Fungi are heterotrophs, meaning that they depend on preformed organic molecules for both carbon and energy. Unlike animals, fungi do not have organs that enable them to ingest food and break it down in a digestive cavity (Chapter 40). Instead, fungi absorb organic molecules directly through their cell walls. This mode of feeding presents two major problems. First, although simple molecules like amino acids and sugars pass readily through the cell wall, more complicated molecules do not. Fungi secrete a diversity of enzymes that break down complex organic molecules like starch or cellulose into simpler compounds that can be absorbed. Fungi, then, digest their food first and take it into the body afterward.

A second challenge is to find food in the environment. Other heterotrophic organisms such as bacteria, protists, and animals commonly move through their habitat, actively searching for food. Fungi, however, have no means of locomotion, and so these organisms use the process of growth itself to find nourishment.