Practice question

Tonicity terminology describes effective osmolality of solution relative to cytoplasm determined by concentration of non-penetrating solutes that cannot quickly equilibrate across membrane. Animal cell cytoplasm about 300 milliosmoles includes potassium, organic phosphates, amino acids and impermeable proteins contributing to Donnan equilibrium. If external medium contains higher total concentration of impermeable species such as sodium chloride, sucrose, mannitol or poorly permeant divalent salts, its osmotic pressure higher per van't Hoff relation pi equals iCRT, water potential more negative by delta psi equals minus RT delta osmoles. Water then tends to leave cell to dilute external compartment attempting to equalize potentials. Such medium characterized as hypertonic compared with cell interior, causing crenation in erythrocytes and plasmolysis in plant cells. By contrast hypotonic medium lower solute causes water entry and swelling, isotonic equal concentration causes no net movement despite ongoing bidirectional exchange driven by thermal kinetic energy. Penetrating solutes like urea cross rapidly via UT transporters and do not determine long-term tonicity. Therefore hypertonic solution definition corresponds to higher solute concentration than that present inside cell, driving osmotic efflux and volume loss.