So our body can use these proteolytic enzymes to catalyze the hydrolysis, the catabolic breakdown of our polypeptides into their constituent amino acids. And only then can our cells can actually ingest these amino acids into our body. So the majority of our proteins are ingested into the cells in the form of our monopeptides, our amino acids. But in some cases, we can also ingest bipeptides and tripepptides into our cells, as we'll see in the next several lectures. Now, proteins have many levels of structure, so we have the largest type of level, known as quaternary. And quaternary means we have several polypeptides that bond together via covalent bonds, our disulfide bridges, to form a single protein structure.