H1-antihistamines work by binding to histamine H1 receptors in mast cells, smooth muscle, and endothelium in the body as well as in the tuberomammillary nucleus in the brain; H2-antihistamines bind to histamine H2 receptors in the upper gastrointestinal tract, primarily in the stomach.
3 NADH molecules and 1 FADH molecule is produced for every turn of the Krebs cycle. One molecule of glucose will result in two turns of the Krebs cycle because two pyruvate molecules are the result of one glucose molecule (pyruvate if fed into the Krebs cycle after it is converted into acetyl-CoA).
FADH2 and NADH are created from FAD and NAD+ through redox (reduction-oxidation) reactions in the Krebs cycle during respiration as seen below here: This cycle gives off small amounts of energy in the form of ATP (adenonsine triphosphate, cell energy) and produces these compounds, FADH2 and NADH.