Bread rises due to a series of biochemical reactions.
Yeast are a sort of fungus. They absorb their food rather than eating like us. The chemical reaction they use to break down sugar is the same one we use, though. They take in oxygen and sugar and produce carbon dioxide and water from it, using a lot of different enzymes to speed up (catalyze) the process. That process is called cellular respiration. It goes on in all our cells. It stores the energy gained by breaking down the sugar in another molecule, ATP. Cells then use ATP to do all sorts of work.
But here's what's different, if there's no oxygen, we die. Yeasts just switch over to another process, fermentation.
Fermentation takes in sugar and breaks it down into carbon dioxide and ethyl alcohol (ethanol). It's a lot less efficient than cellular respiration. Cellular respiration makes about 36 ATP and fermentation makes about 2. Fermentation also uses enzymes.
Here's how all this fits into bread making. Usually, bakers buy the yeast in a dry form. Then they put the amount they need into some warm water. Enzymes are sensitive to temperature. If the water is too hot, it will destroy the enzymes and the yeast will die. If it's too cold, the process is slow. Some sugar or another food source is added. Then the yeast do a lot of cellular respiration and multiply. The bowl will look foamy due to carbon dioxide bubbles.
When the yeast are mixed into the flour to make dough, they don't have access to oxygen anymore and will start doing fermentation. The carbon dioxide that they give off makes the bubbles or holes in bread dough. Baking kills the yeast and evaporates the alcohol.
Why do you think it's important to give yeast oxygen at the start of the process?
Thanks for asking,