As mentioned earlier, dough is fermented by bread ferment or yeast. Yeast is a mono-cell living organism which turn starch and saccharine of flour into carbonic gas and alcohol through a complex process. The produced carbonic gas causes the dough structure to become porous and expanded. This is called rising. The produced alcohol is evaporated during baking and this is one of the reasons which make the bread sponge-like.
The yeast,Due to the break of long chains of starch, makes the digestion of bread easier (as you may have experienced, breads, the dough of which is not fermented properly or in production of which baking soda is used, are not digested easily. However, breads like Sangak which their dough is fermented for a long time can be digested easily).
An acid called Fitnik acid which exists in the husk of wheat is combined with minerals like ferrous and calcium and makes them non-solvent in water and therefore, indigestible for human beings. However, this acid is eliminated via fitaz enzyme which is produced during fermentation materials remain digestible. In breads in which baking soda is used or those with short-time fermentation, the exiting minerals of such breads remain indigestible.
The yeast contains a lot of proteins, minerals and vitamins which are added to bread. The most distinguishing function of yeast for consumers is adding pleasant taste and aroma to bread which is due to creation of aromatic acids produced by yeast.