It takes energy to produce new cellular materials, to maintain the organization of membranes and organelles, and to fuel movement and active transport. The common energy for use in living systems is chemical energy released when chemical bonds are broken.
Metabolism encompasses all of the chemical reactions within the cell.
Catabolic reactions are breakdown reactions that reduce large molecules to smaller components, releasing energy. Digestion and cell respiration are primarily catabolic processes. Anabolic reactions are synthetic reactions that produce larger molecules from small ones, storing energy in chemical bonds. Photosynthesis and the manufacture of proteins are examples of anabolic processes. Metabolic pathways, both anabolic and catabolic, are chains of reactions involving multiple steps.