Chapter 8.2Second Stage of Aerobic Respiration

AP BiologyFall 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 FADH2 respectivly

•Important potential energy for the cell comes from the eight NADH and two FADH2 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 Cycle1. 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 NADH2. In the first step of the Krebs cycle,

acetyl-CoA transfers two carbons to oxaloacetate, forming citrate

The Krebs Cycle3. 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 Cycle5. 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 Cycle9. 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

Review1. T/F The Krebs cycle begins with the

molecule pyruvate 2. T/F The Krebs cycle produces 2 ATP, 6

NADH, 4 FADH23. T/F The Krebs cycle is an anaerobic

process

Answers1. False Begins with Acetyl-CoA2. True3. False Aerobic process