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In a redox pair, oxidation is electron loss and reduction is electron gain. Electron transfer tends to proceed from carriers with more negative standard reduction potentials toward carriers with more positive potentials. Biological carriers—including NAD⁺/NADH, NADP⁺/NADPH, flavins, coenzyme Q, cytochromes, and iron–sulfur clusters—handle electrons in different ways. Dehydrogenases commonly couple substrate oxidation to coenzyme reduction. The lesson follows electron flow through the respiratory chain to oxygen and explains how proton pumping at selected complexes helps establish the gradient that drives ATP synthase. It also introduces specialized redox systems, including cytochrome P450, glutathione, reactive oxygen species detoxification, the neutrophil oxidative burst, and the pentose phosphate pathway.