Water Potential

Osmosis is the net movement of water molecules across a partially (selectively) permeable membrane, from a region of higher water potential to a region of lower water potential. It is a special case of diffusion — only the solvent (water) moves, because solute particles cannot cross the membrane freely.

Water potential (Ψ, the Greek letter "psi") is a measure of the tendency of water to move out of a system. It is measured in units of pressure — kilopascals (kPa).

Water potential has two components:

$\Psi = \Psi_s + \Psi_p$

• Solute potential ($\Psi_s$) — also called osmotic potential. Dissolving solutes lowers water potential, so $\Psi_s$ is always zero or negative. The more solute, the more negative $\Psi_s$.
• Pressure potential ($\Psi_p$) — the physical (hydrostatic) pressure on the water. In a plant cell, the cell wall pushes back on the swollen contents, giving a positive $\Psi_p$.

Key reference point: pure water at standard atmospheric pressure has a water potential of exactly zero ($\Psi = 0$). Because solutes only ever make Ψ more negative, the water potential of any solution is negative.

Direction of movement. Water moves from higher (less negative) Ψ → lower (more negative) Ψ until the two are equal (equilibrium). So a cell with a more negative Ψ than its surroundings will gain water by osmosis.

Comparing the water potential of a solution with that of the cell lets us predict water movement. The three cases (tonicity) are:

• Hypotonic solution — higher Ψ (fewer solutes) than the cell → water moves INTO the cell.
• Isotonic solution — equal Ψ to the cell → no net water movement.
• Hypertonic solution — lower Ψ (more solutes) than the cell → water moves OUT of the cell.

Animal cells (e.g. red blood cells) have no cell wall, so they cannot resist water gain or loss:

• In a hypotonic solution, water floods in and the cell swells and bursts — lysis (haemolysis in red blood cells).
• In a hypertonic solution, water leaves and the cell shrivels — crenation.
• In an isotonic solution, the cell keeps its normal shape — the safe condition.

Plant cells have a strong cellulose cell wall that resists expansion, generating pressure potential:

• In a hypotonic solution, water enters; the cell swells until the wall pushes back — the cell becomes firm and turgid (this turgor pressure supports non-woody plants).
• In an isotonic solution there is no net movement; the cell is flaccid (no turgor) — a wilted state.
• In a hypertonic solution, water leaves; the cytoplasm and membrane shrink away from the wall — plasmolysis (the cell is plasmolysed).

Applications of isotonic conditions.

• Intravenous (IV) fluids and drips are made isotonic with blood plasma (e.g. 0.9% saline, "normal saline"). A hypotonic drip would cause red blood cells to lyse; a hypertonic drip would cause crenation — both are dangerous.
• Tissue and organ storage for transplant uses isotonic, balanced solutions so cells neither swell nor shrink during preservation.
• Agriculture / food preservation uses osmosis the other way: salting or sugaring food creates a hypertonic environment that draws water out of microbial cells (plasmolysing/dehydrating them), preventing spoilage. Over-fertilising soil can likewise make it hypertonic and pull water out of roots ("fertiliser burn").