Protein Digestion


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Protein digestion begins in the stomach, where pepsin breaks proteins down into fragments, called peptides. Further digestion occurs in the small intestine, where a variety of enzymes break peptides down into smaller peptides, and then into individual amino acids. Several of the protein-digesting enzymes found in the small intestine are secreted from the pancreas. Amino acids are absorbed from the small intestine into the blood stream. The liver regulates the distribution of amino acids to the rest of the body. A small amount of dietary protein is lost in the feces.
Protein digestion is a multistep process that begins in the stomach and continues through the intestines. Source: OpenStax Biology 2e

OpenStax Biology 2e

A large part of protein digestion takes place in the stomach. The enzyme pepsin plays an important role in the digestion of proteins by breaking down the intact protein to peptides, which are short chains of four to nine amino acids. In the duodenum, other enzymes—trypsin, elastase, and chymotrypsin—act on the peptides reducing them to smaller peptides. Trypsin elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas and released into the duodenum where they act on the chyme. Further breakdown of peptides to single amino acids is aided by enzymes called peptidases (those that breakdown peptides). Specifically, carboxypeptidase, dipeptidase, and aminopeptidase play important roles in reducing the peptides to free amino acids. The amino acids are absorbed into the bloodstream through the small intestines.

Source: OpenStax Biology 2e


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at:

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