How are proteins digested and absorbed?

Question

What will be an ideal response?

Answer 1

Proteins are crushed and moistened in the mouth, but the real action begins in the stomach. The major event in the stomach is the partial breakdown (hydrolysis) of proteins. Hydrochloric acid uncoils (denatures) each protein’s tangled strands so that digestive enzymes can attack the peptide bonds. The hydrochloric acid also converts the inactive form of the enzyme pepsinogen to its active form, pepsin. Pepsin cleaves proteins—large polypeptides—into smaller polypeptides and some amino acids.
When polypeptides enter the small intestine, several pancreatic and intestinal proteases hydrolyze them further into short peptide chains, tripeptides, dipeptides, and amino acids. Then peptidase enzymes on the membrane surfaces of the intestinal cells split most of the dipeptides and tripeptides into single amino acids. Only a few peptides escape digestion and enter the blood intact.
A number of specific carriers transport amino acids (and some dipeptides and tripeptides) into the intestinal cells. Once inside the intestinal cells, amino acids may be used for energy or to synthesize needed compounds. Amino acids that are not used by the intestinal cells are transported across the cell membrane into the surrounding fluid where they enter the capillaries on their way to the liver.