After animals obtain food, they break it down in the process of digestion. The food is isolated in a specialized organelle or compartment so that it can be broken down chemically without damaging other organelles or structures of the body. Organisms on different branches of the tree of life have evolved very different means of isolating and digesting food. The process in single-celled protists, which obtain food particles by phagocytosis, is intracellular digestion, in which food is broken down within cells. Lysosomes containing hydrolytic enzymes fuse with vacuoles containing food particles, so the enzymes are mixed with the food (Chapter 5). Following their chemical breakdown, the food products of intracellular digestion are available for use by the cell. In most animals, however, the process is extracellular digestion, in which food is isolated and broken down outside a cell in a body compartment. Following digestion, the breakdown products are taken up into the bloodstream in a process called absorption.