Many cellular processes require energy. Most of these reactions are enzymatically coupled to another energy liberating reaction. Metabolic energy sources, like sugars and fats are broken down into smaller, more useful species. Cells contain a number of high energy molecules to use as sources for these reactions.
The most common of these is adensosine triphosphate, ATP. Cleaving one phosphate group from this liberates 7.3 kcal/mole.
Nicotinamide Adenine Dinucleotide
|Flavin Adenine Dinucleotide
Another important part of oxidative metabolism is flavin adenine dinucleotide, FADH2. Oxidation to FAD releases 43.4 kcal/mole, enough to generate nearly six ATPs. Again, some inefficiency in electron transport results in the formation of only two ATPs.