In a plant cell, the cell wall is freely permeable to water and other smaller substances, the cell membrane is semi-permeable to oxygen, carbon dioxide, ions and water. The cell membrane together with the vacuolar membrane determines the movement of the molecules inside and outside the plant cell. The vacuolar sap in the vacuole contributes to the solute potential of the plant cell.
The diffusion of water across a semi-permeable membrane from high concentration of water (or low solute concentration) to low concentration of water (or high solute concentration) is called osmosis. The net direction and rate of osmosis depends on two factors: concentration gradient and pressure gradient. If two solutions are put into a chamber separated by a semi-permeable membrane, water will move from the region of the higher concentration gradient to the region of the lower concentration gradient until equilibrium is reached. At equilibrium, both chambers will have the same water potential. The potential of water molecules to move from a hypotonic solution to a hypertonic solution across a semi-permeable membrane is called osmotic potential.
To demonstrate osmosis, a solution of sucrose in water is put into a funnel and separated from the pure water in a beaker by a semi-permeable membrane. The water being hypotonic will move from the beaker to the funnel containing the hypertonic sugar solution, raising the solution level in the funnel. This water movement will continue until both solutions in the funnel and beaker achieve equilibrium. Now we can apply external pressure to the upper part of the funnel so that no water diffuses from the beaker to the funnel through the membrane. This external pressure that is required to prevent the water from diffusing is known as osmotic pressure.
Osmotic pressure is a function of solute concentration. The higher the solute concentration, the more the diffusion of water. Therefore, more osmotic pressure will be required to stop the entry of water molecules into the solution.
The water potential is equal to its solute potential for a solution that is at atmospheric pressure. Unlike osmotic potential, which is negative, osmotic pressure is a positive pressure. However, numerically, osmotic pressure is equal to osmotic potential, only their signs differ.