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Carbohydrates are often defined as compounds with only C, H, and O and the H and usually the O atoms are in a 2:1 ratio.

Exceptions like deoxyribose exist, but why is it recognized as a carbohydrate?

What distinguishes a carbohydrate from other compounds?

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up vote 12 down vote accepted

The definition of carbohydrates as compounds containing $C$, $H$ and $O$ usually with the empirical formula ${C_m(H_2O)_n}$, is outdated.

More rigorous and a accurate definition would be Polyhydroxy Aldehydes and Ketones composed of Carbon, Hydrogen and Oxygen. Moreover, the nomenclature including Mono-, Di-, Oligo- and Poly- "Saccharide" is preferred over the now obsolete and too-general term "carbohydrate", since the etymological origin of the word "carbohydrate" is from the greek words Carbo- for carbon and hydor- for water which indicates that the carbohydrates are "hydrates" of carbon and have the empirical formula ${C_m(H_2O)_n}$, which is not always true. Instead "saccharide" is derived from the greek word for "sugar" and hence is much more accurate as it does not limit the definition to any empirical formula.

Apart from the polyhydroxy aldehyde or ketone characteristic of carbohydrate, they have a marked structural and functional variation from other biomolecules. This includes the absence of $S$ (as in proteins), $P$ and $N$ (as in proteins and nucleic acids), presence of Glycosidic bond (as compared to peptide bond in proteins, phosphodiester bond in nucleic acid, and ester linkages in fats or glycerides) and much greater structural uniformity as compared to other biomolecules.However, these characteristics are not absolute as is evident from the fact that several monosaccharide derivatives do contain $S$ or $N$ atoms. Actually, the entire group of amino-sugars (Hexosamines etc.) are usually classified under carbohydrates since they are primarily monosaccharide derivatives obtained through enzymatic amination.

These characteristics distinguish this class of compounds without having to assign a empirical formula to it.

ADDENDUM
Adding the clause compounds which can, on simple hydrolysis yield polyhydroxy aldehyde or ketones in the definition helps involve certain non-reducing sugars like Sucrose.

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Nice summary, but haven't you gone a bit far with the "absence of..." section? What, for example, would you classify N-acetyl glucosamine as? Or isopropylthio D galactoside? These are derivatives of saccharides containg N and S respectively, and it makes no sense to not classify them, broadly speaking, with their parent molecules. –  Alan Boyd Oct 9 '13 at 14:37
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@AlanBoyd The absence is not absolute and I forgot to mention that(corrected). However, the compounds you mention are referred to as monosaccharide-derivatives and not monosaccharides as such. These derivatives, usually cannot be classified into one of the traditional groupings which are not defined by strict structural criteria. Broadly speaking they are carbohydrates (i.e. monosaccharides) but while distinguishing, omitting a few exceptions might help us grasp the uniqueness of each group and I guess is what the asker meant with "What distinguishes a..?". –  Satwik Pasani Oct 9 '13 at 16:54
    
@SatwikPasani That's exactly what I meant. Thank you for this rather slick summary - very clever to limit the composition with C, H and O. –  Greek Fellows Oct 10 '13 at 12:26
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Not all carbohydrates are reducing. Sucrose (a disaccharide), for example, contains no aldehyde or ketone group (the anomeric carbons are linked together) but is a carbohydrate (saccharide). Trehalose, a disaccharide of two glucose units, is also most certainly a carbohydrate, but is also non-reducing. –  TomD Oct 11 '13 at 10:11
    
@TomD The addendum tries to resolve this problem. –  Satwik Pasani Mar 7 at 5:32
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