Bitesize has changed! We're updating subjects as fast as we can. Visit our new site to find Bitesize guides and clips - and tell us what you think!

Home > Biology > Genetics and adaptation > Maintaining water balance

Biology

Maintaining water balance

Water balance in plants

Transpiration stream: First Stage

Movement of water from root hair cell across root cortex cell and to xylem vessels

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 [root hair cells: tiny hairs covering the ends of the smallest roots. They give the root a very large surface area through which to absorb water and mineral salts from the soil. 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 [xylem vessels: vein-like tissues which transport water and minerals up a plant.
  • Water may move by diffusion [diffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration through the cell walls and intercellular spaces.

Second Stage

Second stage of transpiration stream - water molecules leave the xylem vessels and move from cell to cell. They move through the spongy mesophyll cells and out through the guard cell, through the stomata into the surrounding air.

In the leaves, water molecules leave the xylem vessels and move from cell to cell. They move through the spongy mesophyll [spongy 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 [stomata: tiny holes in the epidermis (skin) of a leaf - usually on the undersides of leaves. They control water loss and gas exchange by openng and closing. Singular is stoma 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 cells [palisade cells: the upper layer of mesophyll where photosynthesis mostly takes place in the leaves
  • the water carries essential mineral salts [mineral salts: inorganic chemical compounds in solution
  • evaporation from the leaves has a cooling effect

Watch

The Sonoran Desert

Learn how xerophytes living in the Sonoran desert are adpated to survive in the harsh dry environment.

BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.