Yeast needs warmth, moisture and some form of sugar to be active and make carbon dioxide gas.
Mixing carbon dioxide with yeast to create fermentation is reversible in the sense that the process can be stopped and the components (yeast and carbon dioxide) can be separated. However, once the yeast consumes the sugars and produces the carbon dioxide, this chemical reaction cannot be undone to revert back to the original state.
When glucose is added to yeast in solution, the enzymes inside it turn the mixture into ethanol and carbon dioxide, so, for your question, carbon dioxide. It also respires normally (aerobically) and then too produces carbon dioxide.
Yeast fermentation stops producing carbon dioxide when either all the available sugars have been consumed or when environmental conditions become unfavorable for yeast growth, such as high alcohol concentrations or low temperatures.
Yeast produces CO2 gas and sometimes ethenol when it metabolizes sugar.
Carbon dioxide
The ingredient in bread that produces carbon dioxide is yeast.
the yeast will react with the warm water, salt and sugar to produce carbon dioxide and alcohol. The carbon dioxide will make the dough rise and become light whilst the alcohol will evaporate when the dough is being baked. The yeast also provides a lot of the flavour of the crust from the breakdown of the starch molecules i the flour that the yeast feeds on.
Carbon Dioxide and Alcohol (anaerobic respiration)
Yeast will respire the sugar causing the yeast to give off Carbon Dioxide.
The gas produced by baking bread is called Ozone. It is a poisonous gas, if a lot is inhaled, but the little bit made when bread is baked is not harmful.
The proof of active dry yeast is the process of activating the yeast by mixing it with warm water and sugar. This causes the yeast to become active and start fermenting, producing carbon dioxide gas. This gas helps the dough rise and creates a light and airy texture in baked goods.
They'll eat the dough nontheless, making carbon dioxide. Their excrements are good to us and the carbon dioxide makes bubbles in the dough, making it rise.