Answers · Plant responses

Roots, shoots and gravity — answer key

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

1. RQ1 · Practice

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

A full answer includes

I chose: a response in which parts of a plant grow towards or away from gravity.

Also accept

grow towards or away from gravity

Watch for

“A plant part being pulled down by its own weight.” — Gravitropism is growth, not sagging: a root grows down even through soil that pushes back, and a shoot grows up against gravity.
“A response to the direction of the light source.” — That is phototropism. Gravitropism is a response to gravity, and it happens in the dark too.
“Roots growing towards water in the soil.” — Roots can grow towards water, but that is a response to water. Gravitropism is a response to gravity.

2. RQ2 · Practice

Options as numbered on the pupil sheet: 1 — curves upwards, away from gravity; 2 — curves towards the light; 3 — curves away from the light; 4 — curves downwards, towards gravity.

Model answer. In the dark, gravity is the only stimulus with a direction: the root curves down, towards gravity, and the shoot curves up, away from it. Lit from one side, a shoot curves towards the light and a mustard root curves away from it.

A full answer includes

I matched the root on its side in the dark with curving downwards.
I matched the shoot on its side in the dark with curving upwards.
I matched the lit shoot with curving towards the light, and the lit mustard root with curving away from it.

Also accept

root on its side down; shoot on its side up; lit shoot towards the light; lit mustard root away

Watch for

“A root on its side droops down because it is heavy.” — The root curves down by growing near its tip, not by drooping; a root grows down even through soil.
“Roots and shoots both grow towards the light.” — Shoots grow towards the light; the roots of mustard seedlings grow away from it.

3. RQ3 · Practice

Model answer. In a dark cupboard. Light from one side would also make the shoots curve. In the dark, gravity is the only stimulus that comes from one direction, so any curve must be a response to gravity.

A full answer includes

I chose the dark cupboard.
Light from one side would be a second stimulus with a direction.

Also accept

in a dark cupboard

Watch for

“On a windowsill, with the window on one side.” — Light from one side would also make the shoots curve, so you could not tell which curve was caused by gravity.
“Under a lamp shining from one side, so the seedlings can photosynthesise.” — One-sided light is a second stimulus with a direction. A day without light does young seedlings no harm.
“Beside a radiator, so they grow faster.” — Heat from one side is another one-sided condition, and it could dry the seedlings out. Keep the temperature even and take away the light.

4. RQ4 · Practice

Model answer. Gravity still acts on the seedlings, but as the disc turns each side of the root faces down for the same time. No side is pulled towards for longer than another, so the root does not curve.

A full answer includes

I chose: gravity acts on every side of the root in turn, for the same time.

Also accept

gravity acts on every side of the root in turn, for the same time

Watch for

“Gravity does not act on seedlings while they turn.” — Gravity still acts on them. The turning makes it act on every side for the same time.
“The turning damages the root tips, so they stop growing.” — The roots keep growing: they grow straight out sideways, longer than before.
“The roots are held flat by the pins.” — The pins hold the seedlings, not the growing root tips. On a still disc the same pinned roots curve down.

5. RQ5 · Practice

Model answer. The root curves round near its tip until it grows downwards. Roots grow towards gravity: this is gravitropism. The curve forms as the root grows, near its tip, so the root ends up pointing down even though it started pointing up.

A full answer includes

I predicted that the root curves round near its tip until it grows downwards.
I wrote that roots grow towards gravity: gravitropism.
I wrote that the curve forms by growth near the tip, not by drooping.

Also accept

curves round near its tip to grow down: gravitropism

Watch for

“The root keeps growing upwards, the way it now points.” — The way the root points does not decide its growth: it responds to gravity and curves round to grow down.
“The root grows towards the light.” — The box is dark, and roots do not grow towards light. The root is responding to gravity.

6. RQ6 · Practice

Model answer. D. The shoots grow towards the lamp: phototropism. Mustard roots grow away from the light, and gravity keeps them growing downwards, so they curve down and away from the lamp.

A full answer includes

I chose row D.
I did not choose row A: mustard roots grow away from the light, not towards it.

Also accept

Watch for

“Row A: every part of a plant grows towards the light.” — Shoots do; the roots of mustard seedlings grow away from the light.
“Row B: shoots grow away from a bright lamp.” — Shoots grow towards the direction of the light source, however bright it is.
“Row C: only gravity affects the seedlings.” — Gravity acts on them, but light from one side also makes the shoots and the roots curve.

