When food is being consumed, digestion begins in the mouth. Chewing or mastication helps to increase the surface area of food so that enzymes can act more quickly to digest the food. Starch digestion begins in the mouth with salivary amylase.
The food then travels as a bolus to the stomach. In the stomach the parietal cells secrete HCl that activates pepsin from its inactive form pepsinogen. Pepsin digests proteins in the stomach. Also found in the stomach is gastric lipase, which aids in the digestion of fats.
The chyme then enters the small intestine. Upon entering, juices from the pancreas are secreted into the first section of the small intestine, the duodenum. These juices include bicarbonate, trypsin, pancreatic amylase, pancreatic lipase, and nucleases. Bicarbonate changes the pH of the chyme to one that is basic, as pancreatic enzymes work best at a basic pH. Trypsin aids in protein digestion. Pancreatic amylase helps to digest starches down to disaccharides. Pancreatic lipase, like gastric lipase, helps to further digest lipids. Finally, the nucleases help to digest nucleic acids like DNA into their component nucleotides. The pancreas is very important in the digestive process as it secretes enzymes responsible for the digestion of all four macromolecules (proteins, carbohydrates, fats, and nucleic acids).
Again, the pancreatic juices are secreted into the small intestine so they are active within the small intestine. However, there are other enzymes, called brush border enzymes, attached to the lining of the intestine that break macromolecules down into their monomers so that they can be absorbed across the wall of the intestine. One example of a brush border enzyme is lactase. People with a functional version of this enzyme are able to break the disaccharide lactose (milk sugar) into two monosaccharides that can be absorbed: glucose and galactose. People without this enzyme are lactose intolerant.