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6.8-6.10 Citric acid cycle and Oxidative phosphorylation 2014-2015 Student 6.8 Pyruvate is oxidized prior to the citric acid cycle • This process will not proceed without oxygen • The pyruvate formed in glycolysis is transported from the cytoplasm into a mitochondrion where – the citric acid cycle and – oxidative phosphorylation will occur. © 2012 Pearson Education, Inc. 6.8 Pyruvate is oxidized prior to the citric acid cycle • Two molecules of pyruvate are produced for each molecule of glucose that enters glycolysis. • Pyruvate does not enter the citric acid cycle, but undergoes some chemical grooming in which – a carboxyl group is removed and given off as CO2, – the two-carbon compound remaining is oxidized while a molecule of NAD+ is reduced to NADH, – coenzyme A joins with the two-carbon group to form acetyl coenzyme A, abbreviated as acetyl CoA, and – acetyl CoA enters the citric acid cycle. © 2012 Pearson Education, Inc. Figure 6.8 NAD NADH H 2 CoA Pyruvate Acetyl coenzyme A 1 CO2 3 Coenzyme A 6.9 The citric acid cycle • The citric acid cycle – is also called the Krebs cycle (after the GermanBritish researcher Hans Krebs, who worked out much of this pathway in the 1930s), – completes the oxidation of organic molecules, and – generates many NADH and FADH2 molecules. © 2012 Pearson Education, Inc. Figure 6.9A Acetyl CoA CoA CoA 2 CO2 Citric Acid Cycle 3 NAD FADH2 3 NADH FAD 3 H ATP ADP P 6.9 The citric acid cycle • During the citric acid cycle – the two-carbon group of acetyl CoA is added to a four-carbon compound, forming citrate, – citrate is degraded back to the four-carbon compound, – two CO2 are released, and – 1 ATP, 3 NADH, and 1 FADH2 are produced. © 2012 Pearson Education, Inc. 6.9 The citric acid cycle • Remember that the citric acid cycle processes two molecules of acetyl CoA for each initial glucose. • Thus, after two turns of the citric acid cycle, the overall yield per glucose molecule is – 2 ATP, (useable by the cell) – 6 NADH, and – 2 FADH2. • To the electron transport chain © 2012 Pearson Education, Inc. NOTE • Do NOT get overwhelmed by the names of the intermediate compound produced by the citric acid cycle you will NOT be responsible for knowing their names. • You may notice: – Each time a carbon(represented by the gray circles) is lost from a carbon compounds a CO2 is produced. – NADNADH2 and FADFADH2 as the molecules in the citric acid cycle are rearrange and energy is released as high energy electrons. Figure 6.9B_s3 Acetyl CoA CoA CoA 2 carbons enter cycle Oxaloacetate 1 Citrate NADH H NAD 5 NAD NADH 2 H Citric Acid Cycle CO2 leaves cycle Malate FADH2 Alpha-ketoglutarate 4 3 FAD CO2 leaves cycle NAD Succinate ADP Step 1 Acetyl CoA stokes the furnace. P Steps 2 – 3 ATP NADH, ATP, and CO2 are generated during redox reactions. NADH H Steps 4 – 5 Further redox reactions generate FADH2 and more NADH. 6.10 Oxidative phosphorylation • At this point in Cellular Respiration only 4 ATP molecules have been produced – 2 in glycolysis – 2 in the citric acid cycle • Oxidative phosphorylation (most ATP is produced here) – The energy in NADH2 and FADH2 is converted into ATP – involves electron transport and chemiosmosis and – requires an adequate supply of oxygen. © 2012 Pearson Education, Inc. 6.10 Oxidative phosphorylation (continued) • Electrons are released from NADH and FADH2 and travel down the electron transport chain to O2. – ETC is a series of proteins embedded in the inner mitochondrial membrane • Oxygen “catches” the electrons and picks up H+ to form water. © 2012 Pearson Education, Inc. Oxidative phosphorylation (continued) • Energy released by these redox reactions is used to pump H+ from the mitochondrial matrix into the intermembrane space. • In chemiosmosis, the H+ diffuses back across the inner membrane through ATP synthase complexes, driving the synthesis of ATP. Figure 6.10 H Intermembrane space H H H H Mobile electron carriers Protein complex of electron carriers H ATP synthase IV I II FADH2 Electron flow NADH Mitochondrial matrix H H III Inner mitochondrial membrane H NAD FAD 2 H 1 2 O2 H2O H ADP P ATP H Electron Transport Chain Oxidative Phosphorylation Chemiosmosis 6.12 Review: Each molecule of glucose yields many molecules of ATP • Recall that the energy payoff of cellular respiration involves 1. glycolysis, 2. alteration of pyruvate, 3. the citric acid cycle, and 4. oxidative phosphorylation. © 2012 Pearson Education, Inc. 6.12 Review: Each molecule of glucose yields many molecules of ATP • The total yield is about 32 ATP molecules per glucose molecule. • This is about 34% of the potential energy of a glucose molecule. • In addition, water and CO2 are produced. © 2012 Pearson Education, Inc. Figure 6.12 CYTOPLASM Electron shuttles across membrane 2 NADH Mitochondrion 2 NADH or 2 FADH2 6 NADH 2 NADH Glycolysis 2 Pyruvate Glucose Pyruvate Oxidation 2 Acetyl CoA Citric Acid Cycle 2 FADH2 Oxidative Phosphorylation (electron transport and chemiosmosis) Maximum per glucose: 2 ATP by substrate-level phosphorylation 2 ATP by substrate-level phosphorylation about 28 ATP by oxidative phosphorylation About 32 ATP