Transpiration
Losing water to the air
- A plant loses water vapour from its leaves all day.
- This is called transpiration.
- It is a side effect of letting carbon dioxide in for photosynthesis.
Practice
Transpiration is:
Transpiration is the loss of water vapour from a plant's leaves.
How transpiration happens
- Water evaporates from the wet surfaces of the mesophyll cells into the air spaces inside the leaf.
- The water vapour then diffuses out through the stomata.
Practice
Water leaves the leaf by:
Water evaporates from the mesophyll cell surfaces, then the vapour diffuses out through the stomata.
What changes the rate
| Faster when… | Why |
|---|---|
| hotter | water evaporates faster |
| windier | wind carries the vapour away |
| drier air (Supplement) | a bigger difference makes water diffuse out faster |
Practice
Which conditions make transpiration FASTER? (Choose all that apply.)
Heat, wind and dry air all speed transpiration up; still humid air slows it down.
How water rises (Supplement)
- As vapour leaves the leaf, it pulls more water up behind it — the transpiration pull.
- Water molecules stick together by forces of attraction, so they rise as one long column.
- If water is lost faster than it is taken up, the plant wilts (droops).
Practice
Water rises up the xylem because transpiration pulls a connected column of water upward.
The transpiration pull draws water up; the molecules stick together as one column.
You've got it
Key idea
- transpiration = loss of water vapour from leaves: evaporates from mesophyll, diffuses out of stomata
- faster when hotter, windier, or the air is drier
- water rises by the transpiration pull, molecules holding together as a column; too much loss → wilting