5.2.2 The Stages of Aerobic Respiration

Key Stages of Aerobic Respiration

1. Glycolysis:

• Occurs in the cytoplasm.
• Glucose (6C) is split into 2 molecules of pyruvate (3C).
• Produces 2 ATP (net gain) and 2 NADH.
• No oxygen is required for this stage.

2. Link Reaction:

• Occurs in the mitochondrial matrix.
• Pyruvate is decarboxylated ($CO₂$ is removed) and oxidised to form acetyl coenzyme A (acetyl-CoA).
• NADH is produced from the reduction of NAD.

3. Krebs Cycle:

• Takes place in the mitochondrial matrix.
• Acetyl-CoA combines with a 4-carbon molecule to form a 6-carbon compound which is then broken down in a series of steps.
• Produces:
• 2 $CO₂$ per cycle.
• 1 ATP (via substrate-level phosphorylation) per cycle.
• $:highlight[3]$ $NADH$ and $:highlight[1]$ $FADH₂$ per cycle.

4. Oxidative Phosphorylation:

• Occurs at the inner mitochondrial membrane (cristae).
• NADH and FADH₂ donate electrons to the electron transport chain (ETC).
• Electrons pass along the ETC, releasing energy used to pump protons into the intermembrane space, creating a proton gradient.
• Protons flow back into the matrix through ATP synthase, driving the production of ATP from ADP and Pi.
• Oxygen acts as the final electron acceptor, combining with electrons and protons to form water.

Products of Aerobic Respiration

• ATP: Up to 38 molecules per molecule of glucose (theoretical yield).
• Water: Produced during oxidative phosphorylation.
• Carbon Dioxide: Released during the link reaction and Krebs cycle.

Summary Table

Stage	Location	Key Outputs
Glycolysis	Cytoplasm	2 ATP, 2 NADH, 2 Pyruvate
Link Reaction	Mitochondrial Matrix	2 NADH, 2 Acetyl-CoA, 2 CO₂
Krebs Cycle	Mitochondrial Matrix	2 ATP, 6 NADH, 2 FADH₂, 4 CO₂
Oxidative Phosphorylation	Inner Mitochondrial Membrane	34 ATP, H₂O