Yeast

A yeast is any species of fungus that grows primarily in a single-celled form and reproduces by budding or fission; yeasts are eukaryotic microorganisms that originated hundreds of millions of years ago, with at least 1,500 species currently recognized, making up about 1 percent of all described fungal species. Some yeast species can develop multicellular characteristics, forming chains of connected budding cells known as pseudohyphae, or evolving quickly into multicellular clusters with specialized organelle functions; yeast cells vary greatly in size depending on species and environment, typically measuring 3 to 4 micrometers in diameter, though some can grow to 40 micrometers, and most reproduce asexually by mitosis, many through the asymmetric division process known as budding.