Final answer:
The final step in protein digestion and absorption is the active transport of single amino acids into the bloodstream from the epithelial cells of the small intestine, following their enzymatic breakdown from proteins.
Step-by-step explanation:
The final step in protein digestion and absorption occurs when individual amino acids, the breakdown products of peptides, are absorbed into the bloodstream through active transport mechanisms in the cells lining the duodenum and jejunum of the small intestine.
Once proteins are ingested, they start being digested in the stomach through the action of hydrochloric acid and enzymes like pepsin, breaking them into smaller polypeptides. This process continues into the small intestine, where the pancreatic enzymes trypsin and chymotrypsin alongside brush border enzymes like aminopeptidase and dipeptidase further digest the polypeptides into smaller peptides and eventually, individual amino acids. These amino acids are then actively transported into the intestinal epithelial cells, often coupled with sodium. Some are absorbed as dipeptides or tripeptides but are rapidly broken down into single amino acids within the cells before being released into the capillary blood. The deaminated forms of these amino acids can then be used in various metabolic processes, such as the transition reaction and the Krebs cycle.