Answers · Plant responses

Shoots that turn to the light — answer key

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

1. PQ1 · Practice

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 it. It is a growth response, so it takes hours.

A full answer includes

I chose: a response in which parts of a plant grow towards or away from the direction of the light source.

Also accept

grow towards or away from the direction of the light source

Watch for

“A process in which a plant uses light to make glucose.” — That is photosynthesis. Phototropism is a growth response to the direction the light comes from.
“A plant growing taller in bright light than in dim light.” — Phototropism is about the direction of the light, not how bright it is: the shoot curves towards one side.
“A response in which parts of a plant grow towards or away from gravity.” — That is gravitropism, a response to gravity. Phototropism is a response to the direction of the light source.

2. PQ2 · Practice

Model answer. The bean shoots grow towards the direction of the light source, the gap in the door: that is phototropism. The mimosa responds to touch, the root in the dark box responds to gravity, and faster growth in a sunny greenhouse is not a response to the direction of the light.

A full answer includes

I chose the bean shoots growing towards the gap in the door.

Also accept

bean shoots growing towards the gap in the door

Watch for

“The mimosa leaves folding when touched.” — That is a fast response to touch, a movement. Phototropism is growth towards or away from the direction of light.
“The root growing down in the dark box.” — In the dark there is no light to respond to: the root is responding to gravity.
“The tomatoes growing faster in the sun.” — Growing faster in bright light is not a response to where the light comes from. Phototropism means curving towards or away from the light source.

3. PQ3 · Practice

Model answer. Shoots with clear plastic caps carry a cap, like the black-paper group, but their tips are lit. If they bend, the cap itself does not stop the response, so darkness at the tip is the difference that matters.

A full answer includes

I chose the shoots with clear plastic caps.
If they bend, a cap on its own does not stop the bending.

Also accept

shoots with clear plastic caps

Watch for

“Shoots with black paper caps, kept in a dark cupboard.” — In the dark no shoot bends at all, so this group cannot tell you anything about the caps.
“More shoots with black paper caps.” — More shoots make the result more reliable, but they do not test whether the cap itself stops the bending.
“Shoots with their tips cut off.” — Cutting changes two things at once: the tip is gone and the shoot is wounded. It does not test the cap.

4. PQ4 · Practice

Model answer. The dashed line from the base to the tip crosses the outer scale, the one that starts at 0° on the upright line, at 24°. The bend angle is 24°.

Checked automatically

Bend angle: 24 ° (within ±1 °)
- Common wrong answer 156 : That is the other scale, which runs in the opposite direction. Read the scale that starts at 0° on the upright line.

A full answer includes

I read the scale that starts at 0° on the upright line.
I wrote 24°.

Also accept

24°

Watch for

“156°.” — That is the other scale, which runs in the opposite direction. Use the scale that starts at 0° where the shoot started, straight up.
“Read the scale where the curved shoot touches it.” — The shoot is curved, so it touches the scale nowhere useful. Read where the straight line from the base to the tip crosses it.

5. PQ5 · Practice

Model answer. The tip. Only the groups whose tips were missing (group 2) or in the dark (group 3) failed to bend; covering the lower half (group 4) made no difference. So the tip detects the direction of the light.

A full answer includes

I chose the tip.
Groups 2 and 3 had no lit tip and did not bend; group 4 bent with its lower half covered.

Also accept

the tip

Watch for

“The lower half of the shoot.” — Group 4 had its lower half covered and still bent, so the lower half does not need light.
“The whole shoot, equally.” — Covering the lower half made no difference, but covering only the tip stopped the bending: the tip is what matters.
“The roots in the compost.” — The roots were the same in every group, yet only the groups without a lit tip failed to bend.

6. PQ6 · Practice

Model answer. (a) At 6 hours the shoots lit from one side had a bend angle of 16°. (b) At 8 hours it was 22° and at 12 hours 26°, so the increase was 26 − 22 = 4°. The shoots lit from above stayed at 0°.

