Fructose, which is commonly called fruit sugar, is a monosaccharide. This is a biochemical term for a single sugar ring composed of carbon, hydrogen and oxygen. Fructose is closely related to glucose, and like glucose, it is a component of table sugar. Table sugar and fruit are the two most significant natural sources of fructose in the diet, though individuals who eat large quantities of processed foods also get significant fructose through high fructose corn syrup. Fructose has several roles in the body.
Body cells need energy to be able to engage in various processes. For instance, according to Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry," a major use of energy by many cells is to maintain the so-called "resting membrane potential," which allows cells to take in certain substances from the fluid that surrounds them and allows cell-to-cell communication. Like glucose, fructose is a source of energy for the cells. Cells process fructose to extract energy through a process called aerobic respiration, which essentially means burning of fructose in the presence of oxygen to produce ATP, the cellular energy molecule.
The cells can also use fructose to make an important form of storage carbohydrate, called glycogen. According to Dr. Lauralee Sherwood in her book "Human Physiology," the liver and muscles store glycogen, which is made up of long chains of glucose, to provide for cellular glucose needs during emergencies or periods of fasting. The muscles maintain glycogen for their own use, while the liver breaks down glycogen to release glucose into the bloodstream for use by all body cells. Partial breakdown of fructose produces the compounds glyceraldehyde and dihydroxyacetone phosphate. Modification of glyceraldehyde to produce glyceraldehyde-3 phosphate allows for production of glycogen -- the glyceraldehyde-3 phosphate reacts with dihydroxyacetone phosphate to produce a precursor in glycogen synthesis.
In addition to storing energy in the form of glycogen, the body also stores energy in the form of triglyceride, or fat. Fat, says Dr. Gary Thibodeau in his book "Anatomy and Physiology," is an important form of energy storage because it's both light and energetically dense. As such, the body can store a significant quantity of energy without a significant amount of stored weight. Chemical reactions modify fructose to produce the precursors to fat synthesis.