yeast -
any of various single-celled fungi that reproduce asexually by budding or division
fungus an organism of the kingdom Fungi lacking chlorophyll and feeding on organic matter; ranging from unicellular or multicellular organisms to spore-bearing syncytia
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with the 1,500 species currently described estimated to be only 1% of all yeast species. Most reproduce asexually by budding, although a few do so by binary fission. Yeasts are unicellular, although some species with yeast forms may become multicellular through the formation of a string of connected budding cells known as pseudohyphae, or false hyphae, as seen in most molds.
OmegaWiki Dictionary
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yeast Many species of unicellular fungi, most of which belong to the Ascomycetes and reproduce by budding. The genus Saccharomyces is used in brewing and winemaking because in low oxygen concentration it produces zymase, an enzyme system that breaks down sugars to alcohol and carbon dioxide. Saccharomyces is also used in bread-making. Some yeasts are used as a source of protein and of vitamins of the B group.\n(Source: ALL)