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pressure generated by the movement of water across the cell membrane caused by the difference between solute concentrations on each side.

osmotic pressure (Wikipedia)

Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in its pure solvent by osmosis. Potential osmotic pressure is the maximum osmotic pressure that could develop in a solution if it were separated from its pure solvent by a semipermeable membrane.

Progression: (1) a U-tube is filled with water and has a membrane in the middle (2) sugar is added to the left part (3) water crosses the membrane and fills the left side more than the right.
Osmosis in a U-shaped tube

Osmosis occurs when two solutions containing different concentrations of solute are separated by a selectively permeable membrane. Solvent molecules pass preferentially through the membrane from the low-concentration solution to the solution with higher solute concentration. The transfer of solvent molecules will continue until equilibrium is attained.

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