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Carbohydrates- Structure, Properties, Classification and Functions
- The carbohydrates are a group of naturally occurring carbonyl compounds (aldehydes or ketones) that also contain several hydroxyl groups. It may also include their derivatives which produce such compounds on hydrolysis.
- They are the most abundant organic molecules in nature and also referred to as “saccharides”.
- The carbohydrates which are soluble in water and sweet in taste are called as “sugars”.
Structure of Carbohydrates
- Carbohydrates consist of carbon, hydrogen, and oxygen.
- The general empirical structure for carbohydrates is (CH2O)n.
- They are organic compounds organized in the form of aldehydes or ketones with multiple hydroxyl groups coming off the carbon chain.
- The building blocks of all carbohydrates are the simple sugars called monosaccharides. A monosaccharide can be a polyhydroxy aldehyde (aldose) or a polyhydroxy ketone (ketose).
The carbohydrates can be structurally representated in any of the three forms:
- Open chain structure.
- Hemi-acetal structure.
- Haworth structure.
Open chain structure – It is the long straight chain form of carbohydrates.
Hemi-acetal structure – Here the 1st carbon of the glucose condenses with the -OH group of the 5th carbon to form a ring structure.
Haworth structure – It is the presence of pyranose ring structure.
Properties of Carbohydrates
Physical Properties of Carbohydrates
- Stereoisomerism – Compound shaving same structural formula but they differ in spatial configuration. Example: Glucose has two isomers with respect to penultimate carbon atom. They are D-glucose and L-glucose.
- Optical Activity – It is the rotation of plane polarized light forming (+) glucose and (-) glucose.
- Diastereo isomers – It the configurational changes with regard to C2, C3, or C4 in glucose. Example: Mannose, galactose.
- Annomerism – It is the spatial configuration with respect to the first carbon atom in aldoses and second carbon atom in ketoses.
Chemical Properties of Carbohydrates
- Osazone formation: Osazone are carbohydrate derivatives when sugars are reacted with excess of phenylhydrazine. eg. Glucosazone
- Benedicts test: Reducing sugars when heated in the presence of an alkali get converted to powerful reducing species known as enediols. When Benedict’s reagent solution and reducing sugars are heated together, the solution changes its colour to orange-red/ brick red.
- Oxidation: Monosaccharides are reducing sugars if their carbonyl groups oxidize to give carboxylic acids.In the Benedict’s test, D-glucose is oxidized to D-gluconic acid thus, glucose is considered a reducing sugar.
- Reduction to alcohols: The C=O groups in open chain forms of carbohydrates can be reduced to alcohols by sodium borohydride, NaBH4, or catalytic hydrogenation (H2, Ni, EtOH/H2O). The products are known as “alditols”.
Properties of Monosaccharides
- Most monosaccharides have a sweet taste (fructose is sweetest; 73% sweeter than sucrose).
- They are solids at room temperature.
- They are extremely soluble in water: – Despite their high molecular weights, the presence of large numbers of OH groups make the monosaccharides much more water soluble than most molecules of similar MW.
- Glucose can dissolve in minute amounts of water to make a syrup (1 g / 1 ml H2O).
Classification of Carbohydrates
The simple carbohydrates include single sugars (monosaccharides) and their polymers, the oligosaccharides and polysaccharides.
- Simplest group of carbohydrates and often called simple sugars since cannot be further hydrolyzed.
- Colourless, crystalline solid which are soluble in water and insoluble in non-polar solvent.
- These are compound which possess a free aldehyde or ketone group.
- The general formula is Cn(H2O)nor CnH2nOn.
- They are classified according to the number of carbon atoms they contain and also on the basis of the functional group present.
- The monosaccharides thus with 3,4,5,6,7… carbons are called trioses, tetroses, pentoses, hexoses, heptoses etc., and also as aldoses or ketoses depending upon whether they contian aldehyde or ketone group.
- Examples: Glucose, Fructose, Erythrulose, Ribulose.
- Oligosaccharides are compound sugars that yield 2 to 10 molecules of the same or different monosaccharides on hydrolysis.
- The monosachharide units are joined by glycosidic linkage.
- Based on the number of monosaccharide units, it is further classified as disaccharide, trisaccharide, tetrasaccharide etc.
- Oligosaccharides yielding 2 molecules of monosaccharides on hydrolysis is known as a disaccharide, and the ones yielding 3 or 4 monosaccharides are known as trisaccharides and tetrasaccharides respectively and so on.
- The general formula of disaccharides is Cn(H2O)n-1and that of trisaccharides is Cn(H2O)n-2 and so on.
- Examples : Disaccharides include sucrose, lactose, maltose etc.
- Trisaccharides are Raffinose, Rabinose.
- They are also called as “glycans”.
- Polysaccharides contain more than 10 monosaccharide units and can be hundreds of sugar units in length.
- They yield more than 10 molecules of monosaccharides on hydrolysis.
- Polysaccharides differ from each other in the identity of their recurring monosaccharide units, in the length of their chains, in the types of bond linking units and in the degree of branching.
- They are primarily concerned with two important functions ie. Structural functions and the storage of energy.
- Theya re further classified depending on the type of molecules produced as a result of hydrolysis.
- They maybe homopolysaccharidese, containing monosaccharides of the same type or heteropolysaccharides i.e., monosaccharides of different types.
- Example of Homopolysaccharides are starch, glycogen, cellulose, pectin.
- Heteropolysaccharides are Hyaluronic acid, Chondrotin.
Functions of Carbohydrates
Carbohydrates are widely distributed molecules in plant and animal tissues. In plants, and arthropods, carbohydrates from the skeletal structures, they also serve as food reserves in plants and animals. They are important energy source required for various metabolic activities, the energy is derived by oxidation.
Some of their major functions include:
- Living organisms use carbohydrates as accessible energy to fuel cellular reactions. They are the most abundant dietary source of energy (4kcal/gram) for all living beings.
- Carbohydrates along with being the chief energy source, in many animals, they are instant source of energy. Glucose is broken down by glycolysis/ kreb’s cycle to yield ATP.
- Serve as energy stores, fuels, and metabolic intermediates. It is stored as glycogen in animals and starch in plants.
- Stored carbohydrates acts as energy source instead of proteins.
- They form structural and protective components, like in cell wall of plants and microorganisms. Structural elements in the cell walls of bacteria (peptidoglycan or murein), plants (cellulose) and animals (chitin).
- Carbohydrates are intermediates in biosynthesis of fats and proteins.
- Carbohydrates aid in regulation of nerve tissue and is the energy source for brain.
- Carbohydrates gets associated with lipids and proteins to form surface antigens, receptor molecules, vitamins and antibiotics.
- Formation of the structural framework of RNA and DNA (ribonucleic acid and deoxyribonucleic acid).
- They are linked to many proteins and lipids. Such linked carbohydrates are important in cell-cell communication and in interactions between cells and other elements in the cellular environment.
- In animals they are important constituent of connective tissues.
- Carbohydrates those are rich in fiber content help to prevent constipation.
- Also they help in modulation of immune system.
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- Madigan, M. T., Martinko, J. M., Bender, K. S., Buckley, D. H., & Stahl, D. A. (2015). Brock biology of microorganisms (Fourteenth edition.). Boston: Pearson.
- Rodwell, V. W., Botham, K. M., Kennelly, P. J., Weil, P. A., & Bender, D. A. (2015). Harper’s illustrated biochemistry (30th ed.). New York, N.Y.: McGraw-Hill Education LLC.