Several types of sugar are eaten. All come from plants. Sugar is stored in most plant tissues for their energy needs.
Sugar is a loose term scientifically, as it refers to a few categories of carbohydrates: monosaccharides, disaccharides, and oligosaccharides. Almost all sugars are of the formula CnH2nOn, where n is between 3 and 7.
Monosaccharides are the simplest sugar. They include, among others: glucose, fructose, and galactose. These 3 sugars are directly absorbed into the bloodstream during digestion. Though they differ in structure, all share the formula C6H12O6.
Monosaccharides form the building blocks of disaccharides (such as sucrose) and polysaccharides (such as cellulose and starch).
Glucose and fructose together form sucrose (C12H22O11). White (or brown) granulated table sugar is sucrose.
Glucose is the simplest sugar; one of the primary products of photosynthesis. Glucose is the favored fuel for cellular respiration because of its nonspecific reaction with protein amino groups, and its conformational stability.
Fructose is fruit sugar; an isomer of glucose. Fructose is found in flowers, fruits, berries, and most root vegetables.
Commercially, fructose is derived from sugarcane, sugar beets, and corn. Its crystalline form is pure fructose, while high-fructose corn syrup is a mixture of glucose and fructose as monosaccharides. Honey is a glucose-fructose mix.
The 5-ring form of fructose is 1.73 times sweeter than sucrose. The 6-ring form is equivalently sweet. Warming fructose creates the 6-ring form.
Hydrogen bonding – between positively charged hydrogen atoms and electron-rich atoms such as oxygen – determines the sweetness of a sugar. Sweeter sugars have tighter, stronger hydrogen bonds.
Fructose has been favored commercially because its sweetness is perceived quicker than either sucrose or glucose, its taste sensation peaks higher than sucrose while diminishing more quickly, and it has a sweetness synergy effect when combined with other sweeteners.
Galactose is less sweet than glucose. Lactose (C12H22O11), found in milk, is the disaccharide combination of glucose and galactose.
Disaccharides are broken down into monosaccharides during digestion; either hydrolyzed via hydrochloric stomach acid or by enzymes in the intestines.
Oligosaccharides are a carbohydrate (saccharide) polymer, typically comprising 2–10 simple sugars. Oligosaccharides serve several functions, including, by their presence in animal cell membranes, cell-to-cell recognition. Plants bundle up oligosaccharides for transport and storage.