Water vapour leaves the leaves — answer key

1. RQ1 · Practice

Model answer. Transpiration is the loss of water vapour from leaves. The water was taken up by the roots and carried up to the leaves in the xylem; most of it leaves as vapour.

A full answer includes

• The loss of water vapour from leaves.

Also accept

• the loss of water vapour from leaves

Watch for

• “Transpiration is water moving up the stem.” — Water does move up the stem, but transpiration is the loss of water vapour from the leaves.
• “Transpiration is the loss of liquid water from leaves.” — The water leaves as vapour, a gas, not as drops of liquid.

2. RQ2 · Practice

Model answer. The leaves lost water vapour by transpiration. Trapped in the bag, some of the vapour condensed on the plastic as droplets of liquid water.

A full answer includes

• Water vapour lost from the leaves condensed on the plastic.

Also accept

• water vapour from the leaves condensed on the bag

Watch for

• “The droplets dripped out of the leaves as liquid water.” — The leaves lose water vapour, a gas. It turns into liquid only on the plastic.
• “The bag made the water out of the air.” — A bag cannot make water. The water came from the leaves.

3. RQ3 · Practice

Model answer. (a) D, the tap: opening it lets water from the reservoir, C, push the bubble back along the narrow tube; it is closed again before timing. (b) F is the air bubble. (c) The bubble moves towards the shoot, because the shoot takes up water, drawing the water in the narrow tube, and the bubble with it, towards the shoot.

A full answer includes

• (a) D, the tap (below the reservoir C).
• (b) F.
• (c) Towards the shoot, because the shoot takes up water from the narrow tube.

Also accept

• (a) D; (b) F; (c) towards the shoot, as the shoot takes up water

Watch for

• “C is opened to push the bubble back.” — C is the reservoir that holds the water. D is the tap that lets it flow.
• “The bubble moves away from the shoot, because the shoot pushes water out.” — The shoot takes water in. The water in the tube moves towards it.

4. RQ4 · Practice

Options as numbered on the pupil sheet: 1 — Time 2 minutes and read the same edge of the bubble; 2 — Let one air bubble into the narrow tube; 3 — Open the reservoir tap to push the bubble back to the start; 4 — Close the tap.

Model answer. First lift the end of the narrow tube out of the beaker for a moment to let in one air bubble. Then open the reservoir tap so that water pushes the bubble back to the start. Close the tap, so that no water from the reservoir moves the bubble while you time. Then time 2 minutes and read how far the same edge of the bubble has moved.

A full answer includes

• Order: let in one air bubble; open the reservoir tap to push the bubble back to the start; close the tap; time 2 minutes and read the same edge of the bubble.

Also accept

• bubble in; tap open to push it back; tap closed; time and read the same edge

Watch for

• “Time the run with the tap open.” — Water from the reservoir would push the bubble too. Close the tap before timing.
• “Read the right edge of the bubble at the start and the left edge at the end.” — Read the same edge each time, or the bubble's length is added to the distance.

5. RQ5 · Practice

Model answer. (a) The bubble moved 250 − 232 = 18 mm. (b) The rate of water uptake is the distance divided by the time: 18 mm ÷ 2 minutes = 9 mm per minute.

Checked automatically

• (a) Distance moved: 18 mm (within ±2%)
  • Common wrong answer 232 : That is where the bubble stopped. Subtract: 250 − 232.
  • Common wrong answer 482 : Subtract the readings, do not add them.
• (b) Rate of water uptake: 9 mm per minute (within ±2%)
  • Common wrong answer 18 : That is the distance in 2 minutes. Divide by the 2 minutes.
  • Common wrong answer 36 : Divide the distance by the time, do not multiply.

A full answer includes

• (a) 250 − 232 = 18 mm.
• (b) 18 ÷ 2 = 9 mm per minute.

