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Protein digestion primarily begins in the stomach, where the strong acidic environment disrupts protein structure and exposes peptide bonds within polypeptide chains. The stomach enzyme pepsin breaks these bonds to form smaller peptide fragments, creating shorter chains of amino acids that move into the small intestine for further breakdown.

Pepsin is a protease, or protein-digesting enzyme, produced in the stomach that severs links between specific types of amino acids. Working in the acidic stomach environment, pepsin breaks peptide bonds to form shorter polypeptide chains. These fragments then move to the small intestine where additional enzymes continue the breakdown process.

As chyme enters the duodenum, enzymes like trypsin continue breaking peptide bonds into shorter peptides. Peptidases secreted by the epithelial surface of the small intestine then split individual amino acids from the ends of polypeptide chains. The anatomy of the intestines and microvilli allows these amino acids to be easily absorbed into the bloodstream.

After absorption into the bloodstream, amino acids are carried throughout the body for protein synthesis or transported to the liver for metabolism. The liver alters digested amino acids so they can be used to produce energy or make carbohydrates and fats, supporting various metabolic functions.

During protein metabolism, the liver produces a toxic byproduct called ammonia from digested amino acids. The liver converts this ammonia into a much less toxic substance called urea, which is then released into the blood and transported to the kidneys for excretion through urine.

Peptides are shorter chains of amino acids formed when pepsin and other proteases break peptide bonds in polypeptide chains. Amino acids are individual units that result from complete peptide breakdown by peptidases. Only individual amino acids can be absorbed through the small intestine into the bloodstream.

The stomach's highly acidic environment is essential for protein digestion because it disrupts protein structure and exposes peptide bonds within polypeptide chains. This acidic condition allows pepsin to function effectively, breaking these bonds to form smaller peptide fragments that can be further processed in the small intestine.