Carbohydrates: Meaning and Types

In this article we will discuss about the Meaning and Types of Carbohydrates.

Meaning of Carbohydrates:

Carbohydrates are polyhydroxy aldoses, ketoses and their condensation products. Al­doses bear terminal aldehyde or —CHO group (e.g., glyceraldehyde, xylose, ribose, glucose, galactose) while ketoses have an internal ketone or —CO— group (e.g., dihydroxy acetone, erythrulose, ribulose, xylulose, fructose, sedoheptulose).

The term carbohydrate is derived from French term hydrate de carbone indicating that it is a hydrate of carbon or C_n(H₂O)_n or (CH₂O)_n.

Accordingly carbohydrates are also defined as organic substances having car­bon, hydrogen and oxygen where hydrogen and oxygen occur in the ratio of 2: 1 as found in water (H₂O). Carbohydrates are directly produced during photosynthesis of autotrophic plants. They, therefore, constitute major part of the dry weight of plants (up to 80%).

Carbohydrates are also called saccharides (Gk. sakcher- sugar) because their basic components are sugars.

Types of Carbohydrates:

Carbohydrates are of two types, small and complex. 

I. Small Carbohydrates:

Small carbo­hydrates are soluble and have low molecular weight. Commonly they are sweet to taste and are called sugars. Small carbohydrates are further differentiated into monosaccharide’s, derived monosaccharide’s and oligosaccharides.

1. Monosaccharide’s:

They are those sugars or simple carbohydrate monomers which cannot be hydrolysed further into smaller components. They have a general formula of C_nH_2nO_n.

Depending upon the number of carbon atoms, monosaccharide’s are of five types:

(a) Trioses (having 3 carbon atoms, C₃H₆O₃), e.g., Glyceraldehyde, dihydroxyacetone.

(b) Tetroses (having four carbon atoms, C₄H₈O₄), e.g., Erythrose, Threose.

(c) Pentose’s (having five carbon atoms, C₅H₁₀O₅), e.g., Ribose, Xylose, Deoxyribose, Arabinose, Ribulose. Deoxyribose is an exception because it has a formula of C₅H₁₀O₄.

(d) Hexoses (having six carbon atoms, C₆H₁₂O₆), e.g., Glucose, Fructose, Galactose, Mannose.

(e) Heptoses (having seven carbon atoms, C₇H₁₄O₇), e.g., Sedoheptulose, Glucoheptose.

The aldose or ketose group can also react with an alcoholic or nitrogen group of another organic compound. The bond thus formed is called glycosidic bond (С—О—С or С—N— С).

Glycosidic bonds are also established during the condensation of monosaccharide’s for the formation of oligosaccharides and polysaccharides. A molecule of water is usually pro­duced at each condensation. It is, therefore, an example of dehydration synthesis.

Pentoses and hexoses exist in both open chains as well as ring forms. There are two types of ring forms, pyranose and furanose (Fig. 9.4). Pyranose has hexagon structure with five carbons and one oxygen. Furanose has a pentagon structure with four carbons and one oxygen. Both pyranose and furanose types are further distinguished into α and β types.

In α-form the hydroxyl group nearer the oxygen atom of the ring is written below while in β-form it is written above. Many monosaccharide’s have asymmetric carbons and are able to rotate polarised light to right side (D, or dextrorotatory, +) or left side (L, or laevorotatory, -). Amongst sugars D-isomers are more common.

Hexoses are generally white crystalline and sweet substances. Fructose is the sweetest of all naturally occurring sugars. It is also called fruit sugar because of its common occurrence in fruits (exception Grape). Nectar and honey contain fructose.

The same is called laevulose (=levulose) because of its laevorotatory nature. Glucose is blood sugar and common respiratory substrate which is also called grape sugar, com sugar. It is called dextrose being dextrorotatory.

2. Derived Monosaccharide’s:

Monosaccharide’s are modified variously to form a number of different substances.

The important derivatives are:

(i) Deoxy Sugar:

De-oxygenation of ribose produces de-oxyribose. The latter is a constituent of de-oxyribotides found in DNA.

