Think of glucose as a stick of dynamite. If you set it off all at once, you get a huge, uncontrolled release of heat — enough to denature every enzyme in the cell. Respiration is the cell's way of "defusing" that dynamite gradually, stage by stage, using enzymes to control exactly how much energy is released at each step, and capturing that energy in a useful, portable form: ATP.
The overall equation:
Aerobic Respiration
glucose + oxygen → carbon dioxide + water + energy
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 2870 kJ
That energy is used to phosphorylate ADP into ATP (adding a phosphate group). ATP is the "energy currency" that powers everything else in the cell — muscle contraction, active transport, protein synthesis, you name it.
The Four Stages — and WHERE They Happen
| Stage | What happens | Location |
|---|
| 1. Glycolysis | Phosphorylation and splitting of glucose | Cytoplasm |
| 2. Link reaction | Decarboxylation and dehydrogenation of pyruvate | Matrix of mitochondria |
| 3. Krebs cycle | Cyclical pathway of enzyme-controlled reactions | Matrix of mitochondria |
| 4. Oxidative phosphorylation | ATP production via oxidation of hydrogen atoms | Inner membrane of mitochondria |
Why does the rate matter?
The reaction catalysed by the slowest enzyme in the whole pathway determines the overall rate of respiration — this is called the "rate-limiting step." It's the same logic as a factory production line: you're only as fast as your slowest machine.
Meet the Coenzymes
Coenzymes are non-protein helper molecules that carry things between reactions. In respiration, you need to know three:
- NAD (nicotinamide adenine dinucleotide) — picks up hydrogen atoms, becoming "reduced NAD" (NADH). Think of NAD as a taxi that picks up hydrogen passengers and drops them off at the ETC.
- FAD (flavin adenine dinucleotide) — does the same job as NAD, but only in the Krebs cycle, becoming FADH₂.
- Coenzyme A — doesn't carry hydrogen; it carries acetate (a 2-carbon fragment) from the link reaction into the Krebs cycle.
Structure of a mitochondrion (know this cold):
┌─────────────────────────────────────┐
│ OUTER MEMBRANE (smooth, permeable) │
│ ┌─────────────────────────────┐ │
│ │ INTERMEMBRANE SPACE │ │ <- low pH, high [H+]
│ │ ┌───────────────────────┐ │ │
│ │ │ INNER MEMBRANE │ │ │ <- folded (cristae)
│ │ │ - site of ETC │ │ │ <- ATP synthase here
│ │ │ ┌─────────────────┐ │ │ │
│ │ │ │ MATRIX │ │ │ │ <- link reaction + Krebs
│ │ │ │ (ribosomes, │ │ │ │ cycle happen here
│ │ │ │ mtDNA, enzymes) │ │ │ │
│ │ │ └─────────────────┘ │ │ │
│ │ └───────────────────────┘ │ │
│ └─────────────────────────────┘ │
└─────────────────────────────────────┘