Osmotic pressure, turgor pressure and suction pressure are the three osmotic quantities of a plant cell because they control the osmotic behaviour of the cell.
Osmotic pressure (OP):
Osmotic pressure of a solution is defined as the excess hydrostatic pressure which is to be applied to it in order to make its water potential equal to that of pure water.
The turgor pressure (T.P):
When a plant cell is placed in water, water enters into the cell due to endosmosis. As soon as the water enters into the cell, it exerts a pressure on cell inclusions and organelles and ultimately on the cell wall.
This pressure which develops in a cell from time to time due to osmotic entry of water is called turgor pressure. The turgor pressure is variable. It is maximum when the cell is fully turgid and is zero when the cell is flaccid.
Due to increase in turgor pressure the cell wall stretch's to-wards outside. Since the cell wall provides a definite shape to the cell and is elastic in nature, it also exerts a pressure on cell sap from outside i.e. in opposite direction, equal to T.P. The pressure exerted by the cell wall on the cell sap is called wall pressure. The wall pressure resists turgor pressure and prevents bursting of cell wall.