Also accept

• (a) 18 mm; (b) 9 mm per minute

Watch for

• “The rate is 232 mm per minute.” — 232 mm is where the bubble stopped. Subtract to find how far it moved.
• “The rate is 18 mm per minute.” — 18 mm is the distance in 2 minutes. Divide by 2 for the rate per minute.

6. RQ6 · Practice

Model answer. The bubble moves as the shoot takes water in, so a potometer measures water uptake, not water loss. A little of the water taken up is used, for example in photosynthesis, or stays in the cells. But nearly all of it is lost by transpiration, so the rate of uptake is a good estimate of the rate of transpiration, especially when comparing conditions on the same shoot.

A full answer includes

• It measures water uptake (the water the shoot takes in), not water loss.
• Nearly all the water taken up is lost by transpiration, so uptake is a good estimate of the rate of transpiration (a little is used or stays in the cells).

Also accept

• measures uptake, not loss; nearly all uptake is lost by transpiration, so it is a good estimate

Watch for

• “All the water taken up is used in photosynthesis.” — Only a small part is used. Most of it is lost as water vapour.
• “Uptake and transpiration are exactly the same.” — A little water is used or kept, so uptake is a close estimate, not exactly the same.

7. RQ11 · Practice

Model answer. Inside the leaf, liquid water turns into water vapour at the surfaces of the mesophyll cells. This change is called evaporation. The vapour collects in the air spaces between the cells, then leaves the leaf by diffusion through pores called stomata.

A full answer includes

• mesophyll; evaporation; air spaces; diffusion; stomata

Also accept

• mesophyll, evaporation, air spaces, diffusion, stomata

Watch for

• “The vapour is pushed out of the stomata as drops of liquid.” — Nothing pushes it: the water evaporates inside the leaf, and the vapour diffuses out.
• “Water evaporates straight from the veins.” — The veins carry water into the leaf; it evaporates from the surfaces of the mesophyll cells.

8. RQ7 · Practice

Options as numbered on the pupil sheet: 1 — Water vapour diffuses out through the stomata; 2 — Water evaporates from the surfaces of the mesophyll cells into the air spaces; 3 — Water in the xylem of a vein; 4 — Water moves into the mesophyll cells.

Model answer. Water arrives in the xylem of a vein and moves into the mesophyll cells. It evaporates from the surfaces of the mesophyll cells into the air spaces. The water vapour then diffuses out of the leaf through the stomata.

A full answer includes

• Order: water in the xylem of a vein; water moves to the mesophyll cells; water evaporates from their surfaces into the air spaces; water vapour diffuses out through the stomata.

Also accept

• xylem; mesophyll cells; evaporates into the air spaces; diffuses out through the stomata

Watch for

• “The water evaporates straight from the veins.” — It reaches the mesophyll cells first and evaporates from their surfaces.
• “The water goes out through the stomata first, then evaporates.” — It evaporates inside the leaf; the vapour then diffuses out through the stomata.

9. RQ8 · Practice

Model answer. Water evaporates from the surfaces of the mesophyll cells into the air spaces. There is more water vapour in the air spaces than in the air outside, so the vapour diffuses out through the stomata. Nothing pushes or pumps it out, and it does not evaporate straight from the veins.

A full answer includes

• True: water evaporates from the surfaces of the mesophyll cells; water vapour diffuses out through the stomata; there is more water vapour in the air spaces than in the air outside.
• False: drops of liquid are pushed out through the stomata; water evaporates straight from the veins; the guard cells pump water vapour out.

Also accept

• evaporates from mesophyll surfaces; vapour diffuses out through stomata; more vapour inside than outside

Watch for

• “Guard cells pump the vapour out.” — Guard cells open and close the stoma. The vapour leaves by diffusion.
• “Liquid water leaves through the stomata.” — Water leaves as vapour, by diffusion.

10. RQ9 · Practice

Model answer. (a) The lower surface. (b) Water vapour diffuses out of the leaf through the stomata. The lower epidermis has more stomata (16 against 5 in the same area), so more water vapour can diffuse out through it.

