01Osmosis and tonicity
Osmosis is the diffusion of water across a selectively permeable membrane, from low solute concentration to high solute concentration (water follows solute).
Tonicity compares solutions: hypertonic = more solute (cell shrinks, water leaves); hypotonic = less solute (water enters, animal cell may lyse); isotonic = equal (no net movement).
02Water potential (Ψ)
Water moves from high water potential to low water potential. Ψ = Ψs + Ψp. Adding solute makes Ψs more negative. Pure water at atmospheric pressure has Ψ = 0.
Solute potential: Ψs = −iCRT, where i = ionization constant (NaCl = 2, sucrose = 1), C = molar concentration, R = 0.0831 L·bar/mol·K, T = temperature in Kelvin.
Plants love hypotonic surroundings: water enters until the cell wall pushes back (turgor pressure). In hypertonic solutions the membrane pulls from the wall — plasmolysis — and the plant wilts.
Freshwater protists pump out excess water with contractile vacuoles — osmoregulation.
03Notebook box
Worked example: Find Ψs of 0.2 M sucrose at 22 °C. T = 295 K, i = 1. Ψs = −(1)(0.2)(0.0831)(295) = −4.9 bar. Water will move INTO this solution from pure water.
Memory hook: water goes where the salt is.