Explain why NADH synthesized in the cytosol of liver cells and used for mitochondrial energy conversion generates 2.5 ATP per NADH, whereas NADH synthesize in muscle cell cytosol only generates 1.5 ATP per NADH

Answer: FAD is being reduced rather than NAD+

Explanation: This occurs in the G-3-P shuttle system wherein oxidation of transported NADH by respiration generates 1 ATP lesser in myocytes (1.5) than the 2.5 ATP generated as is the case with hepatocytes. This reduction in ATP is as a result of FAD (which enables electrons from cytosolic NADH to be moved against an NADH concentration gradient) rather than NAD + being the electron acceptor G-3-P dehydrogenase of the mitochondria. The resultant effect of this transport is one molecule of ATP generated for every two electrons reduced.