living cells are in a state of ceaseless activity. To maintain its “life,” each cell depends on highly coordinated biochemical reactions. Carbohydrates are an important source of the energy that drives these reactions. This chapter discusses the energy-generating pathways of carbohydrate metabolism are discussed. During glycolysis, an ancient pathway found in almost all organisms, a small amount of energy is captured as a glucose molecule is converted to two molecules of pyruvate. Glycogen, a storage form of glucose in vertebrates, is synthesized by glycogenesis when glucose levels are high and degraded by glycogenolysis when glucose is in short supply. Glucose can also be synthesized from noncarbohydrate precursors by reactions referred to as gluconeogenesis. The pentose phosphate pathway enables cells to convert glucose-6-phosphate, a derivative of glucose, to ribose5-phosphate (the sugar used to synthesize nucleotides and nucleic acids) and other types of monosaccharides. NADPH, an important cellular reducing agent, is also produced by this pathway. In Chapter 9, the glyoxylate cycle, used by some organisms (primarily plants) to manufacture carbohydrate from fatty acids, is considered. Photosynthesis, a process in which light energy is captured to drive carbohydrate synthesis, is described in Chapter 13.