Most fungi are multicellular, but single-celled species are found in most fungal groups. Unicellular, free-living fungi are referred to as yeasts (Figure 29.2). Some fungi have both a yeast life stage and a multicellular life stage. Thus the term “yeast” does not refer to a single taxonomic group, but rather to a lifestyle that has evolved multiple times. Yeasts live in liquid or moist environments and absorb nutrients directly across their cell surfaces.

The ease with which many yeasts can be cultured, combined with their rapid growth rates, has made them ideal *model organisms for study in the laboratory. They present many of the same advantages to laboratory investigators as do many bacteria, but because they are eukaryotes, their genome structures and cells are much more like those of humans and other eukaryotes than are those of bacteria.