Cellular Respiration and the Kreb's Cycle

Cellular Respiration: Overview of Steps

Step 2: Pyruvate Oxidation

• Pyruvate travels from the cytoplasm into the mitochondrial matrix via active transport.
• Three Main Steps of Pyruvate Oxidation:
  1. Decarboxylation: Removal of CO₂.
  2. REDOX Reaction: NAD⁺ is reduced to NADH, which enters the electron transport chain.
  3. Addition of Coenzyme A: Results in the formation of Acetyl-CoA, which is essential for the Kreb’s Cycle.
• Net Energy Yield: 2 NADH molecules per glucose (one from each pyruvate).
• Purpose: To prepare Acetyl-CoA for entry into the Kreb’s Cycle.

Step 3: Kreb’s Cycle (Citric Acid Cycle)

• An eight-step process, with each step catalyzed by a specific enzyme.
• It is cyclic because oxaloacetate, the product of the last step, is the reactant for the first step.
• The cycle rearranges atoms in acetate, resulting in energy capture in the forms of ATP, NADH, and FADH₂.
• All carbon atoms in the intermediates are oxidized to CO₂, which is released as waste.
• Generated NADH and FADH₂ are used in the next stage for ATP synthesis.

Kreb’s Cycle Steps

1. Condensation of Acetyl-CoA and Oxaloacetate:

   • Acetyl group from Acetyl-CoA (2-C) combines with oxaloacetate (4-C) to form citrate (6-C).

2. Isomerization:

   • Citrate (6-C) rearranges to isocitrate (6-C).

3. Decarboxylation and REDOX:

   • Isocitrate (6-C) oxidizes to α-ketoglutarate (5-C), releasing CO₂ and reducing NAD⁺ to NADH.

4. Decarboxylation, REDOX, and CoA Addition:

   • α-ketoglutarate (5-C) oxidized to succinyl-CoA (4-C), with removal of CO₂ and formation of NADH.

5. Condensation, Loss of CoA, and ATP Phosphorylation:

   • Succinyl-CoA (4-C) is converted to succinate (4-C) while forming ATP from ADP + Pi.

6. REDOX (FADH₂ Formation):

   • Succinate (4-C) is converted to fumarate (4-C), reducing FAD to FADH₂.

7. Hydration:

   • Fumarate (4-C) is hydrated to malate (4-C).

8. REDOX:

   • Malate (4-C) is converted back to oxaloacetate (4-C), reducing NAD⁺ to NADH.

Kreb’s Cycle Yield

• Total Energy Molecules (per glucose):
  • NADH = 6
  • FADH₂ = 2
  • ATP = 2
• Reactants and Products (per glucose):
  • 2 Acetyl-CoA → 4 CO₂
• Overall Purpose: To extract all available energy from the glucose molecule, facilitating cellular energy production through ATP.