Intracellular vs. Extracellular Digestion

Some of the simplest forms of life (protists and sponges) have no gut and thus carry out intracellular digestion.  Tiny food particles are taken into the cell directly from the environment by diffusion or endocytosis.  Digestion begins in a food vacuole where lysosomal enzymes break the small particles into constituent nutrients.  Intracellular digestion eliminates the need for a gut or other cavity in which to chemically digest food.   At the same time, however, intracellular digestion limits an animal’s size and complexity—only very small pieces of food can be used. 

Larger animals have evolved structures and mechanisms for extracellular digestion, the enzymatic breakdown of larger pieces of food in a special organ or cavity.  Cells lining the gut cavity secrete enzymes into the cavity.  There, the enzymes break down food materials into constituent nutrients, and the nearby cells absorb these nutrients.