# Diffusion and Osmosis

#### Biology Doubts

Two types of mechanisms, namely, physical processes and biological processes are involved in the entry or exit of various molecules. Physical processes are slow, do not expend energy and occur down the concentration gradient. Only small molecules pass through the plasma membrane by physical processes. These processes include diffusion and osmosis.

### Differences between diffusion and osmosis

Find the table of difference between diffusion and osmosis in the below mentioned pdf

### Basics of osmosis

The phenomenon of the passage of solvent molecules from a region of lower concentration (of the solution) to a region of its higher concentration or from pure solvent to solution through a semi-permeable membrane is called Osmosis.

Due to Osmosis, there is net movement of solvent is from the less concentrated (hypotonic) to the more concentrated (hypertonic) solution, which tends to reduce the difference in concentrations. This effect can be countered by increasing the pressure of the hypertonic solution, with respect to the hypotonic.Thus, Osmotic Pressure () can be defined as the external pressure which must be applied on the high concentration side to maintain equilibrium, and prevent the phenomenon of Osmosis with no net movement of solvent.

[Note: If the applied external pressure is more than the osmotic pressure then the solvent particles will start flowing in the opposite direction i.e., solution to solvent side(or high conc. to low conc. Side). This phenomenon is known as reverse osmosis.]

Osmotic Pressure can be calculated using the Van't Hoff equation for dilute solutions, which is given as  = CRT

where pi Osmotic pressure

C Molar concentration of the solution (Molarity)

Gas constant

T Absolute temperature

### Types of Solution on the basis of Osmotic Pressure

1. Isotonic – Two solutions are said to be isotonic if they have the same osmotic pressures.

2. Hypertonic – A hypertonic solution is the one with high osmotic pressure when compared to another solution which is separated from it by a semi-permeable membrane.

3. Hypotonic – A hypotonic solution is the one with low osmotic pressure when compared to another solution which is separated from it by a semi-permeable membrane.