The human body is made up of cells, each one of which needs to be able to provide for its energy needs by taking up nutritional molecules from the bloodstream and chemically burning them as part of cellular metabolism. Glucose is an important nutrient molecule that cells rely upon for energy, both as a component of diet and when stored for later use in the form of the carbohydrate molecule glycogen.
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Sources of Glucose
Any food that contains carbohydrates serves as a source of glucose. Even if the food itself doesn't contain glucose, your body can break down sugar and starch in the foods you eat into glucose and other simple sugars, and can then convert the other simple sugars into glucose. Thats why carbohydrate-containing foods increase your blood glucose levels -- your body breaks them own into glucose, then absorbs the glucose into your bloodstream.
The function of glucose in the body is an energy molecule. Cells take up glucose absorbed by the digestive tract and either use the glucose or store it for later. While proteins and fat also provide cells with energy, some body cells -- the cells of the brain in particular -- preferentially rely upon glucose, notes Dr. Lauralee Sherwood in her book, "Human Physiology." Further, glucose breaks down into smaller molecules that provide building blocks for many cellular products.
All cells need energy to function. With some cells, the reason for the energy requirement is more apparent than with others -- muscle cells, for instance, require energy in order to shorten and produce movement. However, even cells not involved in producing motion need a regular source of energy, explains Dr. Gary Thibodeau in his book, "Anatomy and Physiology." Cells use energy derived primarily from glucose to produce electrical currents that allow communication, to synthesize hormones and other products, and to grow and divide.
Since the primary utility of glucose in the human body is metabolism, the vast majority of ingested glucose is processed through a series of reactions. The first reaction, called glycolysis, splits glucose into two smaller molecules called pyruvate. The second reaction converts pyruvate to a molecule called acetyl-CoA, which enters the Kreb's cycle. Note Drs. Reginald Garrett and Charles Grisham in their book, "Biochemistry," the Kreb's cycle is one of the most important metabolic processes in a cell, since it ultimately leads to generation of massive amounts of energy from glucose.
Unlike protein and fat, glucose presents a distinct advantage to many cells as a source of energy, because it can be metabolized either with or without oxygen, note Garrett and Grisham. While the Kreb's cycle requires oxygen to function, there is another metabolic branch that allows oxygen-independent energy generation from glucose. This process, called fermentation, doesn't generate as much energy as the Kreb's cycle, but it does allow muscles and other cells to continue to function in low-oxygen conditions.
The human body stores energy in two ways -- as fat, and as a carbohydrate called glycogen. While most of the energy stored in the body is stored as fat, glucose is so important to normal function that the liver and muscle cells store a certain amount of it for times during which blood glucose levels begin to run low. Note Garrett and Grisham, without glucose, the body can't store glycogen. This leads to fatigue and muscle weakness.