Practice question

Energy coupling distinguishes passive and active transport. Simple diffusion of gases, facilitated diffusion via carriers like GLUT and channels like aquaporin, and osmosis driven by water potential are passive, relying on thermal motion and existing gradients without ATP input, negative Gibbs free energy toward equilibrium. Primary active transport directly uses chemical energy from ATP hydrolysis within same protein to move solute against gradient. P-type ATPases like Na+/K+ ATPase exchanging 3 Na+ out for 2 K+ in, V-type H+ ATPase acidifying lysosomes, F-type ATP synthase reversible, and ABC superfamily including CFTR, MDR1, TAP hydrolyze two ATP to pump ions, drugs or peptides uphill creating gradients for secondary use. Because it maintains Na+, K+, Ca2+ and H+ gradients primary active consumes up to forty percent of ATP in neurons and kidney. Secondary active transport uses pre-existing ion gradients, for example Na+/glucose symport. Tertiary active further couples gradients. Inhibitors ouabain and bafilomycin collapse gradients impairing nutrient uptake, pH regulation and volume control rapidly demonstrating energetic dependence.