A substantial amount of the NADPH required for reductive processes (i.e., lipid biosynthesis) is supplied by these reactions. For this reason this pathway is most active in cells in which relatively large amounts of lipids are synthesized, (e.g., adipose tissue, adrenal cortex, mammary glands, and the liver). NADPH is also a powerful antioxidant. (Antioxidants are substances that prevent the oxidation of other molecules. Their roles in living processes are described in Chapter 10.) Consequently, the oxidative phase of the pentose phosphate pathway is also quite active in cells that are at high risk for oxidative damage, such as red blood cells.