Answers · Plant responses

Plant responses — final check key

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

1. PK1 · Final check

Model answer. R, S and T bend: each has a lit tip (the clear cap lets the light through, and the band of foil on S is below the tip). P has no tip and Q has its tip in the dark, so neither detects the direction of the light and neither bends.

A full answer includes

I chose R, S and T.
I did not choose P or Q: neither has a lit tip.

Also accept

R, S and T

Watch for

“S stays straight because it is covered with foil.” — The foil on S is round the middle. Its tip is lit, so it detects the light and bends.
“R stays straight because its tip is covered.” — The clear cap lets the light through, so the tip of R still detects the light.
“P bends because the rest of the shoot is lit.” — P has no tip, the part that detects the direction of the light, so it does not bend.
“Q bends because the light reaches the rest of its shoot.” — The foil cap on Q stops light reaching its tip, so the tip cannot detect the direction of the light and Q does not bend.

2. PK2 · Final check

Model answer. (a) The stimulus is light coming from one direction, the window; the response is phototropism: the shoots grow towards the direction of the light source. (b) Grow identical seedlings with the same soil, water and temperature, but lit evenly from above. If they grow straight, the lean was caused by the light from one side.

A full answer includes

(a) I wrote that the stimulus is light from one direction, the window.
(a) I wrote that the response is phototropism: the shoots grow towards the light.
(b) I suggested identical seedlings lit evenly, for example from above, with everything else kept the same.

Also accept

(a) light from one side, phototropism; (b) identical seedlings lit evenly from above

Watch for

“(a) The stimulus is the warmth of the window.” — The window may be warm as well as bright, so warmth cannot be ruled out just by looking. The control in (b) keeps the temperature the same and changes only the direction of the light: if those seedlings grow straight, the light from one side caused the lean.
“(b) Put the seedlings in the dark.” — Seedlings in the dark change two things: the direction of the light and whether there is any light at all. Light from above changes only the direction.

3. RK1 · Final check

Model answer. P curves upwards: in the dark gravity is the only stimulus with a direction, and shoots grow away from it. Q grows straight out sideways: on the turning clinostat gravity acts on every side for the same time. R curves upwards too: shoots grow away from gravity and towards the light from above, so both stimuli turn it the same way.

A full answer includes

P: I predicted that the shoot curves upwards, away from gravity.
Q: I predicted that the shoot grows straight out sideways.
R: I predicted that the shoot curves upwards, as both gravity and the light from above turn it that way.

Also accept

P up; Q straight out sideways; R up

Watch for

“Q curves upwards, because gravity is still there.” — Gravity is still there, but the turning makes it act on every side for the same time, so the shoot does not curve.
“R grows sideways, because the light is not on its side.” — The light from above and gravity both turn the shoot upwards.

4. RK2 · Final check

Model answer. The fallen tree. Its branch tips grow away from gravity, so they turn to grow upwards: gravitropism. The sunflower responds to light, the tendril responds to touch, and a falling apple is not a growth response at all.

A full answer includes

I chose the fallen tree whose branch tips turn to grow upwards.

Also accept

the fallen tree whose branch tips turn upwards

Watch for

“A sunflower head turning to face the morning sun.” — That is a response to light, not to gravity.
“An apple falling from a tree.” — A falling apple is not a growth response: it falls because of its weight.
“Pea tendrils coiling round a stick.” — That is a response to touch, not to gravity.

5. AK1 · Final check

Model answer. Auxin is made in the shoot tip and diffuses down. Light from the right makes it unequally distributed: more moves to the shaded, left side. Auxin stimulates cell elongation, so the cells on the left side elongate more, the left side grows longer and the shoot curves to the right, towards the window. A hormone controlling growth in this way is chemical control.

A full answer includes

I wrote that auxin is made in the shoot tip and diffuses down.
I wrote that light from the right makes more auxin move to the shaded, left side.
I wrote that the cells on the left elongate more, so the shoot curves to the right.
I wrote that this is chemical control: a hormone changes how much the cells grow.

Also accept

auxin to the shaded left side; left cells elongate more; curve to the right

Watch for

“The lit side has more auxin, so it grows towards the light.” — Auxin moves to the shaded side. The shaded side grows longer, which turns the shoot towards the light.

6. AK2 · Final check

Model answer. (a) As the clinostat turns, every side of the shoot is the lower side for the same time, so the auxin does not collect on one side: it stays evenly distributed, both sides elongate by the same amount and the shoot grows straight out. (b) The response is controlled by a hormone: auxin made in the tip diffuses down, gravity makes it collect on the lower side, and there it stimulates cell elongation. A chemical made in one place that changes growth in another is chemical control.

A full answer includes

(a) I wrote that as the shoot turns, no side stays lowest, so the auxin does not collect on one side.
(a) I wrote that both sides elongate by the same amount, so it grows straight.
(b) I wrote that a hormone, auxin, made in the tip travels to the region below and changes how much the cells there elongate.

Also accept

(a) no side stays lowest, auxin even; (b) a hormone made in the tip controls elongation below

Watch for

“(a) The shoot grows straight because gravity stops acting on it.” — Gravity still acts, but on every side in turn, so the auxin never collects on one side.