Cellular energetics covers how cells capture energy (photosynthesis) and release it (cellular respiration) — the metabolic processes that power all life.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Light reactions (thylakoid): water split, O₂ released, ATP and NADPH produced. Photosystems II and I, electron transport chain, chemiosmosis (H⁺ gradient → ATP synthase). Calvin cycle (stroma): CO₂ fixed by RuBisCO, G3P produced using ATP and NADPH. Carbon fixation → reduction → regeneration of RuBP. C3, C4, and CAM plants differ in how they handle CO₂ fixation to minimize photorespiration.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. Glycolysis (cytoplasm): glucose → 2 pyruvate, 2 ATP, 2 NADH. Pyruvate oxidation → acetyl CoA + CO₂ + NADH. Krebs cycle (matrix): 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂ per glucose. ETC & oxidative phosphorylation (inner membrane): NADH/FADH₂ donate electrons, O₂ final acceptor, chemiosmosis produces ~30-32 ATP total. Total yield: ~36-38 ATP per glucose.
Anaerobic pathways when O₂ is absent. Alcohol fermentation (yeast): pyruvate → ethanol + CO₂. Lactic acid fermentation (muscle cells): pyruvate → lactate. Both regenerate NAD⁺ so glycolysis can continue. Only 2 ATP per glucose (very inefficient compared to aerobic). Obligate anaerobes: cannot survive with O₂. Facultative anaerobes: can switch between aerobic and anaerobic.
They are complementary: photosynthesis captures light energy and stores it in glucose (CO₂ + H₂O → C₆H₁₂O₆ + O₂), while respiration releases that stored energy (C₆H₁₂O₆ + O₂ → CO₂ + H₂O + ATP). The products of one are the reactants of the other. Both use electron transport chains and chemiosmosis for ATP production. In plants, both processes occur simultaneously — photosynthesis in chloroplasts, respiration in mitochondria. During the day, photosynthesis exceeds respiration (net O₂ production). At night, only respiration occurs (net CO₂ production).
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