the biological making and breaking of molecules. Metabolism encompasses catabolism, which is the breakdown of molecules (for energy and/or reusable parts) and anabolism , which is the building of molecules from smaller parts (you can remember this by thinking of anabolic steroids being misused to bulk up muscles). Many metabolic pathways (or at least parts of them) are readily reversible so the same pathway can be used for building or breaking, depending on cellular needs. Metabolic pathways are interconnected into a big sort of metabolic web so that molecules can enter from different sources (different sugars, fats, proteins, etc.) at different points on the path.