About a third of the amino acids we use come from the diet. Dietary enzymes digest the proteins in our food. In the stomach, pepsin breaks down the proteins into smaller peptides. In the small intestine, trypsin, chymotrypsin and elastase cleave these peptides into still smaller peptides, while carboxypeptidases A and B start removing amino acids one at a time from the C-terminal end of each peptide. The resulting amino acids are then absorbed into our system. Since they are water-soluble they need no special transport system.
Two thirds of our amino acids come from breaking down our own proteins. This could happen in the lysosomes or in macromolecular complexes called proteasomes. Proteins containing segments with certain sequences are targeted to the lysosome; proteins with certain other segments go to the proteasome. In other words, the specific sequence of a protein determines not only its function and its location but also its lifetime.
Thought Question: Why do we need to break down our own proteins? Why don't we just keep them?