The movement of particles from the region of higher concentration to the region of lower concentration through a semipermeable membrane is called osmosis. Osmotic pressure is equal to external pressure which must be applied to stop entry of water into an osmotically active solution through a semipermeable membrane. Osmotic pressure is also defined as the maximum pressure which is produced in osmotically active solution when the same is separated from pure water by a semipermeable membrane. Osmotic pressure is dependent upon the concentration of solutes. It is higher in case of xerophytes as compared to mesophytes while mesophytes have higher osmotic pressure than hydrophytes. The value of osmotic pressure is equal to osmotic potential but with a positive sign.
Measurement of osmotic pressure: It is done with the help of instrument called osmometer .Example-Pfeffer’s osmometer, Berkeley and Hartley’s osmometer.
In Pfeffer’s osmometer, a pressure develops due to entry of water into a vessel having osmotically active solution. The pressure is measured with the help of manometer. The value is not accurate because it involves dilution of solution.
In Berkeley and Hartley’s osmometer, pressure is applied to prevent entry of water into a solution. The value of pressure which is just sufficient to prevent entry or exit of water in the apparatus is equal to osmotic pressure of the solution. Thistle funnel experiment can be modified to measure osmotic pressure. Thistle funnel is fitted with a piston to which pressure can be applied .The pressure that keeps the level of solution unchanged is equal to osmotic pressure of solution contained in thistle funnel.
The pressure required to stop the movement of water completely is called osmotic pressure.