Glycolysis
In glycolysis, ATP is formed by the transfer of a high-energy phosphate from 1,3-bisphosphoglycerate to ADR No such high-energy phosphate donor has ever been isolated in mitochondria because

no such phosphate donor exists
the techniques for isolating the phosphate donor are not refined enough
the high-energy phosphate donor is very short-lived and difficult to isolate
None of these

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