Transpiration Questions

Question 1

Four similar sized leaves were cut from the same plant. The cut end of each leaf stalk was sealed. The leaves were then covered in petroleum jelly in the following way:
A - the upper surface only was greased
B - the lower surface only was greased
C - both surfaces were greased
D - neither surface was greased

The mass was re-measured after 24 hours and the % loss of mass of each leaf was found. The results are shown here:

A: 40%
B: 4%
C: 2%
D: 43%

Describe and explain these results.

Answer 1

Leaves A and D lost the most mass, while B and C lost very little mass. This shows that the leaves that lost the most mass were the ones that did not have grease on the lower surface.

Leaves lose most water through the stomata which causes a loss in mass. Leaves B and C had petroleum jelly covering the stomata so they could not lose water that way. Therefore they lost almost no water.

Question 2

Define transpiration.

Answer 2

Movement of water through a plant from roots to leaves

Question 3

Describe how water moves from the soil into a plant and evaporates from the leaf.

Answer 3

• water enters root hair cells by osmosis
• moves through plant via xylem
• evaporates from leaf as water vapour via stomata
• draws water out of leaf cells/xylem

Question 4

Suggest the advantage to the plant of stomata being present mainly on the underside of the leaf.

Answer 4

On the surface the stomata could get blocked by water droplets this would reduce/prevent gas exchange/transpiration.

Question 5

Why do fans cool leaves of a plant down?

Answer 5

• move moisture/ water droplets away from the plant as they evaporate from the leaves
• less moisture leads to a higher concentration gradient, so more water evaporates from the leaves
• this increases the rate of transpiration as the plant draws up more water for respiration
• as water molecules evaporate they take heat away from the plant

Question 6

Why does a plant transpire more in higher light intensities?

Answer 6

• light causes a plant’s stomata to open
• more water is able to evaporate throught the stomata
• this increases the rate of transpiration
• photosynthesis also increases, which uses more water