Barely, rye, and oats can all be used to make bread, but wheat is the most common choice. Other additives like honey, nuts, and even strains of bacteria can be used to add flavors, but one think all leavened breads have in common is yeast. Most bread-making would be impossible without the help of yeast. Yeast is a very simple single-celled organism. It is a fungi and is related to the mushrooms we eat at the dinner table.
Yeasts live a very simple life, they eat sugars and they give off carbon dioxide as they breathe.
The carbon dioxide they produce is very important to bread-making. When we make a loaf of bread, we add in a package of yeast to the dough. The dough is then allowed to set aside for a while. When the dough is rising, the yeast are busy at work. They use their own powerful enzymes to convert flour molecules into glucose or sugar molecules which they eat.