Osmosis is the diffusion of water through a membrane. The overall tendency of a solution to take up water is called its water potential (Ψ). Water potential is the sum of the osmotic potential (Ψπ, usually negative) and the pressure potential (Ψp, usually positive).

The water potential of pure water is zero. Adding solutes to pure water decreases its osmotic potential, and thus lowers its water potential. The osmotic potential of any solution is negative and the greater the solute concentration, the more negative the osmotic potential of the solution.

Pressure potential is the pressure applied to a solution. Applying hydrostatic pressure increases the pressure potential, while applying tension of suction decreases the pressure potential.

Therefore Ψ = Ψπ + Ψp.

Osmosis is the net movement of water through a selectively permeable membrane from a region of higher water potential to one of lower water potential. i.e. the water always moves towards the region of more negative water potential.

Comparing osmosis in plant and animal cells.

In cells with cell walls (eg. plants, fungi, some protists) the net movement of water by osmosis depends on the cell's osmotic potential and an opposing pressure potential exerted by the cell wall on the cell's volume. A plant cell placed in distilled water will take up water until an equilibrium is reached when the tendency of water to enter the plant cell due to osmotic potential is balanced by the repulsion of water from the cell due to pressure potential.

The osmotic movement of water in cells lacking walls (eg. animals) depends on osmotic potential differences on either side of the membrane. The next movement will be from the side with a less negative osmotic potential to the side with a more negative water potential.