The human digestive tract is both mechanical and chemical. Mechanically, the tract breaks down food into smaller pieces that can be chemically digested and moves food along at a constant rate. Chemical digestion, which breaks down nutrient molecules so they can be absorbed, must occur in a relatively short period of time due to the rate at which the intestine moves food through. Enzymes help digestive reactions take place faster than they otherwise would, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry," and come from several sources.
Mouth
While digestion of most nutritional molecules doesn't begin until food reaches the stomach or intestine, certain carbohydrates are digested beginning in the mouth. Saliva secreted into the mouth during the process of eating not only helps to lubricate food so that it's easily swallowed but also contains the digestive enzyme salivary amylase, explains Dr. Lauralee Sherwood in her book "Human Physiology." This enzyme begins digesting amylose, or starch, while food is still in the mouth.
Stomach
Once food reaches the stomach, it interacts with more enzymes. These are secreted by the cells that line the stomach itself, and they work together with gastric acid to help break food molecules into smaller pieces that are then passed into the intestine and eventually absorbed. The most important of the stomach enzymes is called pepsin, which is a protease, or protein-digesting enzyme.
Intestine
The intestine is the site of the majority of enzyme functions in digestion, but only a few digestive enzymes actually come from the intestine itself. The most important of these are the disaccharide-cleaving enzymes, which break sugars into their constituent parts. Intestinal sugar enzymes include sucrase, maltase and lactase.
Pancreas
The vast majority of enzymes that act on food in the small intestine actually come from the pancreas, a digestive accessory organ, explains Dr. Gary Thibodeau in his book "Anatomy and Physiology." Pancreatic enzymes include amylase, which continues the process of starch digestion, proteases that digest ingested proteins, and lipases, which work on fat.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007.
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004.
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007.
