Water is a very important substance obtained and transported throughout plants

Transpiration stream is the movement of water through a plant.

Transpiration stream is maintained by:

1. Osmosis 2. Root pressure 3. Transpiration

Osmosis is the movement of water from high water concentration to low water concentration across a semi-permeable membrane.

Root pressure is the force exerted by water within the xylem tissue of the roots.

Transpiration is the loss of water from the aerial parts of a plant.

Water and mineral uptake:

Water enters the root hairs by osmosis, moving from high water concentration to low water concentration.

Minerals dissolve easily in water and move into the root by diffusion - either by passive transport or by active transport (requires ATP).

Water moves across the ground tissue and into xylem tissue.

Water is then transported up the plant.

Cohesion-tension model of water transport

John Joly and Henry Dixon were two Irish scientists who first proposed the cohesion-tension model of water transport.

Water moving into the xylem tissue of the root causes a pressure build up - this is called root pressure.

Root pressure contributes to the upward movement of water molecules.

Water molecules have hydrogen bonds between them maintaining them in the liquid form - this is cohesion of the water molecules.

Water molecules also tend to stick easily to the sides of the xylem vessels - this is called adhesion of the water molecules.

Transpiration of the water molecules occurs mainly from the leaves - this pulls the column of water molecules upwards through the xylem, creating a tension in the water molecules.

Control of transpiration

Transpiration is controlled by:

Waxy cuticle - prevents direct water loss from the surface of leaves.

Stomata - controls the rate of transpiration by opening and closing.

Lenticels - allows a small amount of transpiration but also allows oxygen in (for respiration) and carbon dioxide out (excretion).