Processing Food

How does digestion occur in animals?

Obtaining food is just the first step. Food must then be broken down, or digested, and absorbed to make energy and nutrients available to body tissues. Some invertebrates break down food primarily by intracellular digestion, but many animals use extracellular digestion to break down food. A variety of digestive systems are shown in Figure 27–2.

Intracellular Digestion Animals have evolved many ways of digesting and absorbing food. The simplest animals, such as sponges, digest food inside specialized cells that pass nutrients to other cells by diffusion. This digestive process is known as intracellular digestion.

Extracellular Digestion Most more-complex animals rely on extracellular digestion. Extracellular digestion is the process in which food is broken down outside cells in a digestive system and then absorbed.

▸ Gastrovascular Cavities Some animals have an interior body space whose tissues carry out digestive and circulatory functions. Some invertebrates, such as cnidarians, have a gastrovascular cavity with a single opening through which they both ingest food and expel wastes. Some cells lining the cavity secrete enzymes and absorb digested food. Other cells surround food particles and digest them in vacuoles. Nutrients are then transported to cells throughout the body.

▸ Digestive Tracts Many invertebrates and all vertebrates, such as birds, digest food in a tube called a digestive tract, which has two openings. Food moves in one direction, entering the body through the mouth. Wastes leave through the anus.

d