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RESPIRATION: SYNTHESIS OF ATP Clickers! Respiration is a series of coupled reactions • Carbon (in glucose) is oxidized • ATP is formed from ADP plus phosphate O2 ADP + Pi CO2 + H2O ATP Synthesis of ATP Anaerobic conditions (fermentation) ! Glycolysis depends on a supply of substrates: glucose, ATP, ADP, Pi, NAD+ ! NAD+, FAD present in only small amounts in cell, and NAD+ and FAD are used up in glycolysis and the citric acid cycle. ! Therefore, NAD+ must be regenerated from NADH to allow continued glycolysis, citric acid cycle operation. ! In air, the electron transport chain regenerates NAD+ and FAD by passing electrons to O2. ! Without air, the electron transport chain cannot oxidize NADH, FADH2; citric acid cycle stops. ! Without air, some cells regenerate NAD+ (from glycolysis only) by passing e- (+ H+) to pyruvic acid ! Result: continued glycolysis, forming 2 ATP per glucose Muscle cells Reduction of pyruvate produces lactic acid Yeast cells Reduction of pyruvate produces ethanol Variations: ! Most plants make EtOH, but are hurt by large amounts; some plants make lactic or malic acid and tolerate these better. ! Most animals make lactic acid, but the acid hurts; goldfish make EtOH and excrete it. Synthesis of ATP Aerobic conditions: electron transport chain ! Electron carriers (4 protein complexes) positioned close together in the membranes of the cristae; FAD, heme are associated with proteins (enzymes) that facilitate transfer of electrons; Q floats in lipid bilayer. ! Carriers have increasing affinity for electrons; thus, electrons move from carrier to carrier in a specific order. Electron transport chain: electrons move from carrier to carrier in a specific order FAD FAD Succinic acid Fumaric acid Synthesis of ATP Aerobic conditions: electron transport chain ! Electron carriers (4 protein complexes) positioned close together in the membranes of the cristae; FAD, heme are associated with proteins (enzymes) that facilitate transfer of electrons; Q floats in lipid bilayer. ! Carriers have increasing affinity for electrons; thus, electrons move from carrier to carrier in a specific order. ! Carriers are positioned in cristae so that H+ moves from inside to outside of membrane as electrons move from NADH to O2. ! H+ moves back to the inside through an enzyme --ATP synthetase--that forms ATP + H2O from ADP + Pi. Electron transport chain: H+ moves from inside to outside of membrane FAD FAD Succinic acid Fumaric acid ATP synthetase: H+ moves back to the inside through an enzyme that forms ATP + H2O from ADP + Pi. ATP synthetase: Adding ATP to the enzyme pumps H+ through the membrane (running backwards relative to ATP synthesis). It also makes the center protein rotate. The ATP synthetase is a rotary pump! (Running forward, it is a turbine.) Calculating the ATP yield of respiration of one glucose molecule (Fig. 9.13) Glycolysis +2 NADH +2 ATP Pyruvate oxidation +2 NADH Citric acid cycle +2 ATP (GTP) +6 NADH +2 FADH2 Electron transport chain -2 NADH +4 ATP -8 NADH +24 ATP -2 FADH2 +4 ATP +36 ATP Rate control: Should respiration run at the same rate whatever the demand for energy? Homeostasis: rate of respiration (fermentation) is controlled by level of ATP Allosteric enzyme: Phophofructokinase is inhibited by ATP and Citrate (see Fig. 9.16) Summary: how free energy flows through the cell ! Cells get free energy in the form of glucose (or other organic molecules) ! Oxidation of glucose releases free energy; much is saved as reduced NADH and FADH2 (and a little ATP) are formed in coupled reactions; the rest lost as heat ! Oxidation of NADH and FADH2 releases free + energy; much is saved as electrochemical (H ) gradient; the rest lost as heat + ! Reversal of electrochemical gradient (H transport) releases free energy; much is saved as ATP; the rest lost as heat ! Hydrolysis of ATP releases free energy; some is saved (in energy of position, new chemical gradients from transport of compounds across membranes, synthesis of polymers, etc.); the rest lost as heat. How did primeval organisms respire? Methanogens may have been early life forms Methanogenesis 4H2 + CO2 --> CH4 + 2 H2O ∆Go = -131 kcal/mol ∆G depends also on concentrations. This reaction works if H2 and CO2 concentrations are high. H+ e- H2 ATP CO2 CH4 H2O H+