The H^-1 from NADH in the cytoplasm must be shuttled into the mitochondrion and one of these shuttles uses glycerol 3-phosphate as the carrier.On the inside of the mitochondrion..the H is placed on an FAD making an FADH2 which enters the ETS and generates its normal 2 ATPs. It is not an NADH that is made inside from this shuttle. The H^-1 from NADH in the cytoplasm may also be carried by malate( malate /aspartate shuttle ) and in this case the malate inside is converted to OAA using an NAD+ so one does generate an NADH inside which goes to the ETS and makes 3 ATP. NADHs enter at the top of the ETC and produces 3 ATPs .FADH2 enter one step lower and "misses " one of the ATP production steps and thus generates 2 ATPs. These two shuttles are the reason one can generate either 36 OR 38 ATP from a mole of glucose depending on which shuttle is used for the extramitochondrial NADH reducing equivalent generated at the glyceraldehyde-3-P to 1,3 bisphosphoglycerate step in glycolysis .
