Q What is osmotic pressure?

Osmotic pressure is the pressure produced in an aqueous solution by a region of lower solute concentration upon a region of higher solute concentration forcing the passage of water from that to this additionally concentrated region. The intensity of the osmotic pressure in units of pressure is equal to the pressure that is necessary to apply in the solution to avoid its dilution by the entering of water by osmosis.

It is possible to apply in the solution another pressure in the contrary way to the osmotic pressure, like the atmospheric pressure or the hydrostatic pressure of the liquid. In plant cells, for example, the rigid cell wall makes opposite pressure against the tendency of water to enter when the cell is put under a hypotonic environment. Microscopically, the pressure contrary to the osmotic pressure does not forbid water to pass throughout a semipermeable membrane but it creates a compensatory flux of water in the opposite way.