontrollably, leaving narrow-leaved crops (grasses, cereals) unaffected.
rooting powder: A powder containing auxin in which plant cuttings are dipped to make them grow roots faster.
malting: The first stage of brewing beer, in which barley grains are made to germinate uniformly — gibberellins can be added to speed this up.

Notes

What plant hormones do

Plants do not have nerves, so every response to the environment is chemical and slow. A plant hormone is a chemical messenger made in one part of the plant that travels to another part and changes how it grows. The three hormones you need for AQA 8461 4.5.4 are auxin, gibberellins and ethene. Each one has a natural role and a commercial use.

A tropism is a directional growth response to a stimulus. Phototropism is growth in response to light; gravitropism (also called geotropism) is growth in response to gravity. A response towards the stimulus is positive; a response away from it is negative. Shoots are positively phototropic and negatively gravitropic — they grow up and towards light. Roots are positively gravitropic and negatively phototropic — they grow down, away from light.

Auxin — direction of growth

Auxin is made in the tips of shoots and roots and diffuses back down through the cells just behind the tip. Auxin causes cells to elongate by stretching, but its effect depends on the organ:

in shoots, auxin stimulates cell elongation;
in roots, auxin inhibits cell elongation.

That single rule explains both tropisms.

Phototropism in shoots. When light shines on a shoot from one side, auxin in the tip is redistributed to the shaded side. The shaded side has more auxin, so its cells elongate more than the lit side. The shoot bends towards the light. This makes sense biologically: bending towards light maximises photosynthesis.

Gravitropism in roots. When a root is laid on its side, gravity causes auxin to gather on the lower side of the root. Because auxin inhibits elongation in roots, the lower side grows less than the upper side. The upper side elongates more, and the root bends downwards into the soil. This helps the root find water and anchor the plant.

Gravitropism in shoots works the other way: if a shoot is laid sideways, auxin again gathers on the lower side, but because auxin stimulates shoot elongation, the lower side grows faster and the shoot bends upwards.

Required-practical-style investigations

AQA expects you to be able to describe how to investigate the effect of light or gravity on the growth of newly germinated seedlings.

Light experiment. Grow cress or mustard seedlings on damp filter paper in petri dishes. Place one dish in all-round light, one in darkness, and one with light coming from a single direction (often a black box with a slit). After a few days the seedlings in one-sided light have bent towards the light; those in darkness are pale, spindly (etiolated) and grow randomly.
Gravity experiment. Germinate seedlings on damp cotton wool, then lay one dish on its side so gravity acts horizontally. After a day or two, shoots have bent upwards and roots have bent downwards, no matter which way the seed was originally pointing.

Control variables: temperature, water, type and number of seeds, time, and the size of the seedlings at the start.

Gibberellins

Gibberellins are a family of plant hormones that initiate seed germination by ending seed dormancy. They also promote stem elongation (very tall growth) and trigger flowering. Commercially, gibberellins are used to:

start seeds germinating on demand — useful for malting barley in brewing, where uniform germination matters;
grow plants that flower out of season;
make larger, seedless fruit such as seedless grapes.

Ethene

Ethene is unusual because it is a gas. Inside the plant it controls cell division and the ripening of fruit. A single ripe banana produces ethene that triggers neighbouring bananas to ripen — the basis of the saying "one bad apple spoils the bunch".

Commercially, fruit is often picked when it is still unripe and firm so that it survives transport without bruising. Once the fruit reaches the warehouse or supermarket, ethene gas is added to the storage room to ripen the fruit just before sale. This means bananas, tomatoes, mangoes and avocados can be sold across the world.

Commercial uses of auxin

Auxin (or synthetic versions of it) has several commercial uses:

Selective weedkillers (herbicides). Most common weeds are broad-leaved plants; crops like wheat and grass are narrow-leaved. Auxin-based herbicides cause broad-leaved weeds to grow uncontrollably and die, while leaving grasses unharmed.
Rooting powder. Cuttings dipped in rooting powder (which contains auxin) grow roots faster, making it easier to clone plants from cuttings.
Tissue culture. Auxin in the growth medium encourages tiny plant tissue samples to grow new roots and shoots — used in cloning and conservation.
Seedless fruit. Treating unfertilised flowers with auxin makes them develop into fruit without producing seeds.

Putting it together for exam answers

A solid four-mark answer about phototropism names auxin, says it redistributes to the shaded side, says the shaded side elongates more, and ends with the shoot bending towards the light. A six-mark commercial-uses answer should pair each hormone with at least one specific application — for example, ethene ripens fruit during transport, gibberellins are used for malting barley, auxin is the active ingredient in selective weedkillers. Use the words stimulates (shoots) and inhibits (roots) precisely — those are the verbs mark schemes look for.

Exam tips

For phototropism, always say auxin moves to the SHADED side and STIMULATES elongation there — examiners want both verbs.
For gravitropism in roots, switch the verb: auxin moves to the LOWER side and INHIBITS elongation — the upper side grows faster and the root bends down.
Name the hormone, name the organ, name the effect. Vague answers like 'the plant grows towards the light' score zero marks.
When asked about commercial uses, pair each hormone with a specific product: auxin → weedkiller / rooting powder; gibberellins → malting / seedless grapes; ethene → ripening bananas in transport.
Gibberellins is plural — it is a family of hormones. Spell it carefully and don't confuse it with 'glucagon' from topic 4.5.3.
Ethene is a gas, not a spray. In commercial use it is released into a sealed storage room.
In experiments, list the control variables: temperature, water, number and type of seeds, time, and starting size of the seedlings.

Mark-scheme phrasing

Common misconceptions

Worked example

Question:

Answer:

Frequently asked questions

Why does auxin do opposite things in shoots and roots?

The same hormone can have different effects on different cells because root cells and shoot cells have different sensitivities to it. At the concentrations found in plants, auxin stretches shoot cells but stops root cells from elongating.

What is the difference between gibberellins and ethene?

Gibberellins are a family of liquid hormones that mainly start seeds germinating, stretch stems and trigger flowering. Ethene is a single hormone — and a gas — that ripens fruit.

Is ethene used to ripen all fruit?

It works best on 'climacteric' fruit such as bananas, tomatoes, mangoes, avocados and apples, which ripen after picking. Strawberries and grapes do not respond — they have to be picked ripe.

Why do plants in a dark room grow tall and spindly?

They are 'etiolated'. With no light to bend towards, the plant invests in stem elongation to try to reach light. The stems are weak and pale because no chlorophyll is made in the dark.

Are commercial plant hormones the same as the ones plants make?

Sometimes they are identical, but many products use synthetic versions that are cheaper to make or more stable. They bind to the same receptors and trigger the same growth effects.

Could a root ever grow upwards?

Yes, if you upturn the plant the root will detect gravity again and curve so its tip points back down. The auxin redistribution happens within hours.