Molecular Structure of Carbohydrates (With Diagram)

In this article we will discuss about the molecular structure of carbohydrates with the help of diagrams.

Carbohydrates are organic compounds consisting of carbon, hydrogen and oxygen. They are present in all cellular organisms. The simple carbohydrates, known as monosaccharides, contain the three component elements C, H and O of which H and O are in the same ratio as that of water.

The number of carbon atoms in simple sugars may be 3, 4, 5, 6 or 7, and the monosaccharides are accordingly known as triose, tetrose, pentose, hexose and heptose, respectively. In biological organisms, these sugars exist generally as esters of phosphoric acid and they serve as intermediates of different metabolic pathways. Among the monosaccharides, pentose’s like ribose and deoxyribose are of special importance as constituents of nucleic acids.

Similarly, glucose is the most important among hexose sugars. It serves almost universally as the respiratory substrate. Derivatives of glucose are often the major constituents of cell wall and cell membrane. Thus, amino sugars like glucosamine and their derivatives are the major constituents of the cell walls of bacteria and fungi.

Some of the monosaccharides and their derivatives are shown in Fig. 8.1:

Another feature of monosaccharides is that, due to the presence of asymmetric C-atoms, they show optical rotation in solution and are present as D- or L- form. Thus, in the simplest monosaccharide, glycerine aldehyde, the C₂ is asymmetric because it has four different substituents — H, OH, CHO and CH₂OH — attached to it. In D-glycerein aldehyde, the asymmetric carbon atom has H on the left side and OH on the right and in the L-form they are opposite.

The monosaccharides play many important roles in metabolism. The two trioses, glycerin aldehyde and dihydroxyacetone, in their phosphorylated form, are involved in the glycolytic pathways as also in the synthetic pathways like the Calvin cycle. The tetrose, erythrose, also as a phophoester, take part in the reductive and oxydative pentose phosphate cycles.

It also serves as an important intermediate in biosynthesis of aromatic compounds. The pentose’s — ribose, deoxyribose, ribulose and xylulose — are important intermediates in the biosynthesis of nucleic acids as well as in other metabolic cycles. The hexoses — glucose and fructose — in their phosphorylated forms, are intermediates in glycolytic pathway. Sedoheptulose is an intermediate in the Calvin cycle and in the Pentose Phosphate cycle.

Some derivatives of sugars are of considerable biological importance. Among these are glucosamine and galactosamine, and the uronic acids. In the amino sugars, the -OH group of carbon atom2 is replaced by an amino (-NH₂) group. The amino group may be modified by addition of an acetyl (-CO-CH₃) group to produce an acetyl amino sugar (Fig. 8.2). Uronic acids are formed by oxidation of the primary alcoholic group at C6 position of hexose to a carboxyl group (-COOH).

Thus, glucose may be converted to glucuronic acid and mannose to mannuronic acid (Fig. 8.2):

N-acetyl glucosamine and N-acetyl muramic acid are constituents of the bacterial cell wall polymer, known as murein. A polymer of N-acetyl-glucosamine is chitin, the major cell wall material of higher fungi. N-acetyl glucosamine is also a constituent of hyaluronic acid present in the animal cells. Glucuronic acid is present in mucopolysaccharides.

Disaccharides are formed by joining two monosaccharide by a glycosidic bond with elimination of a molecule of water. The two monosaccharides may be identical or different. Thus, in maltose there are two glucose molecules, in lactose a glucose and a galactose, in sucrose a glucose and a fructose and so on.

The structures of maltose, lactose and sucrose are shown in Fig. 8.3:

While all monosaccharides possess reducing properties due to the presence of an aldehyde group in C1 position or a keto group in C2 position, disaccharides may be reducing or non-reducing. In the formation of the glycosidic linkage, C1 aldehyde group is involved resulting in the loss of a reducing group in one of the two monosaccharides.

If the reducing group of the other monosaccharide remains free, the resulting disaccharide still remains a reducing sugar. Thus, maltose and lactose possess reducing property. But, in case of sucrose, the aldehyde group (C1) of glucose and the keto group of fructose (C2) take part in glycosidic bond formation. Hence, sucrose is a non-reducing disaccharide.

Polysaccharides are composed of a large number of monosaccharides bound to each other by glycosidic bonds to form long chains which may be branched or linear. They may contain the same repeating units e.g. starch or glycogen contains only glucose, or may contain different monosaccharides or their derivatives.

The monosaccharides may be pentoses or hexoses. Depending on the nature of monosaccharides, polysaccharides are known as glucan, mannan, galactan, araban, xylan etc. The general formula of pentosans is (C₅H₈O₄)_n and that of hexosans is (C₆H₁₀O₅)_n, where n denotes the number of monosaccharide units.

Starch, glycogen and cellulose are all polysaccharides containing glucose. Starch is characteristically formed in green plants as a photosynthetic reserve carbohydrate and serves as a source of carbon to humans. Similarly, cellulose is consumed by herbivorous animals. Glycogen is the main reserve carbohydrate in most animals and many fungi and some bacteria. Both starch and glycogen are branched polymers of glucose. While the straight chain portions of these molecules consist of glucose units linked by 1—>4 glycosidic bonds, the branching points have 1—>6 glycosidic bonds.

The long chains of these molecules are actually spirally coiled (Fig. 8.4):

Mucopolysaccharides are composed of disaccharide units consisting of a uronic acid and an amino sugar. Hyaluronic acid is a mucopolysaccharide present in animal tissues. It is a polymer of glucuronic acid and N-acetyl glucosamine.

The capsules of many bacteria are made of complex polysaccharides e.g. capsule of pneumococci Type III is composed of glucuronic acid and glucose. The backbone of bacterial cell wall is a polymer of N-acetyl glucosamine and N-acetylmuramic acid, (see Fig. 8.2).