All fungi are eukaryotic and have sterols but not peptidoglycan in their cell membrane. Characteristically they also produce sexual and asexual spores. There are over 100,000 species recognised, with 100 infectious agents of man. Moulds are composed of numerous, microscopic, branching hyphae known collectively as a mycelium. Growth occurs from the apical tip, the apical vesicles contain materials and enzymes for the formation of new hyphal wall.
Older hyphae are less biochemically active and contain many storage vacuoles .
Part of the mycelium is involved in gaining nutrients, the vegetative mycelium, and part in growth and reproduction, the aerial or reproductive mycelium. From the reproductive mycelium, spores are formed. Asexual spores are produced by the aerial mycelium of a single organism, whereas sexual spores are formed by the fusion of cells and nuclei from opposite mating strains. There are many types of spore with names such as chlamydospore and sporangiospore. Examples of moulds include Penicillium, the source of penicillin that was the first antibiotic.