Hot, windy or humid — questions

1. identify

A pupil wants to find out how moving air affects the rate of water uptake by a leafy shoot. Which plan is a fair test?

Circle one answer.

• One run in still air, then one run with the fan on, timed as soon as it is switched on
• The same shoot: three runs in still air, then three with the fan on after a few minutes
• One shoot timed in still air, and a fresh shoot of the same plant with the fan on
• The fan and a heater switched on together, so the change in the rate is easier to see

Nudge (only if you are stuck): Change one thing at a time: only one thing should differ between the tests.

2. classify

In the fan investigation, match each variable to its role.

Write the number of the matching option.

1. The variable you measure (dependent variable)
2. Kept the same
3. The variable you change (independent variable)

• Whether the fan is on: ______
• The distance the bubble moves in 2 minutes: ______
• The shoot, the light and the time of each run: ______

Nudge (only if you are stuck): One you change, one you measure, and the rest you keep the same.

3. describe

Use the results table. Which statement describes the effect of moving air on the rate of water uptake?

Circle one answer.

• Moving air made the rate slower, about half as fast
• Moving air made the rate faster, about twice as fast
• The runs vary too much to tell what the fan did
• Moving air made no real difference to the rate

Nudge (only if you are stuck): Compare the fan runs with the still-air runs.

4. calculate

Use the results table. For warmer air, at 30 °C, work out (a) the mean distance the bubble moved in 2 minutes and (b) the mean rate of water uptake.

(a) Mean distance: ______________ unit ________

(b) Mean rate of water uptake: ______________ unit ________

Nudge (only if you are stuck): Add the three distances and divide by 3. Then divide by the 2 minutes.

5. explain

After switching on the fan, the pupil waits 5 minutes before timing the first run. Explain why.

Nudge (only if you are stuck): Does the shoot change its rate the moment the fan starts?

6. explain

Between the tests, the pupil switches the fan off, waits, and times one run in still air again: a check run. It gives 12 mm per minute, the same as at the start. (a) Why is the check run done? (b) What does this result show?

(a) Why the check run is done

(b) What the result shows

Nudge (only if you are stuck): Could the shoot itself have changed during the experiment?

7. explain

Supplement. A leafy shoot transpires more slowly in humid air. Which explanation is right?

Circle one answer.

• The concentration gradient of water vapour out of the leaf is shallow, so the vapour diffuses out more slowly
• The humid air pushes water vapour back in through the stomata
• The humid air closes the xylem, so no water reaches the leaves
• Humid air is always cold, so water stops evaporating from the mesophyll cells

Nudge (only if you are stuck): Compare the water vapour inside the leaf with the water vapour in the air outside.

8. explain

Supplement. Why does a leafy shoot transpire faster in warmer air?

Circle one answer.

• Heat always opens the stomata wider, so more water vapour gets out
• The xylem vessels contract faster when warm, pumping more water up
• Water molecules gain kinetic energy, so they evaporate and diffuse faster
• The warm shoot needs more water, so its cells pull harder on the xylem

Nudge (only if you are stuck): Think about the water molecules themselves.

9. explain

Supplement. Explain why a leafy shoot transpires faster in moving air than in still air. (a) What does moving air do just outside the leaf? (b) How does this affect the diffusion of water vapour?

(a) What moving air does

(b) The effect on diffusion

Nudge (only if you are stuck): In still air, where does the water vapour that leaves the stomata go?

10. identify

Supplement. Choose all the changes that would slow down transpiration from a plant.

Tick every correct statement.

• The air becomes drier
• The air becomes cooler
• The air becomes more humid
• A breeze starts
• The air becomes warmer
• A breeze stops and the air becomes still

Nudge (only if you are stuck): Each factor has a direction: which way speeds it up?

11. explain / evaluate

Supplement. A clear plastic bag tied over the leaves cut the rate of water uptake from 12 to 4 mm per minute, and droplets formed inside the bag. (a) Explain why the rate fell. (b) Give one reason why this is not a fair test of humidity alone.

(a) Why the rate fell

(b) Why it is not a fair test of humidity alone

Nudge (only if you are stuck): What builds up inside the bag? And what else does a bag change?

12. describe / suggest

Another class used a potometer and a fan with three settings. (a) Describe the effect of wind speed on the rate of water uptake. (b) Suggest two things they should keep the same.

(a) The effect of wind speed

(b) Two things to keep the same

Nudge (only if you are stuck): Read down the table. For (b), think about what else could change the rate.

13. explain

Supplement. Gardeners grow new plants from cuttings: shoots with leaves but no roots yet. They keep the cuttings under a clear cover, or spray a fine mist of water into the air around them. Explain why this helps the cuttings to survive.

Nudge (only if you are stuck): A cutting with no roots cannot take up much water. What does humid air do to water loss?

14. predict

A garden centre sells herbs in small pots. Where should the pots stand so that their soil dries out most slowly?

Circle one answer.

• Under a vent that blows cool air
• In a cool corner where the air is still
• By the open door, in a warm breeze
• Next to a heater, where there is no draught

Nudge (only if you are stuck): Which conditions slow transpiration down?