The life support activities of organisms with even the simplest structure involve a large number of complex biochemical reactions, which is metabolism. Generally, all chemical reactions within a cell are called metabolism and consist of two types of reactions: a synthetic reaction that needs energy, an anabolic reaction, and a degradative reaction that releases energy, a catabolic reaction. Each organism needs two sources: energy sources and carbon sources. Energy sources can be the sun's ray energy for photosynthetic organisms or can be organic or inorganic compounds for chemotropic organisms. In a biological system, energy must be available nonstop, and the required energy is temporarily stored in the energy storing molecule, adenosine triphosphate (ATP). ATP is the energy currency in cells because it has high energy phosphate bonds. Metabolic pathways contain chemical reactions created by making and breaking chemical bonds within the cell. Chemical bonds keep atoms together to form molecules.