Tropic responses in plants
Phototropism, gravitropism and the role of auxin.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 14.4.1Describe phototropism and gravitropism and explain the role of auxin.
5 minute read
Tropic responses in plants
Plants respond to stimuli by growing, which is slower than an animal's movement but achieves the same end.
The definitions
A tropism is a growth response in which the direction of growth is determined by the direction of the stimulus.
- Phototropism — response to light. Shoots are positively phototropic (grow towards light); roots are negatively phototropic.
- Gravitropism (geotropism) — response to gravity. Roots are positively gravitropic (grow downwards); shoots are negatively gravitropic.
"Positive" means growing towards the stimulus, "negative" means away.
Why these responses help
A shoot growing towards light reaches more light for photosynthesis. A root growing downwards reaches water and mineral ions and anchors the plant.
The role of auxin
Auxin is a plant hormone made at the shoot tip that stimulates cell elongation. When light comes from one side:
- Auxin moves to the shaded side of the shoot.
- A higher auxin concentration there makes those cells elongate more.
- The shaded side grows longer than the lit side, so the shoot bends towards the light.
The bending is caused by unequal growth, not by the plant "turning".
The root behaves oppositely
In roots, a high auxin concentration inhibits elongation. So on the lower side of a horizontal root, accumulated auxin slows growth there, the upper side grows faster, and the root bends downwards. The same hormone produces opposite effects in the two organs, which is exactly what makes this a favourite exam question.
Think of it like this
A shoot bends towards light the way a marching column turns when the outside rank takes longer strides. Nobody steers — one side simply grows more than the other.
Worked examples
Method, step by step
Explain why a shoot grows towards a light source coming from one side.
- 1Auxin is produced at the shoot tip.
- 2Light causes auxin to move to the shaded side of the shoot.
- 3The higher auxin concentration there stimulates greater cell elongation on that side.
- 4The shaded side therefore grows longer than the lit side, bending the shoot towards the light.
Auxin accumulates on the shaded side, where it stimulates more cell elongation, so that side grows longer and the shoot bends towards the light.
Common misconceptions
- Thinking the plant "moves" towards light. It **grows** unequally; no existing tissue relocates.
- Assuming auxin accumulates on the lit side. It moves to the **shaded** side.
- Applying the shoot rule to roots. In roots, high auxin **inhibits** elongation, producing the opposite bend.
In the exam
- Always name the direction of the stimulus and the side auxin moves to before describing the bend — those are the marking points.
- State the benefit if asked "why is this an advantage": more light for photosynthesis, or reaching water and anchorage.