Cellular respiration is the process by which living organisms oxidise organic molecules in a controlled, stepwise manner to release usable energy in the form of ATP. The overall equation for aerobic respiration is $C_{6}H_{12}O_{6}$ + $6O_{2}$ → $6CO_{2}$ + $6H_{2}O$ + Energy (38 ATP). Unlike combustion, which releases energy all at once as heat, respiration captures approximately 40% of the available energy as ATP through four integrated biochemical stages.

Stage 1 — Glycolysis (Embden-Meyerhof Pathway): Glycolysis occurs in the cytoplasm and is universal — it operates in all living cells under both aerobic and anaerobic conditions. It does not require oxygen at any step. In 10 enzymatic steps, one 6-carbon glucose molecule is broken down into two 3-carbon pyruvate molecules. The pathway has an investment phase (2 ATP consumed to prime glucose) and a payoff phase (4 ATP produced), yielding a net gain of 2 ATP. Additionally, 2 NADH are produced. Three key regulatory enzymes govern glycolysis: hexokinase (step 1), phosphofructokinase (PFK, step 3 — the rate-limiting committed step, allosterically inhibited by high ATP and citrate), and pyruvate kinase (step 10 — substrate-level phosphorylation).

Stage 2 — Oxidative Decarboxylation (Link Reaction): Under aerobic conditions, pyruvate enters the mitochondrial matrix where the pyruvate dehydrogenase complex (requiring TPP, N$AD^{+}$, and CoA as cofactors) converts each pyruvate (3C) into acetyl CoA (2C), releasing 1 $CO_{2}$ and generating 1 NADH per pyruvate. For one glucose (two pyruvates), this yields 2 $CO_{2}$ and 2 NADH.

Anaerobic Branch — Fermentation: When oxygen is absent, pyruvate is not transported into mitochondria. Instead, it undergoes fermentation in the cytoplasm. In yeast (Saccharomyces), pyruvate decarboxylase (using TPP) converts pyruvate → acetaldehyde + $CO_{2}$, then alcohol dehydrogenase reduces acetaldehyde to ethanol using NADH (regenerating N$AD^{+}$). In skeletal muscles and Lactobacillus, lactate dehydrogenase directly converts pyruvate to lactate using NADH. Both pathways yield only 2 ATP per glucose — representing incomplete, inefficient oxidation. The primary metabolic purpose of fermentation is to regenerate N$AD^{+}$ so that glycolysis can continue producing ATP.

Stage 3 — TCA Cycle (Krebs/Citric Acid Cycle): Acetyl CoA (2C) enters the mitochondrial matrix and condenses with oxaloacetate (OAA, 4C) via citrate synthase to form citrate (6C), the first intermediate of the cycle. The cycle proceeds through eight sequential enzymatic steps — citrate → isocitrate → alpha-ketoglutarate → succinyl CoA → succinate → fumarate → malate → OAA — regenerating OAA for the next turn. Per turn, the TCA cycle yields 3 NADH, 1 $FADH_{2}$, and 1 GTP (at the succinyl CoA → succinate step, substrate-level phosphorylation). Since two acetyl CoA molecules enter per glucose, the total TCA yield is 6 NADH, 2 $FADH_{2}$, and 2 GTP, plus 4 $CO_{2}$. The TCA cycle is described as amphibolic — its intermediates serve both catabolic (energy) and anabolic (biosynthetic) functions: OAA is transaminated to aspartate, alpha-ketoglutarate to glutamate, and acetyl CoA provides the building block for fatty acid synthesis.

Stage 4 — Electron Transport System (ETS) and Oxidative Phosphorylation: The ETS is embedded in the inner mitochondrial membrane (cristae). NADH donates electrons at Complex I (NADH dehydrogenase), while $FADH_{2}$ enters at Complex II (succinate dehydrogenase). Electrons flow via ubiquinone (CoQ, lipid-soluble mobile carrier) to Complex III (cytochrome bc1), then via cytochrome c (water-soluble mobile carrier) to Complex IV (cytochrome c oxidase), where they are passed to molecular oxygen ($O_{2}$) to form water. As electrons traverse Complexes I, III, and IV, protons are actively pumped from the matrix into the intermembrane space, creating an electrochemical proton gradient. This gradient drives ATP synthesis via chemiosmosis — $H^{+}$ ions flow back into the matrix through the $F_{0}$-$F_{1}$ ATP synthase (rotary motor), synthesising ATP from ADP + Pi. This mechanism was proposed by Peter Mitchell (Nobel Prize 1978). NADH yields approximately 3 ATP (three proton-pumping complexes) while $FADH_{2}$ yields approximately 2 ATP (bypasses Complex I, so only two pump complexes).

ATP Balance Sheet Summary:

Respiratory Quotient (RQ): RQ = $CO_{2}$ evolved / $O_{2}$ consumed. Carbohydrates: RQ = 1.0; fats: ~0.7 (more $O_{2}$ needed for long hydrocarbon chains); proteins: ~0.8; organic acids (malic acid): ~1.33 (already partially oxidised, less $O_{2}$ needed relative to $CO_{2}$); CAM plants at night: RQ → ∞ ($CO_{2}$ fixed internally, no external gas exchange). Germinating fatty seeds start at <1 and rise toward 1 as fat stores are depleted and carbohydrates become available.