(ii) Amino Sugars:

The monosaccharide’s have an amino group (—NH₂). Glucosamine forms chitin, fungus cellulose, hyalu­ronic acid and chondroitin sulphate. Galactosamine is similarly a component of chondroitin sulphate,

(iii) Sugar Acid:

Ascorbic acid is a sugar acid. Glucuronic acid and galacturonic acid occur in mucopolysaccharides, (iv) Sugar Alcohol. Glycerol is involved in lipid syn­thesis. Mannitol is storage alcohol in some fruits and brown algae.

3. Oligosaccharides:

They are small carbohydrates which are formed by condensation of 2-9 monosaccharide’s. Therefore, oligosaccharides belong to the category of compound carbohydrates (polysaccharides are also compound carbohydrates).

Depending upon the number of monosac­charide molecules condensed to form oligosaccharides, the latter are known as disaccha­rides (e.g., sucrose, lactose), trisaccharides (e.g., raffinose), tetrasaccharides (e.g., stachyose), pentasaccharides, hexasaccnarides, heptasaccharides, etc. Larger oligosaccharides having branched or unbranched chains occur attached to cell membranes.

The smallest and the commonest oligosaccharides are disaccharides. They are formed by the condensation of two monosaccharide molecules. Hexosan disaccharides are the best known, e.g., sucrose, maltose, lactose, trehalose.

Trehalose is found in some fungi and haemolymph of some insects. Sucrose is the commercial sugar which is also called cane sugar because most of it is obtained from Sugarcane (Saccharum officinarum).

A large quantity is also obtained from Sugar Beet (Beta vulgaris). Sucrose is formed by condensa­tion of one molecule each of glucose and fructose. Glycosidic bond is established between carbon atom 1 of glucose and carbon atom 2 of fructose. A molecule of water is released.

The union is in the aldehyde region of glucose and ketone region of fructose. Unlike its components (glucose and fructose), sucrose is a non-reducing sugar. In sucrose, fructose occurs in the furanose (pentagon) form while glucose is in pyranose (hexagon) state (Fig. 9.5). Sucrose is the storage carbohydrate of Sugarcane and Sugar beet.

Maltose or malt sugar is named so because of its occurrence in malted grains of Barley. It is also found in detectable amounts in most germinating seeds and tissues where starch is being broken down. It is a reducing sugar which is formed by condensation of two molecules of glucose (Fig. 9.5). Glycosidic bond (α 1 → 4) is established between carbon 1 of one glucose molecule and carbon 4 of second glucose molecule.

Lactose or milk sugar is named so because it is found naturally in milk. It is a reducing sugar which is formed inside mammary glands by (3 1→ 4 condensation of two hexose molecules, glucose and galactose.

Souring of milk is due to conversion of lactose into lactic acid. Trisaccharide raffinose is also a reducing sugar. It is formed of glucose, fructose and galactose. Sweetening index of sucrose, maltose and lactose is respectively 100, 32 and 16. It is 170 for fructose, 40000 for saccharin and 200,000 for monellin (a protein).

Reducing Sugars:

They are those sugars which can reduce Cu²⁺ ions to Cu⁺ state. The property is found in all those saccharides which possess free aldose or ketose groups. All monosaccharide’s have this ability. Amongst disaccharides, sucrose is non-reducing because both aldose group of glucose and ketose group of fructose are lost due to formation of glycosidic bond between the two.

However, some other disaccharides (e.g., lactose, mal­tose) possess the reducing groups. The reaction is also used in detecting glucose in urine. Two types of tests are used, Fehling’s solution and Benedict’s solution. The latter is more common.

The solution contains blue coloured alkaline solution of copper (cupric) sulphate. Reducing sugar changes into insoluble reddish cuprous oxide on gentle heating. Cuprous oxide separates as precipitate. The final precipitate may appear green, yellowish, and orange to brick red depending upon the amount of reducing sugar.

Functions of Small Carbohydrates:

Some of the important function of small carbohydrates are listed below:

1. Trioses, glyceraldehyde and dihydroxy acetone, are important intermediates of both respiratory and photosynthetic pathways.

2. Erythrose, a tetrose monosaccharide, is not only an intermediate of respiratory and photosynthetic pathways but is also a raw material for the synthesis of lignin, anthocyanin’s and some amino acids (e.g., phenylalanine, tyrosine) and a few aromatic compounds.

3. Pentose sugars, arabinose and xylose form polymers (arabans and xylans) which are constituents of hemicellulose.

4. Ribose is an important pentose sugar which is found in a variety of chemicals like CoA, FAD (flavin adenine dinucleotide), NAD (nicotinamide adenine dinucleotide), NADP and ATP besides being a constituent of nucleotides that form RNA (ribonucleic acid). Deoxyribose (C₅H₁₀O₄) is part of nucleotides that form DNA (deoxyribonucleic acid).

