Stage 2: Krebs Cycle (Citric Acid Cycle)

Location: Mitochondrial matrix

Before Krebs:

• Pyruvate → Acetyl CoA (by pyruvate dehydrogenase)
• Releases CO₂, makes NADH

Process:

• Acetyl CoA (2C) enters cycle
• Combines with oxaloacetate (4C) → citrate (6C)
• Series of redox reactions
• Regenerates oxaloacetate

Energy yield (per glucose = 2 turns):

• 2 ATP (or GTP)
• 6 NADH
• 2 FADH₂
• 4 CO₂ released

Stage 3: Oxidative Phosphorylation

Two parts:

Electron Transport Chain (ETC)

Location: Inner mitochondrial membrane (cristae)

Process:

• NADH and FADH₂ donate electrons
• Electrons pass through protein complexes
• Energy used to pump H⁺ into intermembrane space
• Creates electrochemical gradient (proton-motive force)

Protein complexes:

1. Complex I: NADH → Q
2. Complex II: FADH₂ → Q
3. Complex III: Q → Cytochrome c
4. Complex IV: Cytochrome c → O₂

Final electron acceptor: O₂ → H₂O

Chemiosmosis

Process:

• H⁺ gradient created by ETC
• H⁺ flows back through ATP synthase
• Flow drives ATP synthesis
• ~3 ATP per NADH
• ~2 ATP per FADH₂

Total ATP Yield

From one glucose:

• Glycolysis: 2 ATP + 2 NADH
• Pyruvate → Acetyl CoA: 2 NADH
• Krebs cycle: 2 ATP + 6 NADH + 2 FADH₂
• Oxidative phosphorylation: ~28 ATP

Total: ~32 ATP (varies by cell type)

Anaerobic Respiration (Fermentation)

When O₂ unavailable:

Lactic acid fermentation:

• Pyruvate → Lactate
• Regenerates NAD⁺ for glycolysis
• Occurs in muscles during intense exercise

Alcohol fermentation:

• Pyruvate → Ethanol + CO₂
• Regenerates NAD⁺
• Used by yeast

Energy yield: Only 2 ATP (from glycolysis)

Key Concepts

1. Glycolysis: glucose → 2 pyruvate (2 ATP, 2 NADH)
2. Krebs cycle: completes glucose oxidation (2 ATP, 8 NADH, 2 FADH₂)
3. ETC: electrons from NADH/FADH₂ pump H⁺
4. Chemiosmosis: H⁺ gradient drives ATP synthesis
5. O₂ is final electron acceptor in aerobic respiration
6. Fermentation: anaerobic, regenerates NAD⁺, only 2 ATP