electron carriers

Meaning: Electron carriers are coenzymes that are reduced when a substrate is oxidized, then give up those electrons in another reaction. The oxidized forms (NAD⁺, FAD) are oxidizing agents; the reduced forms (NADH, FADH₂) are reducing agents. NADH and FADH₂ are made in glycolysis, pyruvate dehydrogenase and the citric acid cycle (3 NADH and 1 FADH₂ per turn). They carry electrons to the electron transport chain, which pumps protons so ATP synthase can make ATP. NADPH is the related carrier used in biosynthesis, such as fatty acid synthesis, and is made in the pentose phosphate pathway.

Equation: NAD⁺ + 2e⁻ + H⁺ → NADH, where NAD⁺ is the oxidized carrier, NADH the reduced carrier, e⁻ an electron and H⁺ a proton (amounts in mol).

On the MCAT: You may be asked to count carriers produced by a pathway, or to predict what happens when the electron transport chain is blocked. In that case NADH builds up, NAD⁺ runs low and the citric acid cycle slows. Fermentation questions rest on the fact that it regenerates NAD⁺ so glycolysis can continue. NADH donates at complex I and FADH₂ at complex II, so FADH₂ yields less ATP (about 1.5 versus 2.5).

Common confusion: Carriers do not make ATP directly. ATP comes from the proton gradient they help build. Also keep NADH (mainly catabolic, feeds the chain) separate from NADPH (anabolic reducing power).