Yeast is a living organism, and it respires or breathes, through its cells. During the respiration process, glucose and oxygen are combined and turned into C0₂, also known as carbon dioxide. It's also converted into alcohol which releases energy from the cell(s). The term "respiration" that was used, derives from any chemical process which releases energy from food. As a side note, yeast is a unicellular organism, meaning that they only have one cell, but deriving from multicellular ancestors.
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