This chapter traces the major steps in the breakdown of sugars and shows how ATP, NADH, and other activated carriers are produced along the way. It concentrates on the breakdown of glucose because it generates most of the energy produced in the majority of animal cells. The chapter explains how cells use many of the molecules generated from the breakdown of sugars and fats as starting points to make other organic molecules. It examines how cells regulate their metabolism and how they store food molecules for their future metabolic needs. After digestion, the small organic molecules derived from food enter the cytosol of a cell, where their gradual oxidative breakdown begins. For most animal and plant cells, glycolysis is only a prelude to the third and final stage of the breakdown of food molecules, in which large amounts of ATP are generated in mitochondria by oxidative phosphorylation, a process that requires the consumption of oxygen.