Transpiration and stomataSpec 4.2.3.2
Transpiration is the loss of water vapour from the leaves. Water moves from the roots to the leaves in the xylem in a continuous flow called the transpiration stream.
- Water enters the root hair cells by osmosis.
- Water moves up through the xylem to the leaves.
- Water evaporates from the surface of the mesophyll cells into the air spaces in the leaf.
- Water vapour diffuses out of the leaf through the stomata.
The transpiration stream brings water and mineral ions to the leaves. Leaf cells need water and carbon dioxide for photosynthesis.
Stomata and guard cells
Stomata are small pores, mainly on the underside of the leaf. Each is surrounded by a pair of guard cells. The role of stomata and guard cells is to control gas exchange and water loss.
- When the guard cells take in water they swell and the stoma opens. Carbon dioxide can diffuse in for photosynthesis and oxygen can diffuse out. Water vapour is also lost.
- When the guard cells lose water the stoma closes, which reduces water loss.
There is a trade-off. Closing the stomata saves water but stops carbon dioxide getting in, so photosynthesis slows down.
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