Edexcel GCSE Biology (1BI0)

6.3 Plant hormones

Plants cannot move, but they grow in directions that help them survive. Topic 6.3 of Edexcel 1BI0 is about the plant hormones that control this growth — auxins, gibberellins and ethene — and how growers use them commercially. You need to be able to explain how auxin makes a shoot bend towards light and a root bend downwards, why gibberellins are used to make seedless grapes and to start malting in breweries, why ethene gas is sprayed on fruit after long-distance transport, and why an auxin-based powder makes cuttings root quickly while an auxin-based weedkiller kills dandelions but leaves grass alive. This page works through each idea using the verbatim mark-scheme phrases Edexcel examiners look for in six-mark answers on Paper 2 (1BI0/2H).

From the 1BI0 specification

Plants respond to their environment using hormones. Auxin is produced at the tips of shoots + roots and controls growth: in shoots, auxin accumulates on the shaded side, stimulating cell elongation so the shoot bends towards light (phototropism); in roots, auxin gathers on the lower side and inhibits cell elongation, so the upper side grows faster and the root bends down (gravitropism). Gibberellins promote seed germination, stem elongation and flowering; commercial uses include producing seedless fruit and stimulating malting in brewing. Ethene controls fruit ripening and is used commercially to ripen fruit after transport. Auxins are used commercially in rooting powders (cuttings) and as selective weedkillers (kill broad-leaved weeds, sparing narrow-leaved crops).

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 (in the xylem, phloem or by simple diffusion through cells) to influence growth somewhere else. Three hormones are named on the Edexcel 1BI0 6.3 specification: auxins (which control tropisms — directional growth responses), gibberellins (a family of hormones that start seeds germinating, stretch stems and trigger flowering) and ethene (a gas that ripens fruit). The big idea 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, and that one fact explains both phototropism and gravitropism. Edexcel also has a strong focus on commercial applications, so you need to know exactly how growers use these hormones — auxins in rooting powders for cuttings and as selective weedkillers, gibberellins to grow seedless fruit and start malting, and ethene to ripen fruit after long-distance 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 earlier Edexcel topic 6.2 (animal hormones).
Cell structure and that young plant cells can elongate by taking up water into the vacuole.
Photosynthesis happens in leaves, so reaching the light matters for survival (topic 4).
Sexual vs asexual reproduction (topic 3) — relevant to cuttings, fertilisation and seedless fruit.
Basic experimental design: control variables, repeats, fair tests.

Define plant hormone, tropism, phototropism and gravitropism — clarify positive vs negative tropism.
Build the auxin model in shoots: auxin made at the tip, accumulates on the shaded side, stimulates elongation there, shoot bends towards the 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, seedless fruit, malting) and ethene (gas, ripens fruit after transport).
Map each hormone to its Edexcel-named commercial uses: rooting powder, selective weedkiller, seedless grapes, malting, ethene after transport.
Practise five- and six-mark answers that name the hormone, describe redistribution or mechanism, and finish with the commercial 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 'shoot 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 promote seed germination, stem elongation and flowering. Used commercially to make seedless fruit and to start malting in brewing. (Spelled with an 's' — 'gibberellins' is a family of hormones.)
ethene: A gaseous plant hormone that controls fruit ripening. Used commercially to ripen fruit after long-distance transport.
cell elongation: How 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 causing excessive growth, leaving narrow-leaved crops unaffected.
rooting powder: A powder containing auxins in which plant cuttings are dipped to stimulate root growth and clone the plant rapidly.
cutting: A piece of stem cut from a parent plant that grows into a new genetically identical plant — a form of asexual reproduction.
malting: The first stage of brewing beer, in which barley grains are made to germinate uniformly — gibberellins are 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 named in Edexcel 1BI0 6.3 are auxins, gibberellins and ethene. Each one has a natural role inside the plant and a set of commercial uses you must be able to describe.

A tropism is a directional growth response to a stimulus. Phototropism is growth in response to light; gravitropism (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, into the soil.

Auxin in shoots — phototropism

Auxin is made in the tips of shoots and roots and diffuses down through the cells just behind the tip. Auxin causes young plant cells to elongate (stretch by taking water into the vacuole), but its effect depends on the organ:

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

When light shines on a shoot from one side, auxins move to/accumulate on the shaded side of the shoot. Auxins stimulate cell elongation on the shaded side, while cells on the lit side elongate less. The uneven growth means the shoot bends towards the light. This is positive phototropism, and it makes biological sense: by bending towards light the shoot maximises photosynthesis.

Auxin in roots — gravitropism

When a root is laid on its side, gravity makes the auxin 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, so the root bends downwards into the soil. This helps the root find water and anchor the plant.

If a shoot is laid on its side, auxin again sinks to the lower side, but because auxin stimulates elongation in shoots, the lower side grows faster and the shoot bends upwards.

