Practice question

Membrane transport energetics divide conceptually into active requiring cellular energy input and passive driven solely by existing electrochemical gradients toward equilibrium. Primary active transport directly couples exergonic ATP hydrolysis to ion movement against gradient: Na+/K+ ATPase phosphorylates aspartate forming E1P E2P cycle extruding three sodium importing two potassium, V-type proton pumps rotate c-ring acidifying lysosomes using ATP, ABC transporters dimerize nucleotide binding domains consuming ATP to expel drugs and lipids. Secondary active cotransporters such as SGLT1 sodium glucose symporter and NCX sodium calcium exchanger use previously established sodium gradient to accumulate glucose or extrude calcium without directly hydrolyzing ATP. Vesicular routes endocytosis and exocytosis require ATP for actin polymerization, dynamin GTPase activity, clathrin uncoating by Hsc70 and NSF mediated SNARE disassembly. In contrast facilitated diffusion includes GLUT uniporters operating via alternating access rocker switch and channel proteins like voltage gated ion channels and aquaporins providing continuous aqueous pathway. Both show saturation kinetics Michaelis-Menten like but operate down gradient, no phosphorylation intermediate or nucleotide consumption, powered solely by concentration difference. Therefore facilitated diffusion distinguishes itself by independence from ATP hydrolysis and uphill energy coupling.