Practice question

Osmosis describes net water movement across selectively permeable membrane driven by difference in solute concentration and water chemical potential, not by ATP. Plasma membranes and artificial phospholipid bilayers act as semi-permeable barriers allowing water to partition and diffuse but restricting most solute diffusion due to polarity. Water molecules move randomly but statistically from compartment with lower osmolarity, higher water mole fraction and higher chemical potential, to compartment with higher solute concentration, lower water potential, until hydrostatic pressure compensates or concentrations equalize, generating osmotic pressure described by van't Hoff equation pi equals MRT times total osmolarity. This thermodynamic process is spontaneous and does not consume ATP directly; energy originates from mixing entropy. Although active maintenance of ion gradients by Na+/K+ ATPase and NKCC cotransporter indirectly influences overall tonicity and cell volume, osmosis itself is passive. Aquaporins increase hydraulic conductivity in erythrocytes, kidney proximal tubule and collecting duct but are not obligatory for osmosis to occur, as lipid bilayer alone permits basal water flux albeit slower with permeability coefficient about 10^-3 cm/s. Claiming ATP requirement misinterprets distinction between primary establishment of solute gradient and passive water equilibration following isosmotically.