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. We also have tertiary, which basically refers to the three dimensional structure of our protein. We have second there, which refers to either the beta pleated sheets or the alpha helixes. And we also have primary, which is basically the long sequence of amino acids inside the protein.