Transpiration — final check key

1. RK1 · Final check

Model answer. (a) The bubble moved 250 − 220 = 30 mm. (b) Rate = 30 mm ÷ 2 minutes = 15 mm per minute.

Checked automatically

• (a) Distance moved: 30 mm (within ±2%)
  • Common wrong answer 220 : That is where the bubble ended. Subtract: 250 − 220.
• (b) Rate of water uptake: 15 mm per minute (within ±2%)
  • Common wrong answer 30 : That is the distance in 2 minutes. Divide by 2.
  • Common wrong answer 60 : Divide by the time, do not multiply.

A full answer includes

• (a) 250 − 220 = 30 mm.
• (b) 30 ÷ 2 = 15 mm per minute.

Also accept

• (a) 30 mm; (b) 15 mm per minute

Watch for

• “The rate is 220 mm per minute.” — 220 mm is where the bubble ended. Subtract to find the distance.
• “The rate is 30 mm per minute.” — That is the distance in 2 minutes. Divide by 2.

2. RK2 · Final check

Model answer. (a) Transpiration is the loss of water vapour from leaves. (b) Water evaporates from the surfaces of the mesophyll cells into the air spaces between them. The water vapour then diffuses out of the leaf through the stomata.

A full answer includes

• (a) The loss of water vapour from leaves.
• (b) Water evaporates from the surfaces of the mesophyll cells into the air spaces; the water vapour diffuses out of the leaf through the stomata.

Also accept

• (a) loss of water vapour from leaves; (b) evaporates from mesophyll cell surfaces into air spaces, diffuses out through stomata

Watch for

• “Water leaves the stomata as liquid.” — It leaves as vapour, by diffusion.
• “Water evaporates from the outside of the leaf.” — It evaporates inside the leaf, from the mesophyll cells.

3. RK3 · Final check

Model answer. (a) The interconnecting air spaces between the mesophyll cells give a large internal surface area of wet cell surface. More water evaporates into the air spaces, so more water vapour diffuses out. (b) Water vapour diffuses out through the stomata. The more stomata there are, and the larger (wider open) they are, the faster water vapour is lost.

A full answer includes

• (a) Interconnecting air spaces give a large internal surface area for evaporation, so more water vapour is produced to diffuse out.
• (b) More stomata, and larger (wider-open) stomata, let more water vapour diffuse out, so the loss is faster.

Also accept

• (a) large internal surface area, more evaporation; (b) more and larger stomata, more diffusion out

Watch for

• “Larger stomata let less water out, because they are harder to open.” — Larger, wider-open stomata let more water vapour diffuse out.
• “The air spaces do not affect water loss.” — They give the large internal surface area that the water evaporates from.

4. HK1 · Final check

Model answer. (a) The higher the temperature, the faster the rate of water uptake: from 8 mm per minute at 15 °C to 18 mm per minute at 30 °C. (b) Different shoots have different sizes and numbers of leaves, so they would take up water at different rates anyway. With the same shoot, only the temperature changes, so any difference in rate was caused by the temperature: a fair test.

A full answer includes

• (a) The higher the temperature, the faster the rate (8 mm per minute at 15 °C up to 18 at 30 °C).
• (b) Shoots differ in size and number of leaves, so a different shoot would have a different rate anyway; with the same shoot only the temperature changes.

Also accept

• (a) higher temperature, faster rate; (b) same shoot so only temperature changes

Watch for

• “The rate doubles every 5 °C.” — From 8 to 11 to 15 to 18: it rises by about 3 or 4 each time, not doubling.
• “The same shoot is used to save shoots.” — It is used so that only the temperature changes: a fair test.

5. HK2 · Final check

Model answer. (a) A higher temperature makes transpiration faster: the water molecules have more kinetic energy, so water evaporates from the surfaces of the mesophyll cells faster and the water vapour diffuses out through the stomata faster. (b) A higher humidity makes transpiration slower: the air outside the leaf already holds a lot of water vapour, so the concentration gradient between the air spaces and the air outside is shallow, and water vapour diffuses out more slowly.

