Question 1
Describe how water leaves a leaf.
Show mark scheme for question 1
- water evaporates from the surfaces of the mesophyll cells (1)
- into the air spaces (1)
- water vapour diffuses out through the stomata (1)
Question 2
A student uses a potometer to investigate the effect of wind speed on the rate of transpiration. Describe how the student could do this.
Show mark scheme for question 2
- place a fan at different distances from the leafy shoot / use different fan speeds (1)
- measure the distance moved by the air bubble in a set time (1)
- keep other variables the same, for example temperature / light / same shoot (1)
- repeat and calculate a mean (1)
Question 3
In a potometer experiment, an air bubble moved the distances shown in the table. Calculate the rate of water uptake with the fan on, in mm / min.
| conditions | distance moved by bubble / mm | time / min |
|---|---|---|
| still air | 18 | 6 |
| fan on | 54 | 6 |
Show mark scheme for question 3
- rate = distance ÷ time = 54 ÷ 6 (1)
- 9 mm / min (1)
Question 4
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanExplain how the structure of a leaf allows it to lose water vapour quickly.
Show mark scheme for question 4
- mesophyll cells surrounded by interconnecting air spaces (1)
- gives a large internal surface area for evaporation (1)
- many / large stomata (1)
- water vapour diffuses out through the stomata (1)
Question 5
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanExplain why a plant loses water vapour faster on a hot, dry, windy day than on a cool, humid, still day.
Show mark scheme for question 5
- higher temperature, so water evaporates faster (1)
- molecules have more energy / diffuse faster (1)
- wind blows water vapour away from the leaf (1)
- keeps a steep concentration gradient (1)
- low humidity so the concentration gradient is steeper, so faster diffusion (1)
Question 6
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanExplain how water moves up the xylem to the top of a tall tree.
Show mark scheme for question 6
- water evaporates from mesophyll cells / leaves (1)
- creates a transpiration pull (1)
- water molecules are held together / attracted to each other (1)
- so the column of water is pulled up the xylem (1)
Question 7
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanExplain how and why wilting occurs.
Show mark scheme for question 7
- water lost by transpiration faster than it is taken up (1)
- cells lose water (by osmosis) / lose turgor / become flaccid (1)
- so leaves / stems are no longer supported and droop (1)