These solutes attract water into the cells through a process known as osmosis, which involves water flowing in through semipermeable membranes that prevent the passage of solutes but not of water. The inflow of water swells the cells until a hydrostatic pressure is reached at which no more water will flow in. In cells bathed in fresh water, such as algal cells in a pond, this equilibrium hydrostatic pressure is known as the osmotic pressure (π) of the cell contents, and is commonly about 500 kPa or 0.5 MPa.

For a given osmotic pressure (π) within a cell, the hydrostatic pressure, P, will be lower than if the cell were bathed in free water. This difference is known as the water potential (ψ) of the cell. It is zero in an algal cell in fresh water, but it is always negative in land plants. Its value is the difference between P and π, that is:

Ψ = P - π