FIGURE 12-1 Overview of aerobic oxidation and photosynthesis. Eukaryotic cells use two fundamental mechanisms to convert external sources of energy into ATP. (Top) In aerobic oxidation, “fuel” molecules [primarily sugars and fatty acids (lipids)] undergo preliminary processing in the cytosol, such as breakdown of glucose to pyruvate (stage I), and are then transferred into mitochondria, where they are converted by oxidation with O₂ to CO₂ and H₂O (stages II and III) and ATP is generated (stage IV). (Bottom) In photosynthesis, which occurs in chloroplasts, the radiant energy of light is absorbed by specialized pigments (stage 1); the absorbed energy is used both to oxidize H₂O to O₂ and to establish conditions (stage 2) necessary for the generation of ATP (stage 3) and of carbohydrates from CO₂ (carbon fixation, stage 4). Both mechanisms involve the production of reduced high-energy electron carriers (NADH, NADPH, FADH₂) and the movement of electrons down an electric potential gradient in an electron-transport chain through specialized membranes. Energy released from these electrons is captured as a proton electrochemical gradient (proton-motive force) that is then used to drive ATP synthesis. Bacteria use comparable processes.