of a solution is always less than zero. Hence, water always moves from the less negative potential (–100 kPa) to more
negative potential (–200 kPa).
In cell contents, the water potential is determined by three major sets of internal factors, i.e., matric potential (ym),
solute potential (ys) and pressure potential (yp). a fourth factor, gravity potential (yg) also sometimes influences
the water potential of cell sap, when the contents face tension, due to force of gravity. The water potential in a plant
cell or tissue can be written as the sum of matric potential, solute potential and pressure potential.
yw = ym + ys + yp
Matric potential (ym) is the component of water potential influenced by the presence of matrix, and has got a
negative value. It is significant, in case of dry seeds, young cells etc, but negligible in hydrated or mature cells. It is
not significant in osmosis, so often disregarded. Thus, equation may be simplified as
yw = ys + yp
Solute potential is also known as osmotic potential. It is defined, as the decrease in the water potential of a
solution over its pure state, as a result of the presence of solutes. Solute potential or osmotic potential (ys) is always in
negative value. The more the solute molecules, the lower is the solute potential (ys).
for a solution at atmospheric pressure, water potential (yw) will be equal to solute potential (ys).
yw = ys (at atmospheric pressure)Hydrostatic pressure or pressure potential is the pressure which develops in an osmotic system due to osmotic
entry or exit of water from it. a positive hydrostatic pressure develops in a system or plant cell due to the entry of water
into it, which is also called turgor pressure while negative hydrostatic pressure or tension develops due to loss of
water. Negative hydrostatic pressure develops in xylem due to loss of water during transpiration. This is very important
in transport of sap over long distance in plants.
Due to turgor pressure, the protoplast of a plant cell will press the cell wall to the outside. The cell wall, being elastic,
presses the protoplast with an equal and opposite force. The force exerted by the cell wall over the protoplast is called
wall pressure (WP).
Normally, wall pressure is equal and opposite to turgor pressure, except when the cell becomes flaccid. The
values of these two opposing forces continue to rise, till the cell becomes fully swollen or turgid. In this state, the value
of wall or turgor pressure becomes equal to osmotic potential, yp = ys.
Importance of turgor pressure
(i) It keeps the cells and their organelles stretched which is essential for proper functioning of a cell.
(ii) It provides support to non-woody tissues (e.g., parenchyma).
(iii) It is essential for cell enlargement, during growth.
(iv) It keeps the leaves fully expanded, and properly oriented
to light.
(v) The opening and closing of stomata are caused due
to turgidity of guard cells.
Osmosis
Osmosis is a special type of diffusion of water that occurs
through a semipermeable membrane. Osmosis is movement
of solvent or water molecules from the region of their higher
diffusion pressure or free energy to the region of their lower
diffusion pressure or free energy, across a semipermeable
membrane.