Transpiration — return questions key

1. RR1 · Return

Model answer. Transpiration is the loss of water vapour from leaves.

A full answer includes

• The loss of water vapour from leaves.

Also accept

• loss of water vapour from leaves

Watch for

• “Transpiration is the movement of water up the xylem.” — That movement is caused by transpiration, but transpiration itself is the loss of water vapour from leaves.
• “Transpiration is the plant breathing.” — Breathing is gas exchange; transpiration is the loss of water vapour.

2. RR2 · Return

Model answer. Water 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

• evaporates; mesophyll; diffuses; stomata

Also accept

• evaporates, mesophyll, diffuses, stomata

Watch for

• “Water vapour is pumped out through the stomata.” — It diffuses out. Nothing pumps it.
• “Water evaporates from the epidermis cells.” — It evaporates from the surfaces of the mesophyll cells, inside the leaf.

3. RR3 · Return

Model answer. Rate = distance ÷ time = 36 mm ÷ 3 minutes = 12 mm per minute.

Checked automatically

• Rate of water uptake: 12 mm per minute (within ±2%)
  • Common wrong answer 36 : That is the distance in 3 minutes. Divide by 3.
  • Common wrong answer 108 : Divide by the time, do not multiply.

A full answer includes

• 36 ÷ 3 = 12 mm per minute.

Also accept

• 12 mm per minute

Watch for

• “The rate is 36 mm per minute.” — 36 mm is the distance in 3 minutes. Divide by 3.
• “The rate is 108 mm per minute.” — Divide the distance by the time, do not multiply.

4. RR4 · Return

Model answer. The interconnecting air spaces between the mesophyll cells give a large internal surface area of wet cell surface, so a lot of water evaporates into them. Many stomata, open wide, give many pores for that water vapour to diffuse out through. Together they let the leaf lose water vapour quickly.

A full answer includes

• The interconnecting air spaces between the mesophyll cells give a large internal surface area, so more water evaporates.
• Many (and large, wide-open) stomata let more water vapour diffuse out.

Also accept

• air spaces: large internal surface area, more evaporation; many large stomata: more diffusion out

Watch for

• “The veins let water vapour out of the leaf.” — The veins bring liquid water in; the vapour leaves through the stomata.
• “Tightly packed mesophyll cells let water evaporate faster.” — Loosely packed cells, with air spaces between them, give the large surface for evaporation.

5. HR1 · Return

Model answer. The rate of transpiration increases: the higher the temperature, the faster the rate (within a moderate range; in very hot, dry air a plant can close its stomata).

A full answer includes

• It increases: the higher the temperature, the faster the rate of transpiration.

Also accept

• it increases

Watch for

• “It decreases, because the plant saves water when it is hot.” — Over a moderate range the rate goes up with temperature.
• “It does not change.” — In the experiment, 30 °C gave 20 mm per minute against 12 at 20 °C.

6. HR2 · Return

Options as numbered on the pupil sheet: 1 — The concentration gradient is shallow, so water vapour diffuses out more slowly; 2 — Water molecules have more kinetic energy, so they evaporate and diffuse faster; 3 — Water vapour is carried away from the leaf, keeping the concentration gradient steep.

Model answer. A higher temperature gives the water molecules more kinetic energy, so they evaporate and diffuse faster. A higher wind speed carries water vapour away from the leaf, so the concentration gradient stays steep. A higher humidity makes the concentration gradient shallow, so water vapour diffuses out more slowly.

A full answer includes

• Higher temperature: water molecules have more kinetic energy, so they evaporate and diffuse faster.
• Higher wind speed: water vapour is carried away from the leaf, keeping the concentration gradient steep.
• Higher humidity: the concentration gradient is shallow, so water vapour diffuses out more slowly.

Also accept

• temperature: kinetic energy; wind: carries vapour away, steep gradient; humidity: shallow gradient

Watch for

• “Humidity speeds transpiration, because the air is wetter.” — Wet air means a shallow gradient: transpiration slows.
• “Wind speeds transpiration by giving the molecules more energy.” — Wind carries the water vapour away; temperature gives the energy.

7. HR3 · Return

Model answer. Any two of: the same shoot in the same potometer; the same wind speed (for example still air); the same light; the same time for each run; reading the same edge of the bubble each time.

A full answer includes

• Any two: the same shoot; the same wind speed (still air); the same light; the same time for each run; reading the same edge of the bubble.

Also accept

• two of: same shoot, wind speed, light, run time, bubble edge

Watch for

• “Keep the temperature the same.” — Temperature is the variable you are changing.
• “Use a new shoot at each temperature.” — A different shoot would have a different rate anyway. Keep the same shoot.

8. TR1 · Return

Model answer. As water evaporates from the mesophyll cells and diffuses out of the leaves, the cells take in water from the xylem. This pull, caused by transpiration, draws up a continuous column of water molecules in the xylem, held together by forces of attraction between the water molecules.

A full answer includes

• Water evaporating from the leaves (transpiration) draws water up the xylem as a continuous column of water molecules, held together by forces of attraction between them.

Also accept

• transpiration from leaves draws up a column of water molecules held together by attraction

Watch for

• “The transpiration pull is the roots pulling water from the soil.” — The pull comes from the leaves, where water evaporates.
• “The transpiration pull is the wind pulling water out of the leaves.” — It is caused by evaporation from the leaves, in any air.

9. TR2 · Return

Model answer. A plant wilts when it loses water by transpiration faster than its roots take up water. Its cells become flaccid, so they no longer hold the leaves and stems up.

A full answer includes

• transpiration; take up (absorb); flaccid

Also accept

• transpiration, take up, flaccid

Watch for

• “Its cells become turgid.” — Turgid cells are firm. Wilting cells are flaccid.
• “A plant wilts when it photosynthesises too fast.” — Wilting is about losing water by transpiration.

10. TR3 · Return

Model answer. Forces of attraction between water molecules hold the water together as a continuous column.

A full answer includes

• Forces of attraction between water molecules.

Also accept

• forces of attraction between water molecules

Watch for

• “The lignin in the xylem walls.” — Lignin strengthens the walls; the column holds together because water molecules attract one another.
• “The sugar dissolved in the water.” — Xylem carries water and mineral ions; attraction between the water molecules holds the column.