Tropism — growth towards or away from a directional stimulus
A tropism is a growth movement of part of a plant in response to a directional (unidirectional) stimulus. Growth towards it is positive; growth away is negative. Gravitropism responds to gravity, phototropism to unidirectional light. Both work by unequal cell elongation on opposite sides of a shoot or root. Plants have no nervous system, so they coordinate growth with hormones (auxin) instead.
Four directional responses — shoot versus root
Shoot: positively phototropic (grows towards light) and negatively gravitropic (grows upwards, against gravity). Root: negatively phototropic (grows away from light) and positively gravitropic (grows downwards, with gravity). Both pairings are adaptive — a shoot growing up and towards light puts its leaves where they absorb most light for photosynthesis, while a root growing down anchors the plant and reaches water and mineral ions in the soil.
(Extended) Auxin — made in the tip, acts on the shaded side
Auxin is a plant hormone made in the tip of the shoot. It diffuses back to the elongation zone and stimulates cells to elongate. Under unidirectional light it is transported away from the light, concentrating on the shaded side; those cells elongate more, and the unequal growth bends the shoot towards the light. The same mechanism drives gravitropism: in a horizontal shoot auxin gathers on the lower side, bending it upwards.
Drawn from real examiner reports.
Auxin gathers on the shaded side
(Extended) Auxin accumulates on the shaded side, never the illuminated side. Those shaded cells elongate more, so the shoot bends towards the light. "Auxin builds up on the lit side, which grows faster" reverses two steps and only reaches the right bend by two compounding errors — examiners mark the mechanism step by step, so it scores nothing.
Auxin concentrated on the illuminated side stated instead of the shaded side. (s23 Papers 21/22 Q26; s22 Paper 21 Q26; w22 Paper 43 Q4a i — cambridge-0610.md Section 14)
"Positive" means direction, not benefit
Positive means the organ grows towards the stimulus; negative means it grows away from it. Neither word says anything about how strong, useful or beneficial the response is. Candidates who read "negative" as "weak" or "harmful" then guess the pairings, and lose marks that the terminology itself would have handed them.
Shoot and root pairings get swapped
"Roots are positively phototropic" and "shoots are positively gravitropic" are both reversals and score zero. Self-check by direction first: shoots grow up and towards light, so they are positively phototropic and negatively gravitropic; roots grow down and away from light, so they are positively gravitropic and negatively phototropic. Attach the label last.
Positive/negative tropism direction confused with a value judgement, or shoot/root responses reversed — consistent with the recurring 0610 pattern of confusing directional biological terms (e.g. veins/arteries, rods/cones) noted across sittings. (cambridge-0610.md Section 14, general pattern)
"It poisons the weed" is not the mechanism
(Extended) "Weedkiller poisons the plant" or "it stops photosynthesis" earns nothing. Synthetic auxins are absorbed by broad-leaved weeds and disrupt normal growth control, causing uncontrolled, excessive growth the plant cannot sustain — it outgrows its water and nutrient supply and dies. Name the hormone class and the uncontrolled-growth step explicitly.
Weedkillers are selective, not universal
(Extended) Synthetic auxin herbicides affect broad-leaved plants (most weeds) far more strongly than narrow-leaved plants such as cereals and grasses — which is why a wheat field can be sprayed and the crop survives. Answers that treat the chemical as equally lethal to everything, or that never say why the crop is spared, drop the selectivity mark.
Auxin is redistributed, not destroyed
(Extended) A popular wrong answer is that light "destroys" or "breaks down" the auxin on the lit side. The mechanism required at Cambridge 0610 is redistribution: auxin made in the tip is transported away from the light towards the shaded side. Nothing is broken down, so write "transported to the shaded side", not "destroyed by light".
Give the full five-link chain
"Explain" questions reward the chain, not the verdict: hormone made in the tip → unequal distribution to the shaded (or lower) side → more cell elongation there → unequal growth → bending. Jumping straight from "auxin" to "the shoot bends" loses marks.
The longer side is the outside of the bend
This is the link candidates fumble. The shaded side elongates more, so it becomes the longer, outer edge of the curve and the tip is pushed round towards the light. Sketch the two sides if unsure — the side that grows more is always on the outside of the bend.
Name which stimulus is acting
In complete darkness only gravitropism can operate; with light from one side, phototropism is being tested. State which directional stimulus is present before you describe the mechanism, so the examiner can see you have not merged the two responses into one answer.
Answer both halves of a weedkiller question
(Extended) A selectivity mark scheme has two halves: why the broad-leaved weed dies (uncontrolled growth from the synthetic auxin) and why the narrow-leaved crop survives (lower sensitivity). Answers describing only the weed's death cap below full marks.
A tropic response (tropism) is a growth movement of part of a plant in response to a directional (unidirectional) stimulus. A response towards the stimulus is positive; a response away from the stimulus is negative.
Gravitropism (geotropism): a growth response to the direction of gravity.
Phototropism: a growth response to the direction of (unidirectional) light.
Define gravitropism and phototropism.
(a) Define the terms gravitropism and phototropism.
(b) State the direction of response (positive or negative) of a shoot to light, and of a root to gravity.
