Glucose and fructose are both chemically classified as monosaccharides, meaning they are single sugar units. They taste sweet, are common in foods -- both are constituents of sucrose, or table sugar -- and provide energy to cells. While the body can't convert fructose directly to glucose, it is possible to make glucose from fructose through a longer, indirect pathway.
Cells require a constant supply of energy to keep them running and able to engage in cellular processes. Either glucose or fructose can supply this energy, though some cells -- particularly brain cells -- show a marked preference for glucose. Glucose, too, has importance in terms of stored energy, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." The liver and muscles synthesize a long molecule called glycogen out of glucose, and they can break the glycogen down and release it as needed.
Glucose and fructose have identical chemical formulas, note Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry"; both are C6H12O6. Since in biochemistry, the phrase "break down" refers to chemical alteration of a molecule into one or more smaller molecules, it's not technically possible to "break down" fructose into glucose. Instead, the process could theoretically take place through chemical bond rearrangement. In the human body, however, the conversion is more circuitous -- cells break down fructose into smaller molecules, then rebuild glucose from those pieces.
The process of building glucose from fructose involves partial metabolism of fructose. The fructose molecule undergoes a process called glycolysis, in which the six-carbon sugar is broken into two three-carbon molecules called pyruvate. Note Campbell and Farrell, this process yields a very little bit of energy. Pyruvate then has many different cellular fates available to it; the pathway through which pyruvate proceeds depends upon conditions and cellular energy needs. Pyruvate can be made into glucose when blood glucose levels are low.
The cellular pathway that makes glucose from pyruvate is an important one. According to Garrett and Grisham, many non-sugar molecules break down into pyruvate in the cell. Among these, certain amino acids, glycerol, which is a constituent of fat, and lactate, which is formed during strenuous exertion in which the muscles burn energy without sufficient oxygen. The purpose of the metabolic pathway through which pyruvate can be made back into glucose -- and then broken down to yield energy -- is to ensure that there's plenty of glucose when blood sugar is scarce.
To convert pyruvate to glucose, the cells use a process called "gluconeogenesis," explain Campbell and Farrell. This involves the use of many different enzymes, and ultimately takes place through a series of many steps and chemical transformations. Glucose generated by gluconeogenesis is indistinguishable chemically from glucose ingested in the form of sugar or starch, and the cells can use it in the same ways: It can either be burned for immediate energy or stored, though conditions requiring glucose generation would likely necessitate immediate burning of the glucose.