5. Ribulose 1, 5 bi-phosphate or RuBP (a phosphorylated pentosan) is the acceptor of carbon dioxide in photosynthesis.

6. Glucose is the main respiratory substrate. Fructose can also function similarly.

7. Fats and amino acids are formed from glucose and other sugars.

8. Oligosaccharides attached to cell membranes form cell coat or glycocalyx.

They are important for:

(a) Cell recognition

(b) Cell attachment

(c) Receptor molecules (for receiving and responding to external stimuli)

(d) Antigen specificity for human blood groups (A, B, Rh)

(e) Components of antibodies which are large molecules with attached carbo­hydrates

(f) Glycoproteins of some viral coats for attaching to and invading host cells.

9. Some plants store oligosaccharides as reserve food, e.g., sucrose (sugarcane and sugar beet).

10. Monosaccharide’s are polymerized to form structural carbohydrates (e.g., cellulose, ligncellulose) and storage carbohydrates, starch in plants and glycogen in animals.

II. Complex Carbohydrates:

Polysaccharides:

They are complex carbohydrates which are formed by polymerisation of large number of monosaccharide monomers (more than 10 but usually numerous). Polysaccharides are also called glycan’s. They are long chained.

The right end of a polysaccharide is reducing end while the left end is named as non-reducing end. The chains may be branched or un-branched. Depending upon the composition, polysaccharides are of two types, homopolysaccharides and heteropolysaccharides.

Homopolysaccharides:

Homopolysaccharides or homoglycans are those complex carbohydrates which are formed by polymerisation of only one type of monosaccharide monomers. For example, starch, glycogen and cellulose are composed of a single type of monosaccharide called glucose.

Depending upon monosaccharide unit involved, polysaccharide is called glucan (made of glucose), fructan (made of fructose), xylan (having xylose), araban (made of arabinose), galactan (formed of galactose), etc.

Heteropolysaccharides:

Heteropolysaccharides or heteroglycans are those complex carbohydrates which are produced by condensation of two or more types of monosaccharide’s or their derivatives, e.g., agar, chitin peptidoglycan, arabanogalactans, arabanoxylans, etc. During the formation of polysaccharides a molecule of water is released at each condensa­tion.

This reduces the bulk, makes the polysaccharide almost insoluble and decreases its effect on water potential or osmotic potential of the cell. They are, therefore, ideal for storage and as structural components. Unlike sugars, polysaccharides are not sweet.

Being large sized, they are unable to pass through cell membranes though their components do so. Passage is allowed through active process. Another method of their passage is by means of secretion vesicles of Golgi appa­ratus.

Functions of Polysaccharides:

Some of the important functions of Polysaccharides are listed below:

1. Starch and glycogen are the major storage foods of organic world.

2. On hydrolysis storage carbohydrates provide both energy and carbon chains.

3. Chitin is the structural carbohydrate of fungal walls and exoskeleton of arthropods.

4. Cellulose is the structural substance of cell walls in most of the plants.

5. Cellulose is economically important in the production of furniture, shelter, fuel, paper, textiles, ropes, rayon, cellophane, plastics, shatter proof glass, propellant explosives, emul­sifier and raw material for several fermentation products.

6. For human beings cellulose has a roughage value but it is a food for ruminants, snails and termites.

7. Mucilage present as a protective coating around aquatic plants, bacteria, blue-green algae and some animals is derived from polysaccharides.

8. Mucopolysaccharides in human body have several functions— lubrication of liga­ments and tendons, formation of some types of body fluids like cerebrospinal fluid, synovial fluid, vitreous humor, providing strength and flexibility to skin, connective tissue and car­tilage, binding of proteins in cell walls and holding water in interstitial spaces.

9. Heparin prevents blood clotting inside blood vessels of animals.

10. Some mucopolysaccharides have medicinal and other commercial importance, e.g., husk of Plantago ovata, mucilage of Aloe, and alginic acid, agar, carragenin of marine algae.

11. Pectin’s form matrix of cell wall. As calcium pectate they produce middle lamella or cementing layer between adjacent plant cells. Pectin’s are commercial jellying agents.