What chemical does yeast respire?

Answer 1

At the end of its growth process, yeast will emit carbon dioxide (Co2). However, the process by which it does so is somewhat more involved.

Yeast makes Co2 as it reproduces. Yeast is a tiny, single-cell organism that asexually reproduces; that is, each cell has the capability to make another cell by itself. When yeast is put in an agreeable environment (warm, wet, and sugary), the yeast will reproduce. As it reproduces, yeast releases an enzyme that converts sugar into two things: alcohol and Co2.

Co2 is the chemical yeast respires ("breaths"); it creates air bubbles in food as it bakes, creating air bubbles that let it rise (this is called "leavening"). The alcohol yeast produces usually bakes off in breads and other baked goods; however, it also makes yeast useful in brewing.