Practice question

Human erythrocytes are discoid cells lacking nucleus and organelles, bounded by lipid bilayer containing aquaporin-1 water channels and Band 3 anion exchanger, supported by spectrin-actin cytoskeleton providing elasticity but limited capacity to expand. Cytoplasm osmolarity about 300 milliosmoles dominated by potassium, chloride and hemoglobin remains nearly isotonic with plasma. Placing these cells into hypotonic solution where extracellular solute osmolarity lower than intracellular creates higher extracellular water chemical potential. Water rapidly enters down gradient via aquaporin-1 at billions of molecules per second and to lesser extent through lipid phase, increasing cell volume. Membrane area expands, cell shape transforms from biconcave disc to sphere. Cytoskeletal network stretches. Continued influx beyond roughly 1.4 times normal volume exceeds tensile strength, causing transient ruptures and release of hemoglobin into surrounding medium. Supernatant becomes red, pellet disappears, phenomenon termed hemolysis. In hypertonic solutions opposite efflux shrinks cells producing crenated appearance. No active water pumping compensates, as Na+/K+ ATPase maintains ion gradients slowly but cannot counter immediate osmotic water entry.