Gibberellins

Gibberellins are a family of plant hormones that promote seed germination, stem elongation and flowering. Inside the plant they end seed dormancy, make stems stretch, and trigger the switch to flower production.

Edexcel asks for two named commercial uses:

Seedless grapes. Spraying unfertilised grape flowers with gibberellins makes them develop into fruit. Gibberellins stimulate fruit formation without fertilisation; no fertilisation means no seeds; gibberellins stimulate cell growth in the fruit, so the grapes are still large and juicy; and seedless fruit are more desirable to consumers who do not want to spit pips.
Malting in brewing. In a brewery, barley grains must all germinate at the same time so the starch inside is converted to sugar uniformly. Adding gibberellins to the grains ends dormancy together, speeds up germination, and gives a consistent batch of malted barley for making beer.

Ethene

Ethene is unusual because it is a gas. Inside the plant it controls the ripening of fruit — a single ripe banana releases ethene that triggers neighbouring bananas to ripen.

The Edexcel commercial use focuses on fruit transport. Fruit is harvested unripe; unripe fruit is firmer so less damaged during long-distance shipping; ethene is applied on arrival in special sealed warehouses; the fruit arrives in sellable condition on supermarket shelves. This is why bananas grown in tropical countries can be sold yellow and ready to eat in the UK.

Auxins in rooting powder

Gardeners and growers use auxins to clone plants from cuttings. A piece of stem cut from a parent plant is dipped in rooting powder, which contains auxins. The auxins stimulate root growth in the cutting, so it forms roots quickly and becomes a new independent plant. This is rapid plant cloning — every cutting is genetically identical to the parent. Because cuttings are asexual reproduction, there is no genetic mixing, so any desirable trait of the parent (flower colour, disease resistance, fruit yield) is preserved exactly.

Auxins as selective weedkillers

Most garden weeds — dandelions, daisies, plantains — are broad-leaved plants. Most crops grown as monocultures — wheat, barley, lawn grass — are narrow-leaved. An auxin-based selective weedkiller exploits this difference.

Auxins stimulate/promote growth in shoots. At low concentrations they encourage normal growth, but at the high concentration used in a weedkiller they cause excessive growth in weeds. The weed cannot sustain that growth and dies. Narrow-leaved crops are not affected at the same dose, partly because they take up less of the chemical through their thinner leaves and partly because they tolerate it differently. The result is a clean field of cereal or a tidy lawn.

Required experimental work

Edexcel expects you to be able to investigate plant responses with newly germinated seedlings. Grow cress on damp filter paper in petri dishes. One dish in all-round light, one in darkness, one with one-sided light. After several days the seedlings in one-sided light have bent towards the light; those in darkness are pale, spindly (etiolated) and grow randomly; those in all-round light grow straight up. Control variables: temperature, water, type and number of seeds, time, and seedling starting size.

Putting it together for exam answers

A five-mark Edexcel answer on phototropism must name auxins, say they accumulate on the shaded side, say they stimulate elongation there, contrast with cells on the lit side elongating less, and finish with the shoot bending towards the light. For commercial uses, pair each hormone with at least one specific Edexcel application — ethene with fruit transport, gibberellins with seedless grapes or malting, auxins with rooting powder or selective weedkillers. Use the words stimulates (shoots) and inhibits (roots) precisely — those verbs are what mark schemes look for.

Exam tips

For phototropism, always say auxins ACCUMULATE on the SHADED side and STIMULATE cell elongation there, while cells on the LIT side elongate LESS — Edexcel mark schemes use those exact phrases.
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.
For commercial uses, pair each hormone with a specific Edexcel application: auxins → rooting powder for cuttings / selective weedkiller; gibberellins → seedless grapes / malting; ethene → ripening fruit after transport.
Gibberellins is plural — it is a family of hormones. Spell it carefully.
Ethene is a GAS, not a spray. In commercial use it is released into a sealed storage room on arrival of the unripe fruit.
For the weedkiller question, remember the HIGH CONCENTRATION point — the auxin makes weeds grow so fast they cannot sustain it and die.
Cuttings are asexual reproduction, so every new plant is a clone of the parent — no genetic mixing.

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 shoot cells and root cells respond differently 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 promote seed germination, stretch stems, trigger flowering and produce seedless fruit. Ethene is a single hormone — and a gas — that ripens fruit.

Why are seedless grapes more desirable to consumers?

Customers prefer fruit they can eat without spitting out pips. Seedless varieties also tend to be larger and sweeter because gibberellins stimulate cell growth in the fruit.

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 or more stable. They bind to the same receptors and trigger the same growth effects.

Why does a selective weedkiller not kill the grass?

Auxins at high concentration cause excessive, uncontrollable growth in broad-leaved weeds, which then die. Narrow-leaved crops like grass take up less of the chemical through their thin leaves and are not affected at the same dose.