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Key concept
Tropisms are directional growth responses of plants to stimuli. Phototropism is growth toward or away from light; gravitropism is growth toward or away from gravity. Auxin is a plant growth hormone that promotes cell elongation. Unequal distribution of auxin on the two sides of a shoot or root causes bending.
Tropisms are a Supplement topic linking plant biology to coordination. The key concept is that auxin accumulates on one side of the plant, causing unequal growth that bends the shoot or root. You must explain the mechanism, not just state the direction of bending.
Types of tropism
Phototropism: a growth response to the direction of light. Plant shoots are positively phototropic: they grow toward the light. This positions leaves to maximise light absorption for photosynthesis.
Gravitropism (geotropism): a growth response to gravity. Roots are positively gravitropic: they grow downward into the soil for anchorage and water absorption. Shoots are negatively gravitropic: they grow upward away from gravity toward the light.
How auxin controls phototropism
Auxin is a plant hormone produced at the shoot tip. It moves toward the shaded side of the shoot when light comes from one direction. The higher concentration of auxin on the shaded side causes cells there to elongate more than cells on the lit side.
Because the shaded side grows faster, the shoot bends toward the light. This is positive phototropism. If you remove the tip of the shoot, it no longer responds to light, confirming that auxin production occurs at the tip.
Auxin and gravitropism in roots
In roots, auxin has the opposite effect: it inhibits cell elongation at high concentrations. When a root is placed horizontally, auxin accumulates on the lower side due to gravity. The high auxin concentration inhibits growth on the lower side, while cells on the upper side elongate normally. This causes the root to bend downward: positive gravitropism.
The key distinction: auxin promotes growth in shoots but inhibits growth in roots. This difference explains why shoots grow up and roots grow down even though auxin moves in the same direction (downward under gravity).
Try a focused question (4 marks)
A shoot tip is illuminated from one side. Explain why the shoot bends toward the light.
Show the answer and marking guidance
Auxin moves from the lit side to the shaded side of the shoot tip.
There is a higher concentration of auxin on the shaded side.
Auxin promotes cell elongation, so cells on the shaded side elongate more.
The unequal growth causes the shoot to bend toward the light (positive phototropism).
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying auxin moves toward the light: it moves away from the light (to the shaded side).
Forgetting that auxin promotes growth in shoots but inhibits growth in roots.
Describing the shoot as moving toward light rather than growing toward it (it is growth, not movement).
Claiming the lit side grows faster: the shaded side grows faster because it has more auxin.