Nov 15, 2009

Water Transport

There are two types of vessels in plants :-

Xylem -- These vessels take water and mineral ions from the roots to the stem and the leaves.
Phloem -- takes iorganic substances and sugars from the leaves to the parts of the leaves that require them eg. the flowers, fruits and roots.
Xylem travels only upwards, whereas phloem travels in both directions.

Movement in xylem vessels
The cells which make up a xylem vessel are dead. They are joined together by a sticky substance called lignin, these cells are therefore said to be "lignified". This causes the xylem vessels to be impermeable.
There are three mechanisms which contribute to the movement of water through the xylem vessel.

Capillarity --- Xylem vessels are often very small in plants and therefore water is able to travel up them via capillary action. The water molecules stick to the side of the vessel and slowly "climbs up". However this mechanism does not account for the greater distance that water can travel in trees.
Root pressure ---- Some plants can produce a water potential gradient by actively transporting mineral ions to the top of the plant. The water potential on the top of the plant is much greater than the bottom of the plant, therefore the water moves up to the top.
Cohesion-tension --- As leaves transpire, water evaporates to the dry surroundings outside the plant. Water molecules stick to each other by hyrdogen bonds. This is known as cohesion. So as they leaves transpire the water molecules are all pulled up the plant. Water molecules also stick to the sides of the vessel which helps to speed this mechanism up. This is known as adhesion.