Four stages, two compartments, and one gradient of protons doing almost all the work. Follow a single glucose molecule all the way to water.
Glycolysis runs in the cytoplasm and needs no oxygen and no mitochondrion. Everything after it happens inside the mitochondrion: which is why cells without mitochondria stop at pyruvate.
Glycolysis and the Krebs cycle make surprisingly little ATP directly. What they really produce is reduced NAD and reduced FAD: loaded carriers taking high-energy electrons to the inner membrane.
Oxygen is used only at the very last step, as the terminal electron acceptor. Remove it and the whole chain backs up: including the Krebs cycle, which never touches oxygen itself.
Every number below is per molecule of glucose. Tap a stage or a product to isolate it.
Follow the middle column for the carbon and the sides for everything that comes off it. Notice how little ATP is made until the very last box: and how much of the pathway is really about loading carriers.
Deliver carriers to the membrane and watch protons pile up in the intermembrane space. Every four that come back through ATP synthase makes one ATP: which is where the awkward 2.5 and 1.5 figures come from. Then switch the oxygen off.
The electrons never make ATP directly. Their energy is spent pumping protons out of the matrix, and the gradient that builds up is what ATP synthase actually uses. Deliver a carrier to begin.
Per molecule of glucose, fully oxidised.
Each NADH pumps enough protons for about 2.5 ATP and each FADH₂ about 1.5: not whole numbers, because protons are the currency, not ATP. Cells also spend some of the gradient importing pyruvate and shuttling electrons in, so the honest answer is a range. The older figure of 38 assumed those costs were zero.
The 4 ATP from glycolysis and Krebs are made by substrate-level phosphorylation: a phosphate handed straight from one molecule to ADP. The other 26–28 come from oxidative phosphorylation, driven by the proton gradient. Only the second kind needs oxygen.
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Almost nothing in respiration makes ATP directly. The pathway's real job is to strip hydrogen from glucose, carry it to a membrane, and spend it building a proton gradient: and oxygen's only role is to keep the far end of that chain clear.
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