Osmosis is a special type of diffusion of water that occurs through asemipermeable 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 freeener energyross a semipermeable
membrane.
The direction and rate of osmosis depends upon the sum of two forces pressuregradient (gradient of wo) and
concentration gradient (gradient of y) The net force or gradient is determnined by the difference in the water potential
of solutions, separated by a semipermeable membrane.
A solution which can cause an osmotic entry of water into it, is said to be osmotically active solution. It possesses
a low water potential. Diffusion of water into it will continue across the separating membrane, till an equilibrium is
reached. At equilibrium, water potential becomes equal on both sides of the membrane.
Osmosis is of two types - Endosmosis and Exosmosis.
. Endosmosis is the osmotic entry of water into a cell, organ or system.
. Exosmosis is the osmotic withdrawal of water from a cell, organ or system.
During osmosis, the water (or solvent) molecules move as follows
From the region of
Pure solvent (water)
Dilute solution
High free energy of water molecules
Higher chemical potential (or water potential)Lower chemical potential (or water potential)
Higher diffusion pressure of water
To the region of
Solution
Concentrated solution
>Low free energy of water molecules
Lower diffusion pressure of water
→
A solution having low osmotic concentration, hence low osmotic pressure, but less negative solute potential, as compared
to another solution is known as hypotonic solution. A solution having high osmotic concentration, hence high osmotic
pressure, but more negative solute potential, as compared to another solution is termed as hypertonic solution. The
two solutions, with the same concentration or pressure or potential are named as isotonic solutions.
