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Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next. (2026)
Question
A young plant is placed in a box with light coming through a slit on one side. Explain how and why the shoot bends towards the light. (4 marks)
Solution
Auxin is made in the tip of the shoot.
Auxin moves to the shaded side (away from the light).
Auxin causes cells on the shaded side to elongate more than cells on the lit side.
The shaded side grows longer than the lit side, so the shoot bends towards the light.
Answer
Auxin is produced in the shoot tip (1). It moves to the shaded side of the shoot (1). Auxin causes the cells on the shaded side to elongate more (1). The shaded side becomes longer than the lit side, so the shoot bends towards the light (1).
Question
A seedling is laid on its side. Explain why the root curves downwards. (3 marks)
Solution
Auxin sinks under gravity to the lower side of the root.
In roots, auxin INHIBITS cell elongation.
The upper side (less auxin) elongates more, so the root curves downwards.
Answer
Auxin sinks to the lower side of the root under gravity (1). In roots, auxin INHIBITS cell elongation (1). The upper side elongates more, so the root curves downwards (positive gravitropism) (1).
Examiner note
Make sure to say auxin INHIBITS elongation in roots — this is the opposite of shoots.
Question
Explain how a selective auxin weedkiller kills weeds on a lawn but leaves the grass alive. (3 marks)
Solution
Weeds are usually broad-leaved plants; grass is narrow-leaved.
Auxin at high concentration causes uncontrolled growth in broad-leaved plants, distorting them so they die.
Narrow-leaved grasses are far less sensitive to these auxin doses, so they survive.
Answer
Weeds (dandelions etc.) are broad-leaved; grass is narrow-leaved (1). At the doses used, auxin causes uncontrolled, distorted growth in broad-leaved plants, killing them (1). Narrow-leaved grasses are far less sensitive and survive (1).
Question
Explain how ethene is used to deliver ripe bananas to UK supermarkets from South America. (4 marks)
Solution
Bananas are picked unripe (green, firm) so they don't bruise during transport.
They are shipped in cool, controlled-atmosphere containers (low/no ethene) to slow ripening.
At the destination, the bananas are placed in rooms with ethene gas.
The ethene triggers ripening so the bananas are perfectly ripe by the time they reach supermarket shelves, reducing food waste.
Answer
Bananas are picked unripe (green, firm) so they don't bruise during long transport (1). They are shipped in cool, ethene-free conditions to slow ripening (1). At the destination they are exposed to ethene gas in controlled rooms (1). The ethene triggers ripening so they reach supermarket shelves perfectly ripe, reducing waste (1).
A plant hormone produced in the tips of shoots and roots. Causes cell elongation in shoots and inhibits cell elongation in roots. Responsible for phototropism and gravitropism.
A growth response in plants to the direction of light. Shoots grow towards light (positive); roots grow away (negative).
A growth response in plants to gravity. Roots grow downwards (positive); shoots grow upwards (negative).
A group of plant hormones that promote seed germination, stem elongation and flowering.
A gaseous plant hormone that controls cell division and ripens fruit.
A herbicide containing synthetic auxins. Kills broad-leaved weeds but does not harm narrow-leaved crops or grasses.
A powder containing auxin used on stem cuttings to stimulate quick root growth, allowing vegetative cloning.
The growing of whole plants from a few parent cells in a sterile auxin-rich gel. Produces large numbers of identical clones.
A period after a seed is shed when it does not germinate, waiting for favourable conditions. Can be broken artificially using gibberellin.
A gaseous plant hormone used to ripen fruit during/after transport, allowing fruit to be picked unripe and ripened at destination.
Mistake
Saying auxin elongates cells in both shoots and roots.
Why it happens
Forgetting the opposite effect in roots.
How to avoid it
Memorise: shoots = ELONGATE. Roots = INHIBIT.
Mistake
Saying auxin moves to the LIT side of a shoot.
Why it happens
Misremembering the direction.
How to avoid it
Auxin moves AWAY from the light → to the SHADED side. That's the side that elongates more, bending the shoot TOWARDS the light.
Mistake
Describing RP 8 without mentioning a control.
Why it happens
Focusing only on the variable being tested.
How to avoid it
Always describe a control set-up (e.g. all-round light or upright dish).
Mistake
Pairing the wrong hormone with the wrong use (e.g. 'gibberellin in weedkiller').
Why it happens
Mixing up names.
How to avoid it
Auxin = weedkiller + rooting. Gibberellin = germination + flowering + fruit size. Ethene = ripening.
Mistake
Saying weedkiller 'just kills weeds' without explaining selectivity.
Why it happens
Skipping the broad-vs-narrow leaf reason.
How to avoid it
Always say: broad-leaved plants (weeds) are sensitive to high auxin; narrow-leaved (crops/grass) are not.
Mistake
Saying ethene 'preserves' or 'stops' fruit ripening.
Why it happens
Reversing the direction.
How to avoid it
Ethene = ripens. To slow ripening, remove ethene (cool, ventilated transport).