Water moves across a selectively permeable membrane from higher water potential to lower by osmosis.

Water potential (MegaPascal, or Ψ) is the sum of pressure potential (Ψ_P) and solute potential (Ψ_S):

Ψ = Ψ_P + Ψ_S.

1. The addition of solutes reduces water potential (to a negative value).

2. Application of physical pressure increases water potential.

3. The increased physical pressure can reverse the effects of the solutes .

4. A negative pressure reduces water potential (on the other side).