Protein digestion: Digestion of proteins begins in the stomach. When the food enters the stomach, it mixes with the HCl and the enzyme pepsin secreted by the cells of the stomach that initiate the breakdown of proteins. Pepsin breaks down the peptide bonds. From the stomach as the food moves into the small intestine, the pancreatic enzymes trypsin, chymotrypsin, and carboxypeptidase break down the polypeptides and results in smaller units called peptides. Intestinal juice released in the small intestine also contains several digestive enzymes (peptidases) that act on the peptide bonds. The end product of protein digestion in the small intestines is amino acids.

Fat digestion: Once the food reaches the stomach, the parietal cells release acids (like hydrochloric acid), pepsin, and gastric amylase and lipase to help degrade food into chyme. The chyme is then moved into the duodenum or the upper part of the small intestine, which is the main site for the digestion of fats. Here, the bile salts break up and coat the fat to form finer droplets. These finer droplets have more surface area, and this aids digestion becausea the fat-digesting enzyme pancreatic lipase can only act on the surface of the fat droplet. It breaks down fats into smaller molecules called fatty acids and glycerol.