A full answer includes

• (a) Faster: water molecules have more kinetic energy, so water evaporates from the mesophyll cells and diffuses out through the stomata faster.
• (b) Slower: humid air holds more water vapour, so the concentration gradient between the air spaces and the air outside is shallow, and water vapour diffuses out more slowly.

Also accept

• (a) faster, more kinetic energy; (b) slower, shallow concentration gradient

Watch for

• “Humid air pushes water back into the leaf.” — Nothing is pushed in. Water vapour still diffuses out, only more slowly.
• “Higher temperature makes the stomata pump faster.” — Stomata do not pump. The molecules move faster.

6. HK3 · Final check

Model answer. On a still day, water vapour that diffuses out of the stomata collects just outside the leaves, so the air there becomes humid. Wind carries this water vapour away. The air outside the stomata stays drier, so the concentration gradient of water vapour stays steep, and water vapour diffuses out of the leaves faster.

A full answer includes

• Moving air carries away the water vapour that collects just outside the stomata.
• The concentration gradient of water vapour stays steep, so water vapour diffuses out of the leaves faster.

Also accept

• wind carries water vapour away, keeping the concentration gradient steep, so faster diffusion

Watch for

• “The wind dries the leaves by blowing on them.” — Say what it does to the water vapour and the concentration gradient.
• “The wind pushes water out of the stomata.” — Water vapour diffuses out; the wind keeps the gradient steep.

7. TK1 · Final check

Model answer. (a) In the leaves, water evaporates from the surfaces of the mesophyll cells into the air spaces, and the water vapour diffuses out through the stomata: transpiration. (b) The mesophyll cells take in water from the xylem to replace it, and this transpiration pull draws up the column of water in the xylem. (c) Forces of attraction between water molecules hold the water together as a continuous column, so the whole column moves up, and more water enters the roots from the soil.

A full answer includes

• (a) Water evaporates from the surfaces of the mesophyll cells and the water vapour diffuses out through the stomata (transpiration).
• (b) The mesophyll cells take in water from the xylem; this transpiration pull draws up the column of water in the xylem.
• (c) Forces of attraction between water molecules hold the column together, so the whole column moves up.

Also accept

• (a) evaporation and diffusion out of leaves; (b) transpiration pull draws up the column; (c) attraction between water molecules

Watch for

• “The roots pump the water up.” — The pull comes from the leaves.
• “The xylem cells push the water up.” — Xylem vessels have no cell contents; they cannot push.

8. TK2 · Final check

Model answer. (a) On the hot day the plant lost water by transpiration faster than its roots could take water up. Its cells lost water, their vacuoles shrank and the turgor pressure fell: the cells became flaccid, no longer pressed outwards, and the leaves and stems drooped. (b) After watering, the roots take up water and it reaches the cells, which take it in by osmosis. The cells become turgid again, pressing outwards on their walls and on one another, so the leaves and stems are held up.

A full answer includes

• (a) It lost water by transpiration faster than its roots took water up; its cells lost water, became flaccid and no longer held the leaves and stems up, so they drooped.
• (b) The roots take up water; water enters the cells by osmosis; the cells become turgid again and press outwards, holding the leaves and stems up.

Also accept

• (a) loss faster than uptake, cells flaccid; (b) water in by osmosis, cells turgid

Watch for

• “It wilted because it had no food.” — Wilting is caused by losing water faster than it is taken up.
• “Watering makes the cells burst back into shape.” — They become turgid, not burst: the cell wall stops them bursting.

9. TK3 · Final check

Model answer. Water moves up the xylem as a continuous column, pulled from the top by transpiration and held together by forces of attraction between the water molecules. An air bubble breaks the column. The attraction acts between water molecules, not across a gap of air, so the water below the bubble cannot be pulled up past it.

A full answer includes

• Water is pulled up as a continuous column, held together by forces of attraction between water molecules.
• An air bubble breaks the column: the attraction cannot act across the air, so the water below cannot be pulled up past it.

Also accept

• continuous column held by attraction; air breaks it, so no pull below the bubble

Watch for

• “The bubble floats up and blocks the leaves.” — The bubble breaks the column; the pull cannot reach the water below it.
• “The bubble pushes the water back down.” — It does not push; it breaks the column, so the pull stops acting below it.