Cellular respiration

Glucose comes apart, and ATP captures the energy.

The idea

Glycolysis, in the cytoplasm, splits one six-carbon glucose into two three-carbon pyruvate molecules with a net gain of 2 ATP and 2 NADH — no oxygen required. When oxygen is available, pyruvate enters the mitochondrial matrix, loses CO₂ to become acetyl-CoA, and feeds the Krebs (citric acid) cycle, which per glucose releases 4 CO₂ and loads electrons onto 6 NADH and 2 FADH₂ while making 2 ATP directly. Those carriers deliver their electrons to the electron transport chain on the folded inner membrane (cristae); the chain pumps H⁺ into the intermembrane space, oxygen accepts the spent electrons to form water, and H⁺ flowing back through ATP synthase makes the bulk of the roughly 30–32 ATP per glucose (chemiosmosis). Without oxygen, cells fall back on fermentation — lactate in muscle, ethanol and CO₂ in yeast — which only regenerates the NAD⁺ that glycolysis needs, so the yield stays at 2 ATP.

Work through an example

Tally one glucose: glycolysis 2 ATP + 2 NADH; pyruvate to acetyl-CoA 2 NADH; Krebs cycle 2 ATP + 6 NADH + 2 FADH₂. In the electron transport chain each NADH is worth about 2.5 ATP and each FADH₂ about 1.5, so 10 × 2.5 + 2 × 1.5 = 28 ATP, plus the 4 made directly, gives 32 ATP (30 if the two cytoplasmic NADH enter the mitochondrion by the cheaper glycerol-phosphate shuttle). Glucose holds about 2,870 kJ/mol and each ATP stores about 30.5 kJ/mol, so 32 × 30.5 / 2,870 ≈ 34% of the energy is captured; the rest leaves as heat.

What to watch for

Respiration is not breathing. Breathing (ventilation) moves air, and gas exchange swaps O₂ for CO₂ in the lungs; cellular respiration is the chemical breakdown of glucose that happens in every living cell — including every plant cell, day and night. Older textbooks quote 36–38 ATP per glucose; that was a theoretical maximum that ignored the cost of moving ATP and pyruvate across mitochondrial membranes, and the modern measured figure is about 30–32.