How Does Stomach Acid Help Digest Protein?

Stomach Acid is Complex

Many people consider the generic term "stomach acid" synonymous with the more specific chemical commonly known as hydrochloric acid. Although hydrochloric acid represents a major component of the stomach's digestive mix, a number of other fluids augment the acid's digestive abilities. According to Biology Online, gastric juice, the broad name for the stomach secretions that enable digestion, contains pepsinogen (pepsin), intrinsic factor, gastrin, bicarbonates, and mucus in addition to generous amounts of hydrochloric acid. These compounds work together to enable digestion of virtually anything a person can eat, but certain components more directly attack specific foods to fully and efficiently break the products down.

Pepsin Partially Digests Protein

When protein products enter the human stomach, gastric juices immediately begin degrading the food into a form that the body can easily absorb. The gastric enzyme pepsin most directly attacks protein, as it contains a specific amino acid that can efficiently disassemble specific protein compounds. Protein consists of three very complex strands of molecules, and pepsin breaks down the protein by unwinding the intertwined molecule strings; as the molecule chains become separated, other amino acids in the stomach can more directly attack the bonds they contain. Of the 20 different components in protein, some carry positive or negative electrical charges (the molecules that do not carry positive or negative charges are electrically neutral); for each of these components, a separate amino acid attacks and dissolves the molecule to release its energy. Despite the nearly complete destruction of protein in the stomach, though, some protein strands leave the stomach intact; these strands are more completely broken down in the small intestine.

Trypsin Completes Protein Digestion

After remaining protein strands leave the stomach, a small amount of chemical digestion continues in the intestines. Because pepsin cannot effectively and totally break down protein with a positive charge, positively charged molecules and strands pass into the intestines where they are attacked by trypsin. According to a U.S. Department of Energy explanation of digestion, trypsin and protein are very similar chemical components, but evolution placed pepsin in the more acidic environment of the stomach and trypsin in the alkaline intestinal environment. As trypsin in the intestines attacks positive molecules in protein, the intestine walls absorb the protein and pass it to the bloodstream. After exposure to both pepsin in the stomach and trypsin in the intestines, protein reaches a stage of complete digestion and is distributed throughout the body as available energy.