The Movement of Water and Solutes in Plants

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The Movement of Water and Solutes in Plants

During the process of osmosis, water molecules move from an area that

is hypotonic to an area that is hypertonic. A hypotonic area is one in

which has less solute and a hypertonic area is one which has more

solute.

Plant cells, such as the ones in the epidermis and cortex regions of

the roots of the plant, all have living contents, which are enclosed

by a cell surface membrane and a thick, quite inelastic cellulose cell

wall. The cell wall has special properties, which help the cells

resist during the osmotic uptake of water. If a plant cell is placed

in distilled water, it wonÂ’t swell up and burst like for example, a

red blood cell, but it will take in water until the pressure that the

wall exerts stops any further intake and expansion. When a plant

reaches this condition it is said to be fully turgid. This turgor is

essential for support in the plant, as the plant would wilt if it lost

water.

The term water potential is used to describe the force acting on water

molecules in a solution, when separated from pure water by a membrane,

which only allows water to pass through it. This is a measure of the

potential of water molecules to move from a region of a certain water

molecule concentration to another region of lower water molecule

concentration. Water will move from a region of higher water potential

to one of lower water potential. It is measured in terms of pressure

and the units are either kPa (kilopascals) or MPa (megapascals). The

symbol for water potential is Ψ (Greek letter psi). Pure water has the

highest water potential (0). More negative water potentials indicate

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...the central tissues. These three ways are:

· Apoplast pathway: in which water passes through the continuous

system of adjacent cell walls.

· Symplast pathway: in which water moves from cell to cell, through

the cytoplasm (the cytoplasm of adjacent cells in the cortex is in

contact via the plasmodesmata, which are fine-channels through the

cell walls).

· Vacuolar pathway: in which water moves from vacuole to vacuole.

However, when water reaches the endodermal cells its movement is

stopped by a waterproof layer in the cell walls, called the Casparian

strip. This is impregnated with suberin, a waxy compound, which is

impermeable to water. Therefore, water is prevented from passing

around the endodermal cells through the cell walls, but instead it

passes through the cell surface membrane and cell contents.

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