D2.3.9 (HL)—Movement from higher to lower potential

Water moves from higher water potential to lower water potential because solutes reduce free water movement in cells and solutions.

The direction of net water movement is set by total water potential, not by one component in isolation.

A higher Ψ means water has a greater tendency to leave. Comparing total values incorporates both solute and pressure effects and explains why a concentrated but pressurized cell may not lose water.

Write both total Ψ values, compare them numerically, then state the direction and the condition for equilibrium.

Water moves from −0.2 MPa to −0.5 MPa, but not from a region at −0.8 MPa to −0.5 MPa; the latter direction is reversed.

‘Higher’ means numerically less negative; movement stops when the relevant potentials are equal or other forces intervene.

Water potential is potential energy of water per unit volume, measured in kPa; pure water at standard conditions has water potential of 0 kPa. Water moves from higher water potential to lower water potential; solutes lower water potential by restricting water molecule movement. Water potential equals solute potential plus pressure potential; solute potential is zero or negative and pressure potential is often positive in walled cells. Water entering plant cells increases pressure potential and dilutes solutes; water leaving plant cells lowers pressure potential and makes solute potential more negative.

Concept essentials

Water moves from higher to lower water potential.
Solutes lower water potential by reducing free water movement.
Water movement can stop when water potentials are equal.
Pressure potential can oppose water entry in plant cells.

D2.3.9 (HL)—Movement from higher to lower potential

Water Moves from Higher to Lower Total Potential

HL Water Potential

Concept essentials