Transpiration stream

First Stage

Water taken up by the roots of a plant is transported through a plant to the leaves and lost into the air. The stages of the process are:

• Water enters root hair cells by osmosis.
• The root hair cell is hypertonic to the surrounding soil water. This means that it has a lower water molecule concentration.
• Water then moves from cell to cell through the root cortex by osmosis along a concentration gradient; this means that each cell is hypertonic to the one before it.
• In the centre of the root the water enters the xylem vessels.

Second Stage

In the leaves, water molecules leave the xylem vessels and move from cell to cell. They move through the spongy mesophyllspongy mesophyll: the lower layer of mesophyll which contains numerous air spaces where gas exchange takes place layer by osmosis along a concentration gradient. Water then evaporates into spaces behind the stomata and diffuses through the stomata into the surrounding air.

Water rises from the roots to the leaves through the xylem vessels because of two properties of water molecules:

• Adhesion

  Water rises in the narrow vessels partly because water molecules are attracted to the walls of the vessels.

• Cohesion

  Water molecules are attracted to each other, and as water evaporates from the leaves columns of water are drawn up through the xylem vessels.

The loss of water from the leaves of a plant is called transpiration, and the resulting flow of water through the plant is called the transpiration stream. The transpiration stream is important because:

• it carries water for photosynthesis to the palisade cellspalisade cells: the upper layer of mesophyll where photosynthesis mostly takes place in the leaves
• the water carries essential mineral salts in solution
• evaporation from the leaves has a cooling effect