Metabolism is the sum total of all chemical reactions in an organism, from a bacteria cell to a complex mammal like a human. In science it does not usually refer specifically to the digestion of food. The reason why metabolism can refer to all of the reactions in an organism is because of the conservation of energy in Nature. Nature does not like waste. It makes her extremely unhappy. Thus there are absolutely no superfluous chemical reactions in the body, they all contribute to something. Metabolism can accurately refer to all of them.

The basic subcomponents of metabolism are called pathways. Pathways are sequences of reactions leading towards a single goal. Pathways can either be classified as anabolic or catabolic, but they will always fall into one of those two categories. Pathways are extremely interwoven in an organism, feeding and stimulating each other in a complex pattern. They are, however, fairly similar across species, following certain themes:

  1. Cells maintain a fairly constant internal concentration of substances: inorganic ions, metabolites (basic small molecules important to reactions like pyruvate), and enzymes. The content of substances in the cell is regulated by the cell membrane, which makes sure equilibrium is preserved.
  2. Organisms or cells extract energy from external sources (food) to drive endrogonic reactions (reactions requiring energy)
  3. Metabolic pathways are specified by an organism's genes. DNA is a blueprint for proteins and enzymes, and enzymes catalyze reactions, so one can rightly say that genes determine of which metabolic pathways one is capable.
  4. Cells interact with their environment. Their activity is dependant on their surroundings, their supply of food.
  5. All materials and cells turnover, they recycle themselves and undergo continual degredation and resynthesis. An example of this is the fairly scientific adage Everyone today has at least three atoms of carbon from Julius Caesar. Another is that in seven years, an entire human body's cells have been fully resynthesized. These things recycle themselves quickly and efficiently (remember, Nature's pet peeve is waste).