Citric Acid Cycle Overview & Steps 🔬

Introduction to Cellular Respiration

Glycolysis

• First step of cell respiration.
• Transformation of glucose into two pyruvate molecules.

Aerobic Cellular Respiration

• Occurs if oxygen is present.
• Pyruvate molecules transported to mitochondria matrix.
• Transformed into acetyl coenzyme A via pyruvate decarboxylation.

Citric Acid Cycle (Krebs Cycle, TCA Cycle)

• Series of 8 steps within the mitochondrial matrix.
• Transfers energy from acetyl coenzyme A into electron carriers (NADH, FADH2).

Purpose

• Generate high-energy electron carriers for the Electron Transport Chain (ETC).
• Does not directly produce ATP.
• Each glucose results in two cycles.

Step-by-Step Breakdown of the Citric Acid Cycle

Stage One: Preparation

• Step 1: Acetyl CoA (2C) + Oxaloacetate (4C) → Citrate (6C).
  • Releases coenzyme A and forms 6-carbon citrate.
• Step 2: Citrate is reorganized into isocitrate.
  • Preparation for decarboxylation.
  • Both steps are grouped as Stage One, preparing molecule for reactions.

Stage Two: Decarboxylation and GTP Formation

• Step 3: Isocitrate Decarboxylation.
  • Releases CO₂, NAD+ → NADH + H⁺, forms α-ketoglutarate (5C).
• Step 4: α-Ketoglutarate Decarboxylation.
  • Releases CO₂, NAD+ → NADH + H⁺, forms Succinyl-CoA (4C).
• Step 5: Formation of GTP.
  • Phosphate from Succinyl-CoA transferred to GDP → GTP (→ ATP), forming Succinate.

Stage Three: Regeneration of Oxaloacetate

• Step 6: Succinate → Fumarate.
  • FAD → FADH₂ (1 electron removed).
• Step 7: Fumarate → Malate
  • Addition of water.
• Step 8: Malate → Oxaloacetate
  • NAD+ → NADH + H⁺.
• Regeneration of oxaloacetate for cycle repetition.

Summary of Products (Per One Glucose Molecule)

• 4 CO₂ (2 per cycle × 2 cycles)
• 6 NADH (3 per cycle × 2 cycles)
• 2 FADH₂ (1 per cycle × 2 cycles)
• 2 GTP → 2 ATP (1 per cycle × 2 cycles)
• 4 H⁺ ions
• 2 Coenzyme A (regenerated)

Conclusion

• Citric Acid Cycle's main function: producing high-energy electron carriers (NADH, FADH₂) that fuel the Electron Transport Chain for ATP synthesis, to be discussed next lecture.