A full answer includes

• (a) The lower surface.
• (b) It has more stomata, so more pores for water vapour to diffuse out through.

Also accept

• (a) lower; (b) more stomata, so more water vapour diffuses out

Watch for

• “The upper surface, because it faces the sun.” — Warmth matters, but the vapour has to leave through stomata, and the lower surface has many more.
• “Both lose the same, because they are the same leaf.” — The number of stomata differs, so the loss differs.

11. RQ10 · Practice

Model answer. The mesophyll cells are loosely packed with interconnecting air spaces between them, so a large area of wet cell surface faces the air inside the leaf: a large internal surface area. Water evaporates from all of that surface into the air spaces, so the air spaces fill with water vapour quickly, and more water vapour diffuses out through the stomata.

A full answer includes

• They give a large internal surface area of wet cell surface facing the air.
• So more water evaporates into the air spaces, and more water vapour diffuses out (through the stomata).

Also accept

• large internal surface area, so more evaporation and more water vapour diffuses out

Watch for

• “The air spaces store water for the leaf.” — They hold air and water vapour; the large surface around them is what matters.
• “The air spaces slow water loss.” — They give a large internal surface area, so they speed up evaporation.

12. RT1 · Apply

Model answer. (a) P: 30 ÷ 2 = 15 mm per minute. Q: 10 ÷ 2 = 5 mm per minute. (b) Transpiration is the loss of water vapour from leaves. P has three times as many leaves, so more leaf surface and more stomata lose water vapour, and P takes up water faster to replace it.

A full answer includes

• (a) P: 15 mm per minute; Q: 5 mm per minute.
• (b) P has more leaves, so it loses more water vapour by transpiration (more leaf surface, more stomata), and takes up more water to replace it.

Also accept

• (a) 15 and 5 mm per minute; (b) more leaves, so more transpiration

Watch for

• “Q is faster because it is smaller and water has less far to go.” — The rate depends on how much water vapour the leaves lose. Q has fewer leaves.
• “P is faster because its stem is thicker.” — The leaves are where the water is lost; more leaves, more loss.

13. RT2 · Apply

Model answer. The rate of water uptake falls a lot. Water vapour diffuses out of the leaf through the stomata, and most of them are on the lower surface. With the jelly blocking them, much less water vapour can leave, so transpiration slows and the shoot takes up much less water. A little may still be lost through the few stomata on the upper surface.

A full answer includes

• The rate falls a lot (the bubble moves much more slowly).
• The jelly blocks the stomata on the lower surface, so water vapour cannot diffuse out there; less is lost by transpiration, so less water is taken up.

Also accept

• falls; stomata blocked, so less water vapour diffuses out

Watch for

• “The rate goes up, because water is trapped inside the leaves.” — Water that cannot leave the leaves does not need replacing, so uptake falls.
• “No change, because the jelly is only on one surface.” — Most of the stomata are on that surface.

14. RT3 · Apply

Model answer. Variety X. Its mesophyll has more air spaces, so a larger area of wet cell surface faces the air inside the leaf, and more water evaporates into the air spaces. It also has more stomata, so there are more pores for the water vapour to diffuse out through. X loses water vapour faster, so its roots must take up more water, and it needs watering more often.

A full answer includes

• Variety X.
• More air spaces: a larger internal surface area of wet cell surface, so more water evaporates.
• More stomata: more pores for the water vapour to diffuse out through.
• So X loses water vapour faster, takes up more water and needs watering more often.

Also accept

• X: more air spaces give a larger internal surface area for evaporation; more stomata let more water vapour diffuse out

Watch for

• “Y, because fewer stomata means each one loses more.” — Fewer stomata means fewer pores; the total loss is less.
• “They need the same, because they grow in the same field.” — The conditions are the same, but the leaves are built differently, so the loss differs.