Section A — Glycolysis

Glycolysis (Embden-Meyerhof pathway) is the universal first stage of cellular respiration, occurring in the cytoplasm of all living cells. It does not require oxygen and operates under both aerobic and anaerobic conditions. The pathway converts one glucose molecule (6C) into two pyruvate molecules (3C) through 10 enzymatic steps. The net yield is 2 ATP and 2 NADH per glucose. The investment phase consumes 2 ATP (priming reactions catalysed by hexokinase and phosphofructokinase), while the payoff phase generates 4 ATP (via substrate-level phosphorylation by phosphoglycerate kinase and pyruvate kinase). The rate-limiting step is catalysed by phosphofructokinase (PFK), which is allosterically inhibited by high ATP and citrate (indicating cellular energy sufficiency) and activated by AMP/ADP (energy deficit).

Section B — Anaerobic Respiration (Fermentation)

When oxygen is absent, pyruvate does not enter the mitochondria. Instead, it undergoes fermentation — an incomplete oxidation yielding only 2 ATP. Two types exist: (1) Alcoholic fermentation (in yeast — Saccharomyces): pyruvate → acetaldehyde (pyruvate decarboxylase) → ethanol + $CO_{2}$ (alcohol dehydrogenase); (2) Lactic acid fermentation (in skeletal muscles and Lactobacillus): pyruvate → lactate (lactate dehydrogenase). The critical metabolic purpose is N$AD^{+}$ regeneration — NADH is reoxidised to N$AD^{+}$, allowing glycolysis to continue. Without this regeneration, N$AD^{+}$ would be depleted and glycolysis would halt.

Section C — Oxidative Decarboxylation (Link Reaction)

In aerobic respiration, pyruvate enters the mitochondrial matrix and is converted to acetyl CoA (2C) by the pyruvate dehydrogenase complex. This complex requires three cofactors: TPP (thiamine pyrophosphate — from vitamin $B_{1}$), N$AD^{+}$ (electron acceptor → NADH), and CoA. Per pyruvate: 1 $CO_{2}$ released + 1 NADH produced + 1 acetyl CoA formed. Per glucose (two pyruvates): 2 $CO_{2}$ + 2 NADH + 2 acetyl CoA.

Section D — TCA Cycle (Krebs Cycle)

The TCA cycle operates in the mitochondrial matrix. Acetyl CoA (2C) condenses with OAA (4C) to form citrate (6C). The cycle proceeds through 8 steps, regenerating OAA at the end. Per turn: 3 NADH + 1 $FADH_{2}$ + 1 GTP + 2 $CO_{2}$. Per glucose (2 turns): 6 NADH + 2 $FADH_{2}$ + 2 GTP + 4 $CO_{2}$. The TCA cycle is amphibolic — intermediates supply carbon skeletons for amino acid synthesis (OAA → aspartate; α-KG → glutamate) and fatty acid synthesis (acetyl CoA).

Section E — Electron Transport System and Chemiosmosis

The ETS is located on the inner mitochondrial membrane. Electrons from NADH enter at Complex I; from $FADH_{2}$ at Complex II. They flow via CoQ → Complex III → cytochrome c → Complex IV → $O_{2}$ (→ $H_{2}O$). As electrons flow, protons are pumped (by Complexes I, III, IV) into the intermembrane space. The resulting proton gradient drives $F_{0}$-$F_{1}$ ATP synthase (chemiosmosis — Peter Mitchell, 1961). NADH → 3 ATP; $FADH_{2}$ → 2 ATP. Total ETS yield = 34 ATP per glucose. Grand total = 38 ATP per glucose (or 36 with the glycerol-3-phosphate shuttle).

Section F — Respiratory Quotient (RQ)

RQ = $CO_{2}$ evolved / $O_{2}$ consumed. Used to identify the respiratory substrate: carbohydrates (1.0), fats (~0.7), proteins (~0.8), organic acids (~1.33 for malic acid), CAM plants at night (RQ → ∞). Fats give RQ < 1 because their highly reduced long-chain structure requires more $O_{2}$ relative to $CO_{2}$ produced. Organic acids give RQ > 1 because they are already partially oxidised.