State where each of the four stages in aerobic respiration occurs in eukaryotic cells: glycolysis in the cytoplasm, link reaction in the mitochondrial matrix, Krebs cycle in the mitochondrial matrix, oxidative phosphorylation on the inner membrane of

Respiration – Aerobic Pathway

Objective: State where each of the four stages in aerobic respiration occurs in eukaryotic cells.

Overview of the Four Stages

1. Glycolysis
2. Link Reaction (Pyruvate Oxidation)
3. Krebs Cycle (Citric Acid Cycle)
4. Oxidative Phosphorylation (Electron Transport Chain & Chemiosmosis)

Location of Each Stage

Key Details of Each Stage

1. Glycolysis

Occurs in the cytoplasm where glucose (\$\text{C}6\text{H}{12}\text{O}_6\$) is split into two molecules of pyruvate. The pathway uses 2 ATP and produces 4 ATP (net gain 2 ATP) and 2 NADH.

2. Link Reaction (Pyruvate Oxidation)

Each pyruvate enters the mitochondrial matrix and is decarboxylated to acetyl‑CoA, releasing one molecule of CO₂ and reducing NAD⁺ to NADH.

\$\text{Pyruvate} + \text{CoA} + \text{NAD}^+ \rightarrow \text{Acetyl‑CoA} + \text{CO}_2 + \text{NADH}\$

3. Krebs Cycle

The acetyl‑CoA combines with oxaloacetate to form citrate and undergoes a series of reactions that regenerate oxaloacetate. Each turn yields 3 NADH, 1 FADH₂, 1 GTP (ATP) and 2 CO₂. Since two acetyl‑CoA molecules are produced per glucose, the totals are doubled.

4. Oxidative Phosphorylation

Electrons from NADH and FADH₂ travel through protein complexes embedded in the inner mitochondrial membrane (Complex I–IV). The energy released pumps protons into the intermembrane space, creating an electrochemical gradient. ATP synthase uses this gradient to synthesise ATP (chemiosmosis).

Overall equation for aerobic respiration:

\$\text{C}6\text{H}{12}\text{O}6 + 6\ \text{O}2 \rightarrow 6\ \text{CO}2 + 6\ \text{H}2\text{O} + \approx 38\ \text{ATP}\$