Chapter 8.2 Second Stage of Aerobic Respiration AP Biology Fall 2010

Acetyl-CoA Formation Each pyruvate enters mitochondria One carbon is removed and attaches to oxygen ▫Forming carbon dioxide The two-carbon fragment remaining joins coenzyme A to form Acetyl-CoA ▫Three-carbon Two NAD + are reduced

Acetyl-CoA Formation Occurs in mitochondria 2 Pyruvate  2 Acetyl CoA

Acetyl-CoA Formation

The Krebs Cycle Each three-carbon Acetyl-CoA molecule enters the Krebs cycle separately Joins Oxaloacetate ▫Transfers two-carbon acetyl group to four-carbon oxaloacetate ▫Oxaloacetate already present from previous “turn” of the cycle

The Krebs Cycle H+ and e- are transferred to NAD + and FAD to become NADH and FADH 2 respectivly Important potential energy for the cell comes from the eight NADH and two FADH 2 coenzyme molecules produced during the Krebs cycle

The Krebs Cycle Two molecules of ATP are produced by substrate-level phosphorylation Most of the molecules are recycled to conserve oxaloacetate for continuous processing of acetyl- CoA Carbon dioxide is released as a by-product

The Krebs Cycle 1.One carbon atom is stripped from each pyruvate and is released as CO2 ▫The remaining fragment binds with coenzyme A, forming Acetyl-CoA ▫NAD + picks up hydrogen and electrons forming NADH 2.In the first step of the Krebs cycle, acetyl-CoA transfers two carbons to oxaloacetate, forming citrate

The Krebs Cycle 3.Rearrangement of intermediates citrate converted to isocitrate 4.In rearrangement of intermediates, another carbon atom is released as CO2, and NADH forms as NAD + picks up hydrogen and electrons 5.Another carbon atom is released as CO2. Another NADH forms. ▫The three carbon atoms that entered the second- stage reactions in each pyruvate have now been released

The Krebs Cycle 5.A phosphate group is attached to ADP. At this point, one ATP has formed by substrate-level phosphorylation 6.FADH2 forms as the coenzyme FAD picks up electrons and hydrogen 7.Fumerate reacts to form malate 8.The final steps regenerate oxaloacetate. NAD + picks up hydrogen and electrons, forming NADH

The Krebs Cycle 9.Oxaloacetate has been regenerated and can begin the cycle again Three NADH, one FADH2, and one ATP have formed 2 pyruvate are produced in glycolysis  2 cycles therefore: ▫6 NADH and 2 FADH2 and 2 ATP

Review 1.T/FThe Krebs cycle begins with the molecule pyruvate 2.T/FThe Krebs cycle produces 2 ATP, 6 NADH, 4 FADH2 3.T/FThe Krebs cycle is an anaerobic process

Answers 1.FalseBegins with Acetyl-CoA 2.True 3.FalseAerobic process