Plants Essay, Research Paper

Circulation in

plants 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, wheras 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. Movement through

cells The Apoplastic pathway —

The water and mineral ions moves through the cells walls and through

the spaces between the cells. The symplatic pathway —

The contents of neighbouring cells are joined by thin strands of cytoplasm,

known as plasmodemata. Water and other substances can move from one cell to the

next via these structures. This is very useful because the substances do not

have to cross cell membranes.