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Plant hormones, phototropism and gravitropism

Plant structures and their functions · Plant hormones · note 1 of 2

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Plant hormones, phototropism and gravitropismSpec 6.15B

In short

A tropism is a growth response to a stimulus, and auxins are the plant hormones involved in phototropism and gravitropism. In a shoot lit from one side, auxin moves to the shaded side and makes those cells elongate, so the shoot bends towards the light. In roots, auxin slows growth on the lower side, so roots grow downwards.

Plant hormones are chemicals that control and coordinate plant growth and development. They are made in one part of the plant and move to other parts, where they change how cells grow.

A tropism is a growth response to a stimulus. A growth towards the stimulus is positive. A growth away from the stimulus is negative. Auxins are the plant hormones involved in two tropisms:

  • Phototropism: growth in response to light.
  • Gravitropism: growth in response to gravity.

Auxins in shoots

  1. Auxin is made in the tip of the shoot.
  2. When light shines from one side, auxin moves to the shaded side of the shoot.
  3. In shoots, auxin makes cells elongate (get longer), so the cells on the shaded side grow faster.
  4. The shoot bends towards the light. Shoots are positively phototropic.

If a shoot is lying on its side, auxin collects on the lower side. The cells there elongate more, so the shoot bends upwards, away from gravity. Shoots are negatively gravitropic.

Auxins in roots

Roots respond differently to auxin. In a root that is lying on its side, auxin collects on the lower side, where it slows the growth of the cells. The cells on the upper side grow faster, so the root bends downwards, in the direction of gravity. Roots are positively gravitropic. Roots also grow away from light, so they are negatively phototropic.

Tropisms of shoots and roots
Response to lightResponse to gravity
ShootPositive phototropism (grows towards light)Negative gravitropism (grows upwards)
RootNegative phototropism (grows away from light)Positive gravitropism (grows downwards)
Top: a shoot lit from the right with auxin made in the tip moving to the shaded left side, then the shoot bent towards the light with longer cells on the shaded side. Bottom: a horizontal root with auxin collected on the lower side, then the root bent downwards because the upper side grows faster. (opens full size in a new tab)
Shoots: auxin makes shaded-side cells elongate, so the shoot bends towards light. Roots: auxin on the lower side slows growth, so the root bends down.
Exam tip:

Tell the story in order: where auxin is made, where it moves to, what it does to cell growth there, and which way the plant bends as a result.

Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026

Frequently asked questions

How do auxins cause phototropism?

Auxin is made in the tip of the shoot. When light shines from one side, auxin moves to the shaded side, where it makes the cells elongate. The cells on the shaded side grow faster than those on the lit side, so the shoot bends towards the light. Shoots are therefore positively phototropic.

Why do roots grow downwards?

Roots grow downwards because they are positively gravitropic. In a root lying on its side, auxin collects on the lower side, where it slows the growth of the cells. The cells on the upper side grow faster, so the root bends down in the direction of gravity. In shoots, auxin has the opposite effect.

What is the difference between phototropism and gravitropism?

Phototropism is growth in response to light, and gravitropism is growth in response to gravity. Both are controlled by auxins. Shoots grow towards light and upwards, so they are positively phototropic and negatively gravitropic. Roots grow away from light and downwards, so they are negatively phototropic and positively gravitropic.

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