Living things are truly remarkable. What all animals have in common is the ability to transform fuels—proteins, fats and carbohydrates—into our living, working, functioning selves.
Most biochemistry books emphasize something a little bit different. They say that living things convert fuels into biological energy, but that’s just a small part of the metabolism that is encoded in our DNA. Conversion of our fuels into energy is the minor part of metabolism while conversion of our fuels into everything we are and everything we do is the major part.
There’s a known amount of energy in all of the fuels that we eat. For example, there are 4 kilocalories of energy in a gram of glucose whether it is burned in a fireplace or used by our cells. Whereas all that can happen in the fireplace is the conversion of fuels to heat, our cells can either convert the fuels to biological energy or they can convert the fuels we eat to a menagerie of other compounds. Conversion of fuels to biological energy is called fuel oxidation (catabolism) while conversion of fuels into everything we are made of involves partial oxidation coupled to building new compounds (anabolism).
Thus, the totality of our metabolism consists of the catabolism, anabolism and repair processes to create, run and maintain our bodies. Unlike a fireplace, the processes evolved to minimize dissipation of heat and the processes depend on enzymes and co-enzymes.
Biological metabolism has three basic characteristics.
Versatility. Compounds such as glucose can be converted entirely to CO2 + water vapor + biological energy, or into testosterone, or into heme, or into stored fat, or into phospholipids, etc.
Specificity. Particularly because metabolic pathways largely exist to make things, the breakdown and buildup of particular molecules goes through specific intermediates. Nowhere is this more apparent than in the pentose phosphate pathway (PPP) in which D-glucose is precisely converted to D-ribose-5-phosphate with one specific CO2H removed from the ring to convert a 6-carbon sugar to a 5-carbon sugar that is a building block for RNA and DNA.
Energy efficiency. High energy electrons are captured from fuel oxidation and used for biosynthetic programs. NAD+ is the primary electron acceptor for fuel oxidation, while NADPH is the primary electron donor to make things and to detoxify reactive oxygen species.
The versatility and specificity of biological metabolism depends on having large numbers of highly specific enzyme catalysts. The energy efficiency of biological metabolism largely depends on NAD coenzymes.