Where Does Protein Digestion Begin and End?
The digestion of proteins begins, as does all digestion, with the mechanical process of chewing food, and it ends with the absorption of the proteins' constituent amino acids by the cells of the body. About.com describes the process at length, ranging from mechanical to chemical digestion, and even lists the specific enzymes involved in the chemical digestion of proteins at various stages along the way.
The first step in digesting proteins is accomplished by the teeth, which grind and slice food material through mechanical digestion. The chemical digestion of proteins begins in the acidic crucible of the stomach, where hydrochloric acid is secreted along with a protease called pepsinogen. Exposed to the powerful acid, pepsinogen degrades into the enzyme pepsin, which About.com describes as beginning the process of breaking down the protein into amino acids.
Dictionary.com describes the next step in the process as that when food material enters the duodenum. The pancreas secretes chemicals that include a proteolytic enzyme called trypsin. Trypsin breaks down the proteins further into peptone, which can then be passed to the small intestine.
Once in the small intestine, rhythmic muscular contractions, known as peristalsis, continue the mechanical mixing of food material. The hepatic portal vein then carries nutrient-rich blood to the liver, which WebMD describes as cleansing and purifying blood. The material left after the nutrients are extracted is then passed to the large intestine for excretion.