Hexoses

HEXOSES (from Greek hex-six and osa-suffix indicating belonging to sugars), C₆H₁₂O₆, simple carbohydrates from the group of monosaccharides containing 6 carbon atoms in the molecule. Depending on whether a hexose contains an aldehyde or a ketone group, a distinction is made between aldo-hexoses (containing the group -CH₂OH-CHO) and keto-hexoses (containing the group -CH₂OH-CO-). Hexoses are colorless, sweet-tasting substances that easily form supersaturated aqueous solutions (syrups). They dissolve more with difficulty in alcohol, easily crystallize from it, and do not dissolve in ether. Hexoses give most of the characteristic reactions of aldehydes and ketones, for example, they possess reducing properties, and form hydrazones and osazones with phenylhydrazine. These reactions are used for the detection and quantitative determination of hexoses. To distinguish keto-hexoses from aldo-hexoses, Selivanov's reaction is used (see Selivanov's reaction). Since hexoses also contain alcoholic (-OH) groups, they form ethers with other alcohols (e.g., methyl) and esters with acids. Of the latter, esters with phosphoric acid are of particular importance, playing the role of essential intermediate products of carbohydrate metabolism (see Hexosephosphoric acids). As compounds containing several asymmetric carbon atoms (see), hexoses are distinguished by optical activity and exist in the form of a number of stereoisomers. Many hexoses, as a result of peculiar intramolecular rearrangements, form a number of new isomeric modifications. Hexoses occurring in nature (and only they) are fermented by yeast, breaking down into CO₂ and alcohol. Biological significance of hexoses. Hexoses and the polysaccharides composed of them form the main mass of natural carbohydrates (in plants, alongside them, carbohydrates consisting of pentoses are also contained). By virtue of this, hexoses have paramount importance in the energy economy of living organisms. Plants accumulate solar energy by synthesizing hexoses from CO₂ and water, deposit them as reserve nutritive material (for example, in the form of sucrose or starch), and build their supporting tissues (cellulose) from them. Hexoses are delivered with food to the animal organism (the latter is also capable of synthesizing hexoses from proteins and, probably, also from fats) and serve within it as the principal fuel material, a source of animal heat and muscular energy (upon physiological oxidation of 1 g of hexoses, about 4.1 calories are released). Free hexoses play the role of the "working" form of carbohydrates in the metabolism of living organisms, in which they are transported and utilized, whereas hexose polysaccharides (starch, glycogen, cellulose) play the role of the "reserve" form in which they are stored in reserve. Of all hexoses, only aldo-hexoses—d-glucose, d-galactose, and d-mannose (the latter only in plants)—and keto-hexoses—d-fructose—are found in living organisms. These four hexoses make up the group of so-called "fermentable hexoses." In the living organism (and in alkaline solutions), glucose, mannose, and fructose easily mutually convert into one another. This property is biologically very important, because thanks to it all food hexoses easily transform in the animal body into glucose, in the form of which they are processed by the organism. Galactose in the animal body is found only in the mammary gland in the form of milk sugar and as part of certain lipoids of nervous tissue (see Lipoids).