Checked automatically

(a) Bend angle at 6 hours: 16 ° (within ±1 °)
- Common wrong answer 0 : That is the line for the shoots lit from above. Read the line for the shoots lit from one side.
(b) Increase from 8 to 12 hours: 4 ° (within ±1 °)
- Common wrong answer 26 : That is the angle at 12 hours. Subtract the angle at 8 hours.
- Common wrong answer 48 : That is the sum of the two angles. Subtract them.

A full answer includes

(a) I read 16°.
(b) I subtracted the angle at 8 hours from the angle at 12 hours: 26 − 22.
(b) I wrote 4°.

Also accept

(a) 16°; (b) 4°

Watch for

“(a) 0°.” — That is the line for the shoots lit from above. Read the line for the shoots lit from one side.
“(b) 26°.” — That is the bend angle at 12 hours. The increase is the angle at 12 hours minus the angle at 8 hours.

7. PQ7 · Practice

Model answer. The bend angle of the shoots lit from one side rose from 0° to 26° over 12 hours. It rose slowly at first (3° in the first 2 hours), fastest between 2 and 8 hours (6° to 7° every 2 hours), then levelled off (1° between 10 and 12 hours). The shoots lit from above stayed at 0°: light from above gives them no side to bend towards.

A full answer includes

I wrote that the bend angle increased from 0° to 26°.
I wrote that it increased fastest in the middle, between about 2 and 8 hours.
I wrote that it levelled off towards 12 hours.
I wrote that the shoots lit from above stayed at 0°: they did not bend.

Also accept

rose from 0° to 26°, fastest in the middle, then levelled off; lit from above stayed at 0°

Watch for

“The bend angle increased at the same rate all the time.” — The line is not straight: it rises slowly, then steeply, then flattens. Describe each part with values.
“The shoots lit from above did not grow.” — The graph shows only the bend angle. A bend angle of 0° means the shoots grew straight, not that they did not grow.

8. PQ8 · Practice

Model answer. C. Foil caps keep the tips in the dark, so group 1 stays straight. Foil round the base leaves the tip lit, so group 2 bends. A clear cap lets the light through to the tip, so group 3 bends too.

A full answer includes

I chose row C.
I did not choose row D: a clear cap lets the light reach the tip.

Also accept

Watch for

“Row A: foil caps do not stop the bending; foil round the base does.” — Foil round the base leaves the tip lit, so those shoots bend. Foil caps put the tip in the dark, so those shoots stay straight.
“Row B: any foil on a shoot stops it bending.” — Foil stops the bending only when it covers the tip, the part that detects the light. Foil round the base makes no difference.
“Row D: a clear cap still covers the tip, so it stops the bending.” — A clear cap lets the light through, so the tip still detects the direction of the light and the shoot bends.

9. PQ9 · Practice

Model answer. The pots differ in two ways: the caps, and their distance from the lamp, so the uncovered shoots receive brighter light. If only the uncovered shoots bend, the cause could be the uncovered tip or the brighter light. Stand both pots at the same distance from the lamp, so the caps are the only difference.

A full answer includes

I wrote that two things differ: the caps and the distance from the lamp.
I wrote that a difference in bending could be caused by either, so you cannot tell which.
I wrote that both pots should stand at the same distance from the lamp.

Also accept

two things differ; stand both pots at the same distance from the lamp

Watch for

“It is unfair because the capped shoots get no light at the tip.” — Keeping the tip in the dark is the point of the cap: it is the variable being tested. The unfair part is the different distance from the lamp.
“It is fair as long as each pot has the same number of shoots.” — The same number of shoots helps, but the distance from the lamp still differs, so two variables change at once.

10. PQ10 · Practice

Model answer. (a) 8° is anomalous: it is far from the other four, which are all between 26° and 30°. (b) Leaving it out: (26 + 30 + 29 + 27) ÷ 4 = 112 ÷ 4 = 28°.

Checked automatically

(a) Anomalous result: 8 ° (exact value)
- Common wrong answer 30 : 30° is close to the other values. The anomaly is the one that does not fit the pattern.
(b) Mean without the anomaly: 28 ° (within ±0.5 °)
- Common wrong answer 24 : That mean includes the anomalous result. Leave it out and divide by 4.

