Practice question

Transport classification distinguishes energy-dependent versus independent processes based on need for free energy beyond ambient thermal energy and existing gradients. Simplest criterion examines whether net flux can occur spontaneously down electrochemical potential or requires coupling to external energy store. Simple diffusion of hydrophobic molecules like steroid hormones and non-polar gases occurs directly across lipid without protein and without ATP, driven by entropy. Facilitated diffusion remains energy-independent but overcomes barrier for polar solutes using integral membrane proteins to provide aqueous pore or carrier binding site; flux occurs strictly down gradient without nucleotide hydrolysis, though proteins confer selectivity for glucose versus fructose and saturability following Michaelis-Menten kinetics. Examples include GLUT1 glucose uniport, aquaporin-1 water channel, and CFTR chloride channel. These systems can be regulated by gating, phosphorylation or vesicular trafficking such as vasopressin induced AQP2 insertion, but regulation controls availability not thermodynamic drive. In contrast primary active transport like Na+/K+ ATPase, V-ATPase acidifying lysosomes, F-ATP synthase operating reverse, and ABC exporters hydrolyze ATP directly, while secondary active uses ion gradients. Facilitated diffusion therefore occupies middle ground as protein-assisted passive route enabling efficient nutrient uptake without energy cost.