The glucose molecule is formally classified as a monosaccharide, meaning it's a single sugar unit made up of a ring of carbon, oxygen and hydrogen atoms. Glucose is ubiquitous in nature, and is found both alone and as a component of other, larger molecules. The glucose molecule has many biochemical roles in the human body.
Function
One of the most major functions of glucose in the body is as a source of cellular energy. In fact, while there are many molecules cells can use for energy--with the exception of brain cells, which require glucose nearly exclusively--glucose is so important to cellular function that physicians monitor patient blood glucose as a measure of health. The digestive tract extracts glucose from food and absorbs it into the blood, explains Dr. Gary Thibodeau in his book "Anatomy and Physiology."
Nutritional Function
Glucose's many roles in the body are a result of its chemical features, both as an individual sugar unit and in combination with other units of glucose. Glucose can be taken into the body in several different ways--most commonly, it's consumed combined with fructose in table sugar, or combined with many other glucose units in the form of either starch or cellulose. Glucose ingested as sugar or starch is absorbed, while glucose ingested in the form of cellulose, or fiber, can't be digested, explains Dr. Thibodeau, and stays in the gut.
Physiological Function
Once glucose has been absorbed into the bloodstream, cells can take it up. They either use it right away for chemical energy, or store it. Drs. Reginald Garrett and Charles Grisham, in their book "Biochemistry," note the storage form of glucose is glycogen, a long molecule made of many glucose units that is similar to starch. The liver and muscles store glucose as glycogen. Glucose can also be converted to fat, which is a much more nutritionally dense mechanism for storing energy.
Significance
Because glucose is so important to the body, glucose levels in the blood are carefully regulated. The pancreas releases hormones to increase and decrease blood glucose levels so cells get the energy they need. Dr. Lauralee Sherwood notes in her book, "Human Physiology," the pancreatic hormone insulin causes cells to take up blood glucose, decreasing blood levels, while the hormone glucagon causes the liver to break down glycogen, and release glucose into the bloodstream. Hormones keep glucose levels constant so all cells have access to energy, and hormone release is triggered by glucose levels.
Expert Insight
Glucose isn't just the brain's favorite fuel--it's also the fuel muscle cells can burn under anaerobic conditions, meaning conditions in which oxygen supply is low. Drs. Garrett and Grisham explain fat and other fuels require oxygen to yield energy. Intense muscular efforts require muscle cells to burn fuel faster than the body can supply oxygen, meaning oxygen supplies quickly run low. Muscles can burn glucose in the absence of oxygen for a short time, which supplies energy even after oxygen has run out.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
