Glucose is one of the most important sources of cellular nutrition. Your cells can derive energy from glucose, and can also store it in the form of either glycogen -- a carbohydrate -- or fat, for later use. In order to extract and absorb the glucose from your food, you use a variety of digestive enzymes.
Glucose in Food
There are many sources of glucose in food, though normally you consume glucose chemically bonded to other molecules of glucose, or other related sugars. Despite the fact that glucose is ubiquitous in nature, it doesn't often occur on its own. Glucose is a monosaccharide, or single sugar unit. When you eat, you take in carbohydrates, which can consist of one or more monosaccharides. Most carbohydrates in the diet, however, consist of at least two -- and often more -- monosaccharides, chemically bonded together.
Sources of Glucose
The most common sources of glucose in the human diet are starch and table sugar. The starch molecule is called amylose, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry," and consists of long chains of glucose molecules, chemically bonded to one another. You also obtain significant quantities of glucose when you eat table sugar, or sucrose. Table sugar consists of a molecule of glucose chemically bonded to a molecule of the monosaccharide fructose.
Digesting Sources of Glucose
Whenever you consume glucose that's chemically bonded to other molecules -- whether they're molecules of glucose or of some other monosaccharide -- digestive enzymes in your body have to separate the monosaccharides from one another before your intestine can absorb them. An enzyme is a chemical that helps reactions take place faster than they otherwise would, note Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry."
Digestive Enzymes
Depending upon the source of glucose, you use different digestive enzymes to liberate the monosaccharides from one another. Digestive enzymes are very specific, and the enzyme that digests starch, for instance, can't digest table sugar. The common enzymes that digest sources of glucose include amylase, which digests starch, sucrase, which digests table sugar, and lactase, which digests milk sugar. Even though fiber also contains glucose, you lack cellulase -- the enzyme that digests fiber -- so you can't access or absorb the glucose in dietary fiber.
Sources of Enzymes
You secrete digestive enzymes that break apart sources of glucose from several different body organs. Your salivary glands secrete saliva that contains amylase, for instance. You secrete minimal quantities of starch-digesting enzymes from the lining of the stomach. The vast majority of carbohydrate digestion takes place in the intestine, which contains sugar-digesting enzymes as well as more amylase, notes Dr. Lauralee Sherwood in her book "Human Physiology."
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
