(The word metabolism can also refer to the sum of all chemical reactions that occur in living organisms, including digestion and the transport of substances into and between different cells, in which case the above described set of reactions within the cells is called intermediary metabolism or intermediate metabolism).Metabolic reactions may be categorized as "catabolic" – the "breaking down" of compounds (for example, the breaking down of glucose to pyruvate by cellular respiration); or "anabolic" – the "building up" (synthesis) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids).