Aerobic Respiration
Aerobic respiration is the complete oxidation of organic molecules (mainly glucose) in the presence of oxygen, resulting in the release of carbon dioxide (CO₂), water (H₂O), and a large amount of energy. This energy is conserved in the form of ATP.
In eukaryotic cells, aerobic respiration occurs in the mitochondria, which is why mitochondria are called the powerhouses of the cell.
The Link Reaction (Gateway Step)
The Link Reaction connects glycolysis (cytoplasm) with the Krebs cycle (mitochondria) and occurs in the mitochondrial matrix.
Nature of Reaction: Oxidative decarboxylation
What happens?
- Pyruvic acid (3C) loses one molecule of CO₂ (decarboxylation)
- Remaining 2C fragment is oxidised, reducing NAD⁺ → NADH + H⁺
- The 2C fragment binds with Coenzyme A (CoA) to form Acetyl-CoA
Enzyme: Pyruvate dehydrogenase complex
Equation (per pyruvate):
Pyruvic acid + CoA + NAD⁺ → Acetyl-CoA + CO₂ + NADH + H⁺
Per glucose (2 pyruvates):
- 2 Acetyl-CoA
- 2 CO₂
- 2 NADH + 2H⁺
Krebs Cycle (TCA / Citric Acid Cycle)
The Krebs cycle is also called:
- Tricarboxylic Acid (TCA) cycle
- Citric Acid cycle (because citric acid is the first stable product)
Location: Mitochondrial matrix
(Exception: Succinate dehydrogenase is embedded in the inner mitochondrial membrane)
Steps of the Krebs Cycle
1. Condensation:
- Acetyl-CoA (2C) combines with oxaloacetic acid (OAA, 4C)
- Forms citric acid (6C)
- Enzyme: Citrate synthase
2. Oxidation & Decarboxylation:
- Citric acid is progressively oxidised
- CO₂ is released at two steps
- NAD⁺ and FAD are reduced to NADH and FADH₂
3. Regeneration:
- OAA (4C) is regenerated
- Cycle is ready to accept the next acetyl-CoA
Products of Krebs Cycle
Per ONE turn (per acetyl-CoA):
- 2 CO₂
- 3 NADH + H⁺
- 1 FADH₂
- 1 ATP (or GTP) → substrate-level phosphorylation
Per glucose (2 turns):
- 4 CO₂
- 6 NADH + H⁺
- 2 FADH₂
- 2 ATP/GTP
Link Reaction + Krebs Cycle (per glucose)
- CO₂: 6 molecules
- NADH: 8 molecules
- FADH₂: 2 molecules
- ATP/GTP: 2 molecules
These reduced coenzymes carry high-energy electrons to the Electron Transport System (ETS).
Significance of the Krebs Cycle
- Major source of reducing power (NADH, FADH₂)
- Central hub connecting carbohydrate, fat, and protein metabolism
- Provides biosynthetic intermediates (amino acids, fatty acids, etc.)
- Essential for aerobic respiration
Important Highlights
- Maximum CO₂ release occurs in the Krebs cycle
- ATP here is formed by substrate-level phosphorylation
- Krebs cycle functions only when oxygen is available (indirectly)
Memory Capsules – Krebs Cycle Quick Recall
Location: Mitochondrial matrix
Entry Molecule: Acetyl-CoA (2C)
First Product: Citric acid (6C)
Per Acetyl-CoA:
- 3 NADH
- 1 FADH₂
- 1 ATP/GTP
- 2 CO₂
Per Glucose (2 turns):
- 6 NADH
- 2 FADH₂
- 2 ATP
- 4 CO₂
Exam Traps:
- Krebs cycle ≠ ETS
- Oxygen not directly used, but required to regenerate NAD⁺/FAD
- Succinate dehydrogenase is membrane-bound
One-Line Logic: Krebs cycle = CO₂ release + electron harvesting
💡 Questions and Answers
Q1. Why is the Krebs cycle called the citric acid cycle?
A1: Because the first stable compound formed after acetyl-CoA enters the cycle is citric acid, a 6-carbon molecule.
Q2. Why does the Krebs cycle operate only under aerobic conditions?
A2: Because NADH and FADH₂ produced in the Krebs cycle must be oxidised in the Electron Transport System, which requires oxygen as the final electron acceptor.
Q3. Where does substrate-level phosphorylation occur in the Krebs cycle?
A3: It occurs during the conversion of succinyl-CoA to succinic acid, producing GTP (or ATP) directly.
Q4. What is the importance of NADH and FADH₂ produced in the Krebs cycle?
A4: They carry high-energy electrons to the ETS, where maximum ATP is generated during oxidative phosphorylation.
Q5. How many CO₂ molecules are released from one glucose molecule during the Krebs cycle and link reaction combined?
A5: A total of 6 CO₂ molecules are released:
- 2 from the link reaction
- 4 from the Krebs cycle