Metabolism and Energy Transformations
Core concepts
Understand metabolism as the sum of catabolic (energy-releasing) and anabolic (energy-consuming) reactions. Know ATP as the universal energy currency and the ATP/ADP cycle. Master glycolysis (cytosol, glucose to pyruvate, net ATP and NADH), the link reaction, the Krebs cycle (mitochondrial matrix), and oxidative phosphorylation (inner membrane, electron transport chain and chemiosmosis). Know anaerobic pathways: lactic acid fermentation in animals and ethanol fermentation in yeast. For photosynthesis, know the light-dependent reactions (thylakoid membranes, photosystems, water splitting, ATP and NADPH) and the Calvin cycle (stroma, carbon fixation, RuBP). Know the factors limiting rates: light, CO2, temperature, enzyme concentration.
Traps
Glycolysis does not need oxygen and does not occur in mitochondria. The Krebs cycle does not directly use O2, but the electron transport chain does. Fermentation regenerates NAD+ rather than making much ATP. In photosynthesis, oxygen comes from water, not CO2; carbon in glucose comes from CO2, not from the soil. Distinguish gross from net photosynthesis (respiration subtraction). Do not confuse the light reactions' location (thylakoid) with the Calvin cycle's location (stroma).
Worked example / rule of thumb
Rule of thumb: "energy in, energy out; count ATP and track electrons." Worked example: one glucose yields net 2 ATP and 2 NADH in glycolysis; if oxygen is absent, pyruvate is reduced to lactate or ethanol and the NADH is reoxidized, so the net yield stays at 2 ATP. If a question asks where ATP synthesis is greatest, answer the inner mitochondrial membrane via oxidative phosphorylation.