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Chapter 12 – Cellular Energetics Chapter 12 – Cellular Energetics 12.1 First Step of Harvesting Energy from Glucose: Glycolysis 12.2 The Structure and Functions of Mitochondria 12.3 The Citric Acid Cycle and Fatty Acid Oxidation 12.4 The Electron-Transport Chain and Generation of the Proton-Motive Force 12.5 Harnessing the Proton-Motive Force to Synthesize ATP 12.6 Photosynthesis and Light-Absorbing Pigments 12.7 Molecular Analysis of Photosystems 12.8 CO2 Metabolism During Photosynthesis Cellular Energetics 12.1 First Step of Harvesting Energy from Glucose: Glycolysis • Aerobic oxidation – Cells use a four-stage process to convert energy released by the of glucose/fatty acid oxidation into ATP terminal phosphoanhydride bond. • Glycolysis – Stage 1: Cytosolic enzymes convert glucose to two molecules of pyruvate and generate two molecules each of NADH and ATP. • In the absence of oxygen (anaerobic conditions), cells can metabolize pyruvate to lactic acid or ethanol and CO2 to convert NADH back to NAD+ required for glycolysis. Cellular Energetics 12.2 The Structure and Functions of Mitochondria • Endosymbiont Hypothesis – Mitochondria and chloroplasts evolved from bacteria that formed a symbiotic relationship with ancestral cells containing a eukaryotic nucleus. • Mitochondria have two distinct membranes (outer and inner) and two distinct subcompartments. • Mitochondria use aerobic oxidation of carboncontaining molecules to generate ATP. • Mitochondria undergo fusion and fission under regulation by the state of the cell. Cellular Energetics 12.3 The Citric Acid Cycle and Fatty Acid Oxidation • In glucose oxidation stage II, the three-carbon pyruvate molecule is first oxidized to generate one molecule each of CO2, NADH, and acetyl CoA, which is oxidized to CO2 by the citric acid cycle. • Most of the energy released in glucose oxidation stages I and II is temporarily stored in reduced NADH and FADH2, which carry high-energy electrons that subsequently drive electron-transport chain (stage III). • Oxidation of short- to long-chain fatty acids occurs in mitochondria with production of ATP; oxidation of very long chain fatty acids occurs primarily in peroxisomes and produces heat not ATP. Cellular Energetics 12.4 The Electron-Transport Chain and Generation of the Proton-Motive Force • Stage III flow of electrons from NADH/FADH2 through the electron transport chain complexes provides energy to drive H+ transport across the inner mitochondrial membrane generating a protonmotive force (voltage and pH gradients). • Reduction potentials of the electron carriers favor unidirectional, “downhill,” electron flow from NADH and FADH2 to O2 to form H2O. Cellular Energetics 12.5 Harnessing the Proton-Motive Force to Synthesize ATP • Chemiosmotic hypothesis – The proton-motive force across the inner mitochondrial membrane is the immediate source of energy for ATP synthesis. • Bacteria, mitochondria, and chloroplasts all use the same chemiosmotic mechanism and a similar ATP synthase to generate ATP. • ATP synthase (F0F1 complex) catalyzes ATP synthesis as protons flow through the inner mitochondrial membrane down their electrochemical proton gradient and rotate the F1 γ subunit. Cellular Energetics 12.6 Photosynthesis and Light-Absorbing Pigments • Plant photosynthesis principal end products are O2 and polymers of six-carbon sugars (starch and sucrose). • Light-capturing and ATP-generating photosynthesis reactions occur in chloroplast thylakoid membranes. • Four photosynthesis stages: (1) light absorption, generation of high-energy electrons, and O2 formation from H2O; (2) electron transport leading to reduction of NADP+ to NADPH and pmf generation; (3) synthesis of ATP; and (4) conversion of CO2 into carbohydrates (carbon fixation). Cellular Energetics 12.7 Molecular Analysis of Photosystems • In the single photosystems, cyclic electron flow from light-excited special-pair chlorophyll a molecules generates a proton-motive force used mainly to power ATP synthesis by the F0F1 complex in the plasma membrane. • The two plant photosystems PSI and PSII have different functions: PSII converts H2O into O2, and PSI reduces NADP+ to NADPH. • Light energy absorbed by chloroplast light-harvesting complexes (LHCs) is transferred to chlorophyll a molecules in the PSI and PSII reaction centers. Cellular Energetics 12.8 CO2 Metabolism During Photosynthesis • Calvin cycle fixes CO2 into organic molecules in a series of reactions that occur in the chloroplast stroma. • In C3 plants, a substantial fraction of the CO2 fixed by the Calvin cycle can be lost during photorespiration, which is favored at low CO2 and high O2 levels. • C4 plants fix CO2 in outer mesophyll cells into fourcarbon molecules that are shuttled to the interior bundle sheath cells for use in the Calvin cycle, decreasing the loss in respiration.