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Learn4BI1Paper 1B · Paper 1 (core content)Structure and functions in living organismsTropisms
Structure and functions in living organisms · Subtopic 21 of 21

Tropisms and Plant Hormones

A tropism is a growth response of a plant to a directional stimulus. Phototropism: shoots grow towards light (positive), helping leaves absorb light. Gravitropism: roots grow down with gravity (positive), shoots grow up (negative). These are controlled by auxin, a hormone made in the shoot tip. In a shoot, auxin moves to the shaded side, where it makes cells elongate more, so the shoot bends towards the light.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 30 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Define phototropism and gravitropism.
  • Explain how auxin causes a shoot to bend towards light.
  • Interpret experiments on plant responses.
Pause and think

A potted plant left on a windowsill ends up leaning towards the window. Why is that useful to it?

Section 2 of 14

Core idea

Start herePlants that bend

Sunflower buds track the Sun across the sky; roots always find their way down. Plants have no nerves, so how do they respond?

The big idea

Plants respond by growing. Auxin, made in shoot tips, gathers on the shaded side and makes those cells elongate, bending the shoot towards light.

1. How auxin works

Phototropism

Unequal auxin causes unequal growth.

Worked exampleClassic experiments

Seedlings lit from one side: (a) untreated, (b) tip covered with foil, (c) tip removed.

Goal
Predict and explain.
Strategy
Where is light detected and auxin made?
  1. (a) bends towards light\text{(a) bends towards light}
    Normal response.
  2. (b) grows straight\text{(b) grows straight}
    Tip can't detect light.
  3. (c) no growth\text{(c) no growth}
    No auxin source.
AnswerThe tip detects light and makes auxin.
✓ CheckMatches Darwin's results ✓.
Key takeawayGravitropism Roots grow towards gravity (positive), shoots away from it (negative), helping seedlings orient correctly underground.
Pause and think

A seedling is laid on its side in the dark. Predict how its shoot and root grow.

Summary

  • Tropisms are growth responses.
  • Auxin on shaded side.
  • Roots + gravitropism.
Key words
Phototropism
Growth response to light direction.
Gravitropism
Growth response to gravity.
Auxin
Plant hormone controlling cell elongation.
HistoryDarwin's seedlings. In 1880 Charles Darwin and his son Francis covered grass seedling tips and showed that the tip detects light but the bending happens lower down: evidence of a messenger, later named auxin.
Fun factPlants in space. On the International Space Station, without gravity, roots grow in all directions; astronauts use light to steer seedling growth instead.
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