6.3 Water Potential and Osmotic Systems

Tonicity compares a solution with a particular cell and depends on effectively non-penetrating solutes. A hypotonic environment has higher water potential and produces net water entry; a hypertonic environment produces net loss; isotonic conditions give no net volume change. Individual water molecules still cross at isotonic equilibrium.

An animal cell in a strongly hypotonic solution swells and may lyse because the plasma membrane cannot resist unlimited expansion. In a hypertonic solution it loses water and crenates. Medical replacement fluids are formulated near physiological osmotic conditions so cells do not undergo dangerous volume changes, though real clinical solutions must also match appropriate ions and chemistry.

A plant cell in a hypotonic environment gains water. The vacuole expands, the protoplast presses against the wall and positive pressure potential rises until internal and external water potentials match. The cell becomes turgid. In a hypertonic solution, water loss reduces pressure, the cell becomes flaccid and the plasma membrane may pull away from the wall in plasmolysis. The wall prevents bursting but does not prevent water movement.

Potato cylinders can be placed in a concentration series, blotted consistently and reweighed. Percentage mass change controls for small differences in starting mass. Replicates reveal random variation and support a mean. The x-intercept of percentage mass change against concentration estimates the external solution producing no net water movement; it does not prove that every cell has identical solute concentration.