FreeScienceLessons · AQA BiologyPlant Hormones — auxins and tropisms
Key facts
Auxins and tropisms
Auxins are plant hormones that control growth. Made in shoot tips.
Phototropism — growth in response to light. Shoots grow towards light (positive).
Gravitropism (geotropism) — growth in response to gravity. Roots grow towards gravity (positive). Shoots grow away from gravity (negative).
Mechanism: light hits one side → auxin moves to shaded side → cells on shaded side elongate more → shoot bends towards light.
In roots: auxin inhibits growth (opposite to shoots). More auxin on lower side → less growth on lower side → root bends downward.
Commercial uses of plant hormones
Rooting powder — contains auxins to stimulate root growth in cuttings.
Selective weedkillers — contain synthetic auxins that cause uncontrolled growth in broad-leaved weeds but not grasses.
Ripening fruit — ethene (a plant hormone) triggers fruit ripening. Used commercially to ripen fruit during transport.
Gibberellins — stimulate seed germination, stem elongation, used to produce seedless fruit.
Exam questions — 3 questions · 11 marks
⚡ Extended questions included. Read the hint. Write in full sentences. AI marks against the AQA mark scheme.
1 markPhototropism mechanismAQA 8461 P2 style
Explain why a shoot grows towards a light source.
A The side of the shoot facing the light photosynthesises more, producing more energy for growth
B Auxin moves to the shaded side of the shoot, causing greater cell elongation on that side, bending the shoot towards the light
C The light side of the shoot loses water by evaporation, causing it to shrink
D Auxin is destroyed by light, so cells on the light side grow faster due to lack of auxin inhibition
1 markGibberellins useAQA 8461 style
A fruit grower wants to produce larger seedless grapes. Which plant hormone should they use?
A Auxin
B Ethene
C Gibberellin
D Cytokinin
4 marksCompare root and shoot responseAQA 8461 P2 Higher style
A seedling is placed on its side in the dark. Explain, using your knowledge of auxins, how the shoot and root will respond over the next few hours. (4 marks)
Hint: Same auxin distribution (lower side) → opposite effects because roots and shoots respond differently to auxin. Shoot: more auxin = more growth (upward). Root: more auxin = less growth (downward).