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Yeast are microorganisms. They are single-celled fungi. There are about 1,500 different species of yeast. Most reproduce asexually, by budding. Some use binary fission to reproduce asexually.
A particular species of yeast, Saccharomyces cerevisiae, has been used for thousands of years. It is used for baking bread because the yeast makes carbon dioxide as a by-product of it eating sugar, which helps the bread rise (leaven). Yeast is also used for making beer and other alcoholic drinks by a process known as fermentation. This process produces both ethyl alcohol and carbon dioxide. Ethanol is also used as fuel, and to make other organic chemicals.
Yeast can be used to make electricity. Yeast is also a model organism for studying cell biology. Some yeasts can cause infections in humans (they are pathogens).
Dry yeast (usually, yeast is sold like this)
Media related to Yeast at Wikimedia Commons