Osmosis explains how cells gain or lose water across selectively permeable membranes. Water potential provides a quantitative framework for predicting the direction of net water movement in biological systems, especially when solutes and pressure are involved.

Core ideas: osmosis and water potential

Osmosis depends on a selectively permeable membrane

Osmosis is “net” movement because water molecules move randomly in both directions; a difference in water potential creates an overall bias in one direction.

Osmosis across a semipermeable membrane: solute is unable to cross, but water diffuses until the gradient is reduced. The diagram makes the “net movement” idea explicit by showing water shifting toward the side with higher solute (lower free-water concentration). Source

Water potential predicts direction of water movement

In cells, $\Psi$ is lowered mainly by dissolved solutes and can be increased by physical pressure against a cell wall. This makes water potential especially useful for understanding osmosis in plant tissues and other pressurised systems.

Linking “hypotonic/hypertonic” to water potential

Tonicity language as a shortcut

The syllabus framing is that osmosis moves water from high to low water potential, and this is often described as water moving from hypotonic to hypertonic regions.

Red blood cells in hypotonic, isotonic, and hypertonic solutions show how tonicity predicts water movement and resulting cell shape changes. The figure highlights swelling/lysis in hypotonic conditions, no net change in isotonic conditions, and shrinking (crenation) in hypertonic conditions. Source

• Hypotonic region: lower solute concentration (effectively higher $\Psi$ for water), so water tends to leave it.

• Hypertonic region: higher solute concentration (effectively lower $\Psi$ for water), so water tends to enter it.

These terms are most reliable when comparing solutions separated by a membrane that is permeable to water but not to the solutes creating the gradient; otherwise, tonicity can change as solutes redistribute.

Components of water potential in biology

Solutes lower water potential

Dissolving solute reduces the “free” water molecules available to move, lowering $\Psi$. This component is typically called solute potential and is always negative (or zero in pure water).

Pressure can raise water potential

Physical pressure (for example, in walled cells) increases the tendency for water to move out of that compartment. In plants, turgor pressure can make the cell’s $\Psi$ less negative, reducing further water uptake.

Plant water status under different water-potential conditions: when external $\Psi$ is lower, water exits cells and the plant wilts; when external $\Psi$ is higher, water enters cells and builds turgor pressure. This visual ties pressure potential ($\Psi_p$) to whole-plant rigidity and explains why walled cells behave differently from animal cells. Source

Because water moves from higher to lower $\Psi$, a compartment with more negative $\Psi_s$ can still fail to gain water if it also has sufficiently high positive $\Psi_p$.

What actually happens at the membrane

Direction and “net” movement

Across a selectively permeable membrane:

• If the outside solution has lower $\Psi$ than the cytosol, net water movement is out of the cell.

• If the outside solution has higher $\Psi$, net water movement is into the cell.

• If $\Psi$ is equal on both sides, there is no net osmosis (dynamic equilibrium).

Biological significance for cells

Osmosis affects cell volume and thus cell function because membranes are flexible:

• Water influx can increase membrane tension.

• Water efflux can reduce cytoplasmic volume and disrupt normal biochemical conditions.

In walled cells, pressure potential can build as water enters, helping stabilise cell volume and limiting further net uptake once the internal and external $\Psi$ values match.