Transpiration and stomata

Plant structures and their functions · Transport in plants · note 3 of 4

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Transpiration and stomataSpec 6.9

Transpiration is the loss of water vapour from a plant, mainly through the stomata in the leaves. It pulls a continuous stream of water, called the transpiration stream, through the plant. This is how water and mineral ions are transported from the roots to the leaves.

Water is absorbed from the soil by the root hair cells.
Water and dissolved mineral ions move up through the xylem to the leaves.
Water evaporates from the surface of the cells inside the leaf, into the air spaces.
Water vapour diffuses out of the leaf through the stomata, into the air.

A plant showing water taken in by root hair cells, moving up the xylem in the root, stem and leaf, then a magnified leaf section where water evaporates from cells into the air spaces and water vapour diffuses out through a stoma. — The transpiration stream: water moves up the **xylem**, evaporates into the air spaces and diffuses out through the stomata.

Stomata

Stomata are tiny pores, found mainly on the underside of a leaf. Each stoma is surrounded by two guard cells which control whether the stoma is open or closed.

Open stomata let carbon dioxide diffuse into the leaf for photosynthesis, and let oxygen and water vapour diffuse out.
When the guard cells take in water they swell and the stoma opens.
When the guard cells lose water they become limp and the stoma closes. This reduces the amount of water lost by transpiration, for example when the plant is short of water.

Surface views of a stoma: open, with swollen guard cells, carbon dioxide going in and oxygen and water vapour going out; closed, with limp guard cells and no pore. — Guard cells that take in water swell and **open** the stoma; guard cells that lose water go limp and **close** it.