Cellular respiration is the metabolic process by which cells convert glucose and oxygen into ATP, the universal energy currency. Complete aerobic respiration yields approximately 30–32 ATP per glucose molecule.
The overall reaction:
C6H12O6 + 6O2 → 6CO2 + 6H2O + ~30–32 ATP
Note: The theoretical maximum yield of 36–38 ATP assumes 100% coupling efficiency; the actual yield is lower (~30–32 ATP) due to proton leakage and the cost of transporting substrates into the mitochondria.
Location: Cytoplasm | Net yield: 2 ATP, 2 NADH, 2 pyruvate
Glycolysis consists of 10 enzymatic steps split into two phases:
| Phase | Steps | ATP Change | Purpose |
|---|---|---|---|
| Energy Investment | 1–5 | −2 ATP | Phosphorylate glucose |
| Energy Payoff | 6–10 | +4 ATP, 2 NADH | Extract energy |
Rate-limiting enzyme: Phosphofructokinase-1 (PFK-1), allosterically inhibited by ATP and activated by AMP/ADP.
Location: Mitochondrial matrix | Yield per glucose: 2 acetyl-CoA, 2 NADH, 2 CO₂
Pyruvate dehydrogenase complex (PDC) converts pyruvate to acetyl-CoA:
Pyruvate + CoA + NAD+ → Acetyl-CoA + NADH + CO2
Warning: Thiamine (Vitamin B1) is an essential cofactor for PDC. Thiamine deficiency (Wernicke encephalopathy) critically impairs energy metabolism in neurons.
Location: Mitochondrial matrix | Yield per turn: 3 NADH, 1 FADH2, 1 GTP, 2 CO₂
The cycle turns twice per glucose molecule (once per acetyl-CoA):
- Acetyl-CoA (2C) condenses with oxaloacetate (4C) → citrate (6C)
- Two sequential decarboxylations release 2 CO₂
- Electron carriers (NADH, FADH2) are reduced
- Oxaloacetate is regenerated
Tip: The mnemonic "Citrate Is Krebs' Starting Substrate For Making Oxaloacetate" maps to: Citrate, Isocitrate, alpha-Ketoglutarate, Succinyl-CoA, Succinate, Fumarate, Malate, Oxaloacetate.
Location: Inner mitochondrial membrane | Yield: ~26–28 ATP
The electron transport chain (ETC) has four complexes:
| Complex | Name | Electron Donor | Protons Pumped |
|---|---|---|---|
| I | NADH dehydrogenase | NADH → NAD+ | 4H+ |
| II | Succinate dehydrogenase | FADH2 → FAD | 0H+ |
| III | Cytochrome bc1 | CoQ → Cyt c | 4H+ |
| IV | Cytochrome c oxidase | Cyt c → O2 | 2H+ |
ATP synthase (Complex V) uses the proton gradient (chemiosmosis) to synthesize ATP.