The digestive system breaks nutrient molecules into smaller molecules that can be absorbed by the cells that line the small intestine, so that these molecules end up in the bloodstream and become available to cells. While all nutrients--proteins, carbohydrates, and fats--break down in the presence of acid alone, this process is quite slow. The acid in the stomach, therefore, is aided by digestive enzymes. An enzyme is a chemical that helps a reaction take place faster than it otherwise would. Enzymes are crucial to digestive function.
Amylase
Carbohydrates are digested by a variety of enzymes, but the most important with regard to complex carbohydrates--or starches--is called amylase. The scientific name for starch, which is carbohydrate that is digestible but doesn't taste sweet, is amylose. Amylase enzymes break apart the constituents of amylose, which is otherwise far too large for the cells of the small intestine to absorb. Amylase is secreted by the mouth, the stomach and the small intestine, notes Dr. Lauralee Sherwood in her book "Human Physiology."
Protease
Proteins are large molecules made up of many--often many hundreds of--smaller molecules called amino acids. The long chains of amino acids can't be absorbed by the intestine. Instead, notes Dr. Gary Thibodeau in his book "Anatomy and Physiology," proteins must be broken down into their constituent amino acids, and the amino acids then absorbed into the bloodstream. The enzymes responsible for this are called proteases, meaning protein-digesting enzymes. Pepsin is the major protease produced by the stomach, while the small intestine utilizes trypsin and chymotrypsin, which are produced in and released by the pancreas.
Lipase
Fat digestion is quite challenging, since enzymes are water soluble but fats are not. For fat-digesting enzymes to be able to digest dietary fat, fat must first be made water soluble. This takes place through the action of bile salts, explains Dr. Sherwood. These salts, which are produced by the liver and stored by the gall bladder, are released into the small intestine and break fat droplets into smaller droplets that easily become surrounded by water. Lipases, or fat-digesting enzymes, from the pancreas then go to work on the dietary fat droplets, breaking fat molecules into smaller pieces that are easily absorbed.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
