For example, the high acidity of our stomach kills all bacterial cells and those cells end up in the feces that we excrete to the outside of our body to the outside environment of our body. Now, feces also contains something called roughage. Roughage, for example, consists of some type of indigestible material such as cellulose. Remember we do not have those special types of cells, bacterial cells that give us the ability to break down cellulose. But certain animals, certain herbivores, for example cows, they do have those bacterial cells that produce special types of proteolytic enzymes that can cleave the bonds in cellulose to produce the individual sugars which can then be used to basically produce ATP molecules. Now, feces also contains proteins, for example proteolytic enzymes that have not been absorbed by our body that were used to basically break down the different types of macromolecules into their constituents inside our mouth, inside our stomach and inside the small intestine.