A full answer includes

(a) I chose 8°.
(b) I added the other four: 26 + 30 + 29 + 27 = 112.
(b) I divided by 4 and wrote 28°.

Also accept

(a) 8°; (b) 28°

Watch for

“(b) 24°.” — That mean includes the anomalous 8°: 120 ÷ 5 = 24°. Leave the anomaly out and divide by 4.
“(a) 30°, because it is the largest.” — The largest value is close to the others. An anomaly is a value that does not fit the pattern of the rest.

11. PQ11 · Practice

Model answer. Increase in length = 34 − 21 = 13 mm. Mean rate = 13 ÷ 24 = 0.5417… = 0.54 mm per hour to 2 significant figures.

Checked automatically

Mean rate of growth: 0.54 mm per hour (2 significant figures)
- Common wrong answer 1.4 : That uses the final length. Find the increase first: 34 − 21.
- Common wrong answer 0.88 : That uses the starting length. Find the increase first: 34 − 21.
- Common wrong answer 13 : That is the increase in length. Divide it by the time taken.

A full answer includes

I found the increase: 34 − 21 = 13 mm.
I divided by 24 hours: 13 ÷ 24 = 0.5417…
I gave 0.54 mm per hour, to 2 significant figures.

Also accept

0.54 mm per hour

Watch for

“1.4 mm per hour.” — That is 34 ÷ 24: it uses the final length, not the increase. Subtract the starting length first.
“0.88 mm per hour.” — That is 21 ÷ 24, the starting length divided by the time. Use the increase in length.

12. PQ12 · Practice

Model answer. A plant cannot want or decide anything. The shoot curves because its shaded side grows more than its lit side, so it grows towards the direction of the light: phototropism. Growing towards the light helps the leaves absorb more light, which is why the response is useful, not what causes it.

A full answer includes

I wrote that a plant has no wants and makes no choices.
I wrote that the shaded side of the shoot grows more than the lit side.
I wrote that this uneven growth makes the shoot curve towards the light.

Also accept

no wants; the shaded side grows more, so the shoot curves towards the light

Watch for

“It is incorrect because plants do not need light.” — Plants do need light, for photosynthesis. What is wrong is saying the plant wants it: the curve is caused by uneven growth.
“The shoot bends like a muscle pulling it over.” — A shoot has no muscles. It curves because one side grows longer than the other.

13. PT1 · Apply

Model answer. The shoots grow straight up. As the turntable turns, every side of each shoot faces the window for the same time, so the light has no one direction that lasts. No side grows more than another, and the shoots do not curve.

A full answer includes

I predicted that the shoots grow straight up.
I explained that every side faces the light for the same time, so no side grows more than the others.

Also accept

straight up: every side is lit for the same time

Watch for

“They curve towards the window.” — They would if the pot stood still. On the turntable the window is on a different side every few minutes, so no side stays shaded.
“They grow in a spiral, following the turn.” — Each side is lit in turn for the same time, so the growth evens out and the shoots grow straight.

14. PT2 · Apply

Model answer. Cutting changes more than the light reaching the tip: the shoot is wounded and may stop growing, and a shoot that does not grow cannot bend. A safer test keeps the tip but stops light reaching it, with a foil cap, and adds shoots with clear caps as a control.

A full answer includes

I wrote that cutting also wounds the shoot, and may stop it growing at all.
I wrote that the shoot might not bend for that reason, not because it cannot detect light.
I suggested keeping the tip but covering it with a foil cap, with clear-capped shoots as a control.

Also accept

cutting also wounds the shoot; cap the tip with foil, with a clear-cap control

Watch for

“The conclusion is safe because the shoots did not bend.” — They did not bend, but more than one thing was changed, so you cannot tell which change stopped them.
“It is not safe because the shoots needed more light.” — The light was the same as for shoots that bend. The problem is that cutting changes two things at once.