Carbohydrates — Classification

What Is a Carbohydrate

Common substances such as cane sugar, glucose and starch belong to one large family of naturally occurring organic compounds called carbohydrates. They were first noticed to follow a general formula $\ce{C_x(H2O)_y}$ and were therefore named "hydrates of carbon." Glucose, for instance, has the molecular formula $\ce{C6H12O6}$, which neatly rearranges to $\ce{C6(H2O)6}$.

That formula, however, is only a loose guide and not a definition. Acetic acid, $\ce{CH3COOH}$, fits $\ce{C2(H2O)2}$ yet is plainly not a carbohydrate, while rhamnose, $\ce{C6H12O5}$, is a genuine carbohydrate that does not satisfy the formula at all. The reliable, chemical definition rests instead on functional groups.

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that produce such units on hydrolysis.

Many carbohydrates taste sweet, and these are also called sugars (Greek sakcharon, sugar — hence the alternative name saccharides). The familiar table sugar is sucrose; the sugar in milk is lactose. With this definition fixed, every classification scheme below simply sorts carbohydrates by a different property.

Classification by Hydrolysis

The principal classification of carbohydrates is based on their behaviour on hydrolysis — that is, how many monosaccharide units they yield when water is added across their linkages. On this basis they fall into three broad groups.

About twenty monosaccharides occur naturally. Oligosaccharides are sub-classified by exact count into disaccharides, trisaccharides, tetrasaccharides and so on; of these, disaccharides are by far the most common. A disaccharide may release two identical units or two different ones — sucrose gives glucose plus fructose, while maltose gives two glucose units.

Monosaccharides: The Building Blocks

A monosaccharide is a carbohydrate that cannot be hydrolysed further into a simpler polyhydroxy aldehyde or ketone. They are the monomers from which every larger carbohydrate is assembled. Glucose, the most abundant organic compound on earth, is the monomer of starch, cellulose and glycogen alike.

Two further descriptors are stacked onto every monosaccharide: the functional group it carries and the number of carbon atoms in its chain. These combine into compact names such as "aldohexose" or "ketotriose," which encode both pieces of information at once.

Aldoses and Ketoses

If a monosaccharide contains an aldehyde group it is an aldose; if it contains a keto group it is a ketose. The carbon count is then prefixed: triose (3 C), tetrose (4 C), pentose (5 C), hexose (6 C), heptose (7 C). Combining the two gives the full name.

Thus glucose is an aldohexose bearing a terminal $\ce{-CHO}$ group, and fructose is a ketohexose bearing a $\ce{>C=O}$ keto group at C-2. Both share the molecular formula $\ce{C6H12O6}$; they differ only in where the carbonyl sits — a classic NEET discrimination point.

The D and L Notation

Monosaccharides are optically active, and their absolute configuration is labelled with the prefixes D or L. These letters indicate the relative configuration of a stereoisomer compared with a reference compound — for sugars, that reference is glyceraldehyde, which has a single asymmetric carbon and so exists as two enantiomers.

In the (+) isomer of glyceraldehyde, written with the most oxidised carbon at the top, the $\ce{-OH}$ on the asymmetric carbon lies on the right; this is assigned the D configuration. Any sugar that can be chemically correlated to D-(+)-glyceraldehyde is D; those correlated to L-(–)-glyceraldehyde are L, with the $\ce{-OH}$ on the left. For a monosaccharide, the comparison is made only at the lowest asymmetric carbon — the one farthest from the carbonyl.

Most naturally occurring monosaccharides — glucose, fructose, ribose and 2-deoxyribose among them — belong to the D-series. The NIOS chapter notes that L-forms (such as L-fructose or L-xylose) are written with the corresponding hydroxyl on the left.

Oligosaccharides and Polysaccharides

Oligosaccharides yield two to ten monosaccharides on hydrolysis. The dominant members are disaccharides, in which two monosaccharide units are joined through an oxygen atom by a glycosidic linkage, formed with the loss of one water molecule.

Polysaccharides contain a large number of monosaccharide units joined by glycosidic linkages and are the most commonly encountered carbohydrates in nature. They serve either as food storage or as structural material. Starch is the storage polysaccharide of plants and consists of amylose (a linear C1–C4 linked chain of α-D-glucose) and amylopectin (a branched chain, with C1–C6 linkages at the branch points). Cellulose, the structural polysaccharide of plant cell walls, is a straight chain of β-D-glucose units. Glycogen is the animal storage polysaccharide, structurally similar to amylopectin but even more highly branched.

Sugars Versus Non-Sugars

A second, taste-based classification cuts across the hydrolysis scheme. Sweet-tasting carbohydrates are called sugars — this includes monosaccharides such as glucose and fructose and disaccharides such as sucrose and lactose. Polysaccharides like starch, cellulose and glycogen are not sweet and are therefore grouped as non-sugars, even though they remain fully carbohydrate by chemical definition.

Reducing Versus Non-Reducing Sugars

The final classification turns on chemical reactivity. Carbohydrates that reduce Fehling's solution and Tollens' reagent are called reducing sugars; those that do not are non-reducing. The deciding factor is whether a free aldehyde or keto (anomeric) group is available.

All monosaccharides — whether aldose or ketose — are reducing sugars. Among disaccharides the picture splits. In maltose and lactose a free aldehyde group can be generated at the anomeric carbon, so both are reducing. In sucrose, the reducing groups of both glucose (C1) and fructose (C2) are locked into the glycosidic bond, leaving no free carbonyl — hence sucrose is the standard example of a non-reducing sugar.