7. RQ7 · Practice

Model answer. It is gravitropism: the shoots grow away from gravity, upwards. It cannot be phototropism, because inside a dark sack there is no light to give a direction; gravity is the only stimulus with a direction.

A full answer includes

I wrote that it is gravitropism: the shoots grow away from gravity, upwards.
I wrote that there is no light in the sack, so it cannot be a response to light.

Also accept

gravitropism: there is no light in the sack

Watch for

“It is phototropism, because shoots always grow towards light.” — There is no light in the dark sack. The shoots are responding to gravity.
“The shoots float upwards because they are light.” — Shoots do not float: they grow upwards, away from gravity, even against the weight of the potatoes above them.

8. RQ8 · Practice

Model answer. Two things differ: whether the clinostat turns, and the temperature. Cold slows growth, and a curve forms by growth, so set 2 may have hardly curved because it hardly grew. Put both clinostats in the same room, so the turning is the only difference.

A full answer includes

I wrote that two things differ: the turning and the temperature.
I wrote that the cold slows growth, so set 2 may have hardly curved because it hardly grew.
I wrote that both clinostats should be in the same room, at the same temperature.

Also accept

two things differ; keep both at the same temperature

Watch for

“Set 2 hardly curved because a still clinostat removes gravity.” — A still clinostat leaves gravity acting from below, so its seedlings should curve. The cold is the likely reason they hardly did.
“The experiment is fine because both sets were in the dark.” — Darkness removes light, but the temperature still differs, so two variables change at once.

9. RQ9 · Practice

Model answer. Decrease in angle = 76 − 28 = 48°. Time taken = 6 − 2 = 4 hours. Mean rate of decrease = 48 ÷ 4 = 12° per hour.

Checked automatically

Mean rate of decrease of the angle: 12 ° per hour (within ±0.5 ° per hour)
- Common wrong answer 8 : Divide by the time between the readings: 6 − 2 = 4 hours.
- Common wrong answer 48 : That is the decrease in angle. Divide it by the time taken.
- Common wrong answer 19 : Find the decrease in angle first: 76 − 28.

A full answer includes

I found the decrease in angle: 76 − 28 = 48°.
I found the time taken: 6 − 2 = 4 hours.
I divided: 48 ÷ 4 = 12° per hour.

Also accept

12° per hour

Watch for

“8° per hour.” — That is 48 ÷ 6: divide by the time between the two readings, 4 hours, not by the last time.
“19° per hour.” — That is 76 ÷ 4: find the decrease in angle first.

10. RQ10 · Practice

Model answer. Each root ends with its tip pointing down, but it has two bends. After the first turn the growing region near the tip curves down. After the second turn that downward tip points sideways again, and the new growth curves down once more. The older parts of the root no longer grow longer, so they keep the shape they had.

A full answer includes

I predicted that the tip of each root points down at the end.
I predicted two bends in each root, one made after each turn.
I wrote that only the region near the tip curves; the older part keeps its shape.

Also accept

two bends; the tip points down

Watch for

“The roots straighten out again after the second turn.” — Only the region near the tip is still growing. The first bend stays, and a second bend forms after the second turn.
“The roots point sideways at the end, because they were turned back.” — After the second turn the new growth curves down again, towards gravity, so the tips end up pointing down.

11. RT1 · Apply

Model answer. The roots and the shoots grew in many directions, not all down and up. Without gravity acting from one direction, gravitropism has nothing to respond to, and in the dark there is no light to give a direction either.

A full answer includes

I predicted that the roots and the shoots grew in many different directions, not down and up.
I explained that without gravity there is no direction for gravitropism, and in the dark no light either.

Also accept

in many directions: no gravity and no light to respond to

Watch for

“The roots still grew down, because roots always grow down.” — Roots grow down because they respond to gravity. With almost no gravity, there is no "down" for them to respond to.

12. RT2 · Apply

Model answer. Gravity acts straight down everywhere, whatever the slope of the ground. The shoots grow away from gravity, so they grow vertically upwards: gravitropism. Light from the sky above also keeps them growing upwards.

A full answer includes

I wrote that gravity acts straight down, whatever the slope of the ground.
I wrote that the shoots grow away from gravity, so they grow vertically: gravitropism.

Also accept

gravity acts straight down; shoots grow away from it

Watch for

“Trees grow at right angles to the ground they stand on.” — Shoots respond to gravity, which acts straight down, not to the slope of the soil.