Monosaccharide

Monosaccharides, known as simple sugars, are organic compounds that typically follow the formula (CH₂O)ₓ and contain at least two carbon-carbon bonds. They are classified as either polyhydroxy aldehydes or polyhydroxy ketones. These substances are colorless, crystalline solids that dissolve in water and are usually sweet-tasting. Based on the number of carbon atoms, they are named trioses (3), tetroses (4), pentoses (5), hexoses (6), heptoses (7), and so on. Common examples include glucose (also called dextrose), fructose (levulose), and galactose. Monosaccharides serve as the fundamental units for larger carbohydrates: they link together to form disaccharides like sucrose, lactose, and maltose, as well as polysaccharides such as cellulose and starch. Everyday table sugar is actually the disaccharide sucrose, which forms when one molecule of D-glucose and one of D-fructose undergo a condensation reaction. Glucose is especially important in metabolism, where it is broken down through glycolysis and the citric acid cycle to supply energy for living organisms. Maltose, meanwhile, results from the dehydration condensation of two glucose molecules.

In terms of structure, each carbon atom attached to a hydroxyl group—except those at the ends of the chain—is chiral, meaning many isomeric forms exist for a given formula, each with distinct properties, particularly in biology. Additionally, most monosaccharides can adopt at least one cyclic form, especially in water when they have more than four carbons. The combination of multiple chiral centers and chain-ring equilibria makes their chemistry quite complex. With few exceptions, such as deoxyribose, monosaccharides follow the formula (CH₂O)ₓ, where x is conventionally 3 or more. Glucose, a hexose, is used for energy and to build starch, glycogen, and cellulose. Ribose and deoxyribose are pentoses found in RNA and DNA, respectively. Heptoses like mannoheptulose and sedoheptulose are ketoses. Monosaccharides with eight or more carbons are rare and unstable.

In their linear form, monosaccharides have an unbranched carbon backbone with a single carbonyl group and hydroxyl groups on the other carbons. The general formula is H(CHOH)ₙ(C=O)(CHOH)ₘH, where n + 1 + m = x, giving the elemental formula CₓH₂ₓOₓ. Carbon atoms are numbered starting from the end nearest the carbonyl. If the carbonyl is at position 1, the compound is an aldehyde, called an aldose; if it is between two carbons, it is a ketone, called a ketose. Biologically relevant ketoses usually have the carbonyl at position 2. These classifications combine into names like aldohexose or ketotriose. Open-chain monosaccharides are named with a Greek prefix for carbon count (tri-, tetr-, pent-, etc.) and the suffix “-ose” for aldoses or “-ulose” for ketoses; if the carbonyl is not at position 2, its position is indicated by a numeric infix (e.g., pent-3-ulose).

Stereoisomers arise when monosaccharides have the same molecular graph but differ in the spatial arrangement of their atoms. This occurs at chiral carbons—those bonded to four distinct groups. In an open-chain monosaccharide, every carbon except the first, last, and (in ketoses) the carbonyl carbon is chiral. For example, the triketose glycerone has no chiral carbon and exists as a single stereoisomer, while the aldose glyceraldehyde has one chiral carbon and thus two enantiomers (mirror-image forms). Monosaccharides with four or more carbons can have multiple chiral centers, so the number of possible stereoisomers is at most 2ᶜ, where c is the number of chiral carbons. The Fischer projection is used to draw these structures, specifying the handedness of each chiral carbon by the left or right placement of hydroxyl groups. Most stereoisomers are chiral, meaning they are distinct from their mirror images, which differ by reversing all chiral hydroxyl positions. While mirror-image isomers behave identically in non-chiral environments, they often have very different biochemical roles.