Practice question

Fick's laws provide quantitative framework for diffusion in solutions and across biological membranes integrating concentration, distance and membrane properties. First law states net diffusive flux J, amount per area per time, is proportional to concentration difference divided by distance, expressed mathematically as J equals minus D diffusion coefficient times dC/dx concentration gradient, where negative sign denotes direction from high to low. D depends on temperature increasing Brownian motion, solute Stokes radius inversely via Stokes-Einstein relation, viscosity of medium and partition coefficient for membranes. Within membrane context, permeability P equals D times partition coefficient K divided by thickness delta. Factors accelerating diffusion include larger gradient, higher temperature, smaller molecular weight, greater lipophilicity, larger surface area and thinner barrier. Fick's law predicts linear relationship between gradient and flux until transporter saturation intervenes, applicable to alveolar gas exchange, glucose entry via simple diffusion at very high concentrations, and morphogen gradient formation in development. Unlike carrier-mediated transport, Fickian diffusion shows no saturation, no stereospecificity beyond partition, no competition and cannot move solute against gradient without energy input, providing baseline against which facilitated processes are compared for selectivity advantage.