Form 5 · Common mistakes

Responses in Plants, common mistakes

The mistakes SPM students make on Responses in Plants, why each one loses marks, and the correct version.

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The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Saying a plant 'moves' towards light like an animal.	Growth is confused with movement of the whole body.	A plant does not relocate; cells on the shaded side of the shoot elongate more than cells on the lit side, so the shoot grows and bends towards the light.
Writing that auxin accumulates on the lit side of an illuminated shoot.	Students expect the hormone to build up where the stimulus is strongest.	Auxin accumulates on the shaded side, so those cells elongate more and the shoot bends towards, not away from, the light.
Confusing geotropism with phototropism in an answer.	Both are tropisms examined in the same section, so the names get mixed up.	Geotropism is a growth response to gravity; phototropism is a growth response to light, and a single organ can show both responses at once.
Saying roots grow towards light.	Students assume every organ responds to light the same way a shoot does.	Roots are negatively phototropic, so they normally grow away from light and, at the same time, positively geotropic into the soil.
Assuming every downward-growing root is responding only to geotropism.	Geotropism is the response taught first, so it is applied automatically.	A root usually grows down mainly because of geotropism, but hydrotropism becomes the stronger influence when the soil is very dry and a clear moisture gradient exists to one side.
Calling the leaf-folding of Mimosa pudica a tropism, or naming it thigmotropism.	The stimulus is touch, so it looks like it should be thigmotropism.	The leaf-folding is a nastic movement (seismonasty) caused by a rapid change in turgor pressure; its direction does not depend on the direction of the touch, so it cannot be a tropism.
Saying a nastic movement's direction depends on which side is stimulated.	Students apply the logic of a tropism to a nastic response.	A nastic movement gives the same response regardless of the direction the stimulus comes from; only a tropism's direction depends on the stimulus's direction.
Saying light destroys or breaks down auxin on the lit side of a shoot.	An old, now-outdated explanation for phototropism is sometimes remembered instead of the current one.	Auxin is not destroyed; it is redistributed, moving laterally to the shaded side so that side elongates faster.
Assuming a high concentration of auxin always promotes growth, in both roots and shoots.	Auxin's effect on shoot cells is taught first and generalised to every organ.	Root cells and shoot cells respond differently to the same auxin concentration: it promotes elongation in shoot cells but inhibits elongation in root cells at that concentration.
Describing statoliths as a type of hormone.	Statoliths are introduced alongside auxin, so the two get muddled.	Statoliths are dense, starch-filled organelles inside root cap cells that settle with gravity; auxin is the hormone whose distribution they are thought to influence.
Assuming a synthetic plant hormone works the same way at any concentration applied.	Students treat the hormone like an on/off switch rather than a dose-dependent chemical.	The effect of a synthetic auxin depends on its concentration: a low concentration promotes rooting or fruit development, while a much higher concentration is needed to act as a selective weedkiller, and an excessive dose can damage the crop itself.
Saying a selective weedkiller kills every plant in the field.	Weedkiller sounds like it should be non-selective.	A selective weedkiller is chosen to make certain broad-leaved weeds grow uncontrollably until they die, while leaving the narrow-leaved crop largely unaffected.

How to avoid these mistakes

List the tropisms with their stimulus and one clear example of each.
Explain phototropism using auxin distribution, step by step, from stimulus to bending.
Note the main agricultural uses of plant hormones with a specific example for each use.
Sketch how statoliths settling in the root cap are linked to auxin redistribution in geotropism.
Build a comparison table contrasting tropisms and nastic movements by direction and mechanism.
Practise describing a phototropism or geotropism experiment, including the setup, variables and expected result.

Frequently asked questions

How does a shoot bend towards light?

When light shines on one side of a shoot, the hormone auxin produced at the tip moves to and accumulates on the shaded side rather than the lit side. Auxin makes cells elongate, so the cells on the shaded side grow longer than those on the lit side. This unequal growth causes the shoot to bend towards the light, a positive phototropic response that helps the plant capture more light for photosynthesis.

What is the difference between phototropism and geotropism?

Phototropism is a plant's growth response to the direction of light: shoots grow towards light (positive) and roots grow away from it (negative). Geotropism is a growth response to gravity: roots grow downwards towards gravity (positive) and shoots grow upwards away from it (negative). Both responses are controlled by the same hormone, auxin, redistributed to different effect in shoots and roots.

How are plant hormones used in farming?

Plant hormones are applied to promote root growth in cuttings before planting, to produce seedless fruits by triggering ovary growth without fertilisation, to control the timing of fruit ripening for transport and sale, and, at higher concentrations, as selective weedkillers that make certain broad-leaved weeds grow uncontrollably until they die. These uses all come from understanding how auxin controls plant growth. Because the effective dose is so small, commercial products specify exact concentrations to avoid damaging the crop being treated.