Cellular respiration

MDCAT Biology · Biology · Bioenergetics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Respiration releases the energy of glucose in small steps and stores it as ATP. In aerobic respiration there are four stages: glycolysis in the cytoplasm, then the link reaction and the Krebs cycle in the mitochondrial matrix, then the electron transport chain on the inner mitochondrial membrane.

The first three stages make only a little ATP directly. Their main job is to load hydrogen onto NAD and FAD. The electron transport chain then passes these electrons to oxygen, which forms water, and the energy released drives oxidative phosphorylation, where most ATP is made.

Without oxygen only glycolysis runs, so the yield is 2 ATP per glucose.

Rules to remember

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP)
Glycolysis: glucose (6C) → 2 pyruvic acid (3C); 2 ATP used, 4 made, net 2 ATP; 2 NADH; no CO₂; no oxygen needed
Link reaction (per pyruvic acid): pyruvic acid → acetyl CoA (2C) + CO₂ + NADH
Krebs cycle (per turn): 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP; oxaloacetic acid (4C) is regenerated; two turns per glucose
Electron transport chain: each NADH gives 3 ATP, each FADH₂ gives 2 ATP; H⁺ is pumped into the intermembrane space and flows back through ATP synthase; oxygen is the final electron acceptor
Per glucose: 10 NADH (30 ATP) + 2 FADH₂ (4 ATP) + 4 ATP made directly = 38 ATP; 36 ATP in most eukaryotic cells, because 2 ATP are spent moving the NADH of glycolysis into the mitochondrion
Anaerobic: yeast, pyruvic acid → acetaldehyde + CO₂ → ethanol; muscle, pyruvic acid → lactic acid. Fats enter as acetyl CoA; amino acids are deaminated first

Common mistake

Forgetting that every stage after glycolysis happens twice per glucose, because one glucose gives two pyruvic acid molecules. Double the link reaction and Krebs cycle numbers.

Worked example

How many ATP are obtained from one acetyl CoA passing through one turn of the Krebs cycle, including the electron transport chain? (NADH = 3 ATP, FADH₂ = 2 ATP.)

One turn gives 3 NADH: 3 × 3 = 9 ATP.
One turn gives 1 FADH₂: 1 × 2 = 2 ATP.
One ATP is made directly in the cycle.
Total = 9 + 2 + 1 = 12.

Answer: 12 ATP per acetyl CoA (24 ATP for the two acetyl CoA from one glucose).

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