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).
The Krebs cycle is like a wheel. Every time it makes one full rotation, energy is created and released. As you can see above, the NAD+ and FAD are brought in at key points throughout the cycle and are attached to other electrons resulting in the formation of NADH and FADH2. This energy is then shuttled off to be used by the cell, mostly for the continuation of cellular respiration.