Lesson 8 of 11 · 10 min
The respiratory balance sheet
NCERT §12.5
Kavya adds up every ATP her glucose has paid out. The total on paper is neat, and she soon learns why a living cell never quite reaches it.
The lesson in notes
In short
The net ATP from one glucose can be calculated, but only as a theoretical exercise resting on four assumptions.
Assumption one: the pathway runs as one orderly sequence, glycolysis then the TCA cycle then the ETS, each substrate forming the next.
Assumption two: the NADH made in glycolysis is moved into the mitochondria and undergoes oxidative phosphorylation.
Assumption three: no intermediate is drawn off to make another compound. Assumption four: only glucose is respired, with no other substrate entering at any intermediate stage.
None of these holds in a living cell: all the pathways run at once, substrates enter and leave as needed, ATP is used as needed, and enzyme rates are controlled in many ways.
On these assumptions the net gain is 38 ATP per glucose in aerobic respiration: 10 NADH × 3 = 30, 2 FADH₂ × 2 = 4, plus 2 net ATP from glycolysis and 2 from the TCA cycle.
Fermentation, by contrast, nets only 2 ATP per glucose.