Final answer:
Protein digestion starts in the stomach with HCl and pepsin and is completed in the small intestine with help from pancreatic enzymes such as trypsin and chymotrypsin.
Step-by-step explanation:
Protein digestion begins in the stomach, where hydrochloric acid (HCl) and the enzyme pepsin break down proteins into smaller polypeptides. This initial stage includes the hydrolysis of protein linkages by the action of gastric juices. As the partially digested proteins move into the small intestine, they encounter pancreatic enzymes such as trypsin and chymotrypsin, which further break down the polypeptides.
The cells of the small intestine also contribute with enzymes like aminopeptidase and dipeptidase, aiding in breaking down peptides to individual amino acids that are then absorbed into the bloodstream.
It is essential to note that essential amino acids cannot be synthesized by the body and must be obtained through the diet, either from complete proteins, which contain all nine essential amino acids, or through a combination of complementary proteins that together provide all the necessary amino acids. Protein-energy malnutrition conditions such as kwashiorkor and marasmus, while rare in the U.S., remain significant issues in less developed countries.