Answers · Plant responses

Plant responses — return questions key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. PR1 · Return

Model answer. Phototropism is a response in which parts of a plant grow towards or away from the direction of the light source. Shoots grow towards the light. It is a growth response, so it takes hours.

A full answer includes

I wrote that it is a response in which parts of a plant grow towards or away from the direction of the light source.
I wrote that shoots grow towards the light.
I did not write that the plant moves like a muscle.

Also accept

growth towards or away from the direction of the light source; shoots grow towards it

Watch for

“Phototropism is a plant using light to make food.” — That is photosynthesis. Phototropism is a growth response to the direction of light.

2. PR2 · Return

Model answer. (a) The tip: only shoots whose tips were in the dark failed to bend. (b) The curve forms a little way below the tip, in the region of the shoot that is still growing longer.

A full answer includes

(a) I wrote the tip.
(b) I wrote a little way below the tip, where the shoot is still growing longer.

Also accept

(a) the tip; (b) just below the tip, where the shoot is growing

Watch for

“(b) At the very tip.” — The tip detects the light, but the curve forms below it, where the shoot is still growing longer.
“(a) The base.” — A foil collar below the tip leaves the tip lit, and those shoots still bend, so the lower part of the shoot does not detect the light.

3. RR1 · Return

Model answer. Gravitropism is a response in which parts of a plant grow towards or away from gravity. Roots grow towards gravity, downwards; shoots grow away from it, upwards.

A full answer includes

I wrote that gravitropism is a response in which parts of a plant grow towards or away from gravity.
I wrote that roots grow towards gravity, downwards.
I wrote that shoots grow away from gravity, upwards.

Also accept

towards or away from gravity; roots towards, shoots away

Watch for

“Roots grow down because they are heavier than shoots.” — Roots curve down by growing, even through soil, and a shoot grows up against gravity. Both are growth responses.

4. RR2 · Return

Model answer. Gravity cannot be switched off. A slowly turning clinostat makes gravity act on every side of the seedlings for the same time, so they do not curve. Everything else is the same as for seedlings on a still clinostat, which do curve, so the curve must be caused by gravity acting from one direction.

A full answer includes

I wrote that the clinostat turns slowly, so gravity acts on every side for the same time.
I wrote that seedlings on it do not curve, while seedlings on a still one do.
I wrote that this shows the curve is caused by gravity acting from one direction.

Also accept

gravity on every side for the same time; no curve, so the curve on a still disc is caused by gravity

Watch for

“A clinostat switches gravity off.” — Gravity still acts. The turning spreads it evenly over every side.

5. AR1 · Return

Model answer. (a) Auxin is made in the shoot tip. (b) It diffuses through the plant from the shoot tip into the region below. (c) It stimulates cell elongation.

A full answer includes

(a) I wrote the shoot tip.
(b) I wrote that it diffuses from the shoot tip.
(c) I wrote that it stimulates cell elongation.

Also accept

(a) shoot tip; (b) diffuses from the tip; (c) stimulates cell elongation

Watch for

“(c) It makes the cells divide.” — Auxin stimulates elongation: the cells grow longer.
“(a) In the roots.” — In a shoot, auxin is made in the tip.

6. AR2 · Return

Model answer. Gravity makes the auxin unequally distributed: more of it collects on the lower side of the shoot. Auxin stimulates cell elongation, so the cells on the lower side elongate more than those on the upper side. The lower side grows longer and the shoot curves upwards, away from gravity.

A full answer includes

I wrote that gravity makes auxin collect on the lower side.
I wrote that the cells on the lower side elongate more.
I wrote that the lower side grows longer, so the shoot curves upwards, away from gravity.

Also accept

auxin to the lower side; lower side elongates more; curves up

Watch for

“The upper side grows more and lifts the shoot.” — The lower side, with more auxin, elongates more. The longer side ends up on the outside of the curve, underneath.