Questions and Answers about BIOLOGY

by Ken Miller and Joe Levine


QUESTION: My Biology colleagues are in debate over the amount of ATP produced in the Krebs cycle, 36 or 38. We found some information that said 38 ATP in prokaryotes and 36 in eukaryotes. Could you help us? [from Angelique, a teacher in Northern Michigan].

Sure. In prokaryotes the usual figure is 38. Per glucose, you get 2 net ATPs from glycolysis. A total of 10 NADH's is made (in glucolysis and the Krebs cycle) per glucose, and 2 FADH2's. We generally say that you get 3 ATPs per NADH and 2 per FADH2, so that these carriers produce a total of 30+4 = 34 ATPs..... and the Krebs cycle itself produces 2 GTPs, which are equivalent to 2 more ATPs, for a total of 38.

Everything is the same in eukaryotes except that the 2 NADHs produced in glycolysis (in the cytoplasm) must be brought into the mitochondrion at a cost of some energy, usually estimated to be 1 ATP per NADH.... so in eukaryotes we usually say you get just 36 ATPs. Clear enough?

The fun part, however, is that none of this is as precise as textbooks make it seem. The 3 ATPs per NADH is an estimate made from lab experiments under carefully-defined conditions, and it's quite likely that the actual values vary considerably in the cell itself!

Hope this helps.

Ken Miller (1/31/04)

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(A web site developed by Ken Miller and Joe Levine to provide scientific and educational support for teachers and students using our textbooks)