FAD is an electron acceptor in metabolism. The active sites of FAD are Nitrogen's 1 and 4. As one electron from the double bond between C3 and N4 flips up to react with, form the bond with, the atomic hydrogen, the other flips down between C2 and C3. The same thing happens from N1. B 12 has to come from either organ meats or supplements.
Pure vegetarians HAVE TO take supplements to ward off developing pernicious anemia.
B 12 is taken up from the diet. IF binds the B 12 for further uptake. The B 12 -IF complex travels to the small bowel. It is at this point that the mechanism of uptake is a bit murky. Either the B 12 is released from the IF and is taken up across the bowel mucosa OR the whole B 12 -IF complex is taken up by pinocytosis after which the B 12 is released.