Carbohydrate

Biomolecules are the complex organic molecules which build up living organism and are essential for their growth and maintenance. For examples proteins, carbohydrates, lipids and nuclic acids.

Carbohydrates:

Carbohydrates are a class of naturally occurring organic compounds of carbon, hydrogen and oxygen. These contain the polyhydric aldehydes, polyhydric ketones and large polymeric substances which can be broken down to polyhydric aldehydes and ketones. These include glucose, fructose, cellulose, sucrose, starch, etc.

Classification of Carbohydrates: Carbohydrates may be classified into:

• Sugars: These are crystalline substances which are sweet and water soluble. e.g. glucose, fructose and cane sugar.
• Non-sugar: These are tasteless, insoluble in water and amorphous. e.g. starch, cellulose, etc.

Carbohydrates are also classified as–

• Monosaccharide: These are the simplest carbohydrates which cannot be hydrolysed into simpler compounds. Therefore, they represent the simplest single carbohydrate units. The general formula of (CH₂O) _n. If the carbohydrates contain aldehydic group, they are named as aldose and if they have ketonic group, they are called ketoses. The common examples are ribose (C₅H₁₀O₅), glucose (C₆H₁₂O₆), fructose (C₆H₁₂O₆) etc.
• Oligosaccharides: These are carbohydrates which give two to ten monosaccharide molecules on hydrolysis. These are further classified into disaccharides, trisaccharides, tetrasaccharides, etc. depending upon the number of monosaccharide units present in their molecules. e.g. Disaccharides: Sucrose, lactose, maltose. All these have the molecular formula C₁₂H₂₂O₁₁.Trisaccharides: raffinose (C₁₈H₃₂O₁₆).Tetrasaccharides: stachyose (C₂₄H₄₂O₂₁).
• Polysaccharides: These are carbohydrates which are polymeric and can be hydrolysed to give a large number of monosaccharide units. e.g. cellulose, starch, glycogen, etc. The general formula of starch and cellulose is (C₆H₁₀O₅) _n. These get hydrolysed to give monosaccharides.

(C₆H₁₀O₅) _n   +  nH₂O ———→  n C₆H₁₂O₆

Starch / cellulose                                Glucose

Monosaccharides:

Monosaccharides are polyhydric aldehydes and ketones which cannot be hydrolysed into simpler carbohydrates. The monosaccharides with an aldehydic group (—CHO) are called aldoses while those with ketonic group (—C = O) are called ketoses. They contain up to six carbon atoms. The general formula of (CH₂O) _n. They are further classified as trioses, tetroses, pentoses, hexoses, etc. depending upon the number of carbon atoms they contain. Some examples of various categories of monosaccharides are given below:

3          Aldotrioses: C₃H₆O₃               CH₂OH CHOH CHO             Glyceraldehyde

4          Aldotetroses: C₄H₈O₄             CH₂OH (CHOH)₂ CHO             Erythrose, Threose

5          Aldopentoses: C₅H₁₀O₅          CH₂OH (CHOH)₃ CHO             Arabinose,Ribose,Xylose, Lyxose

6          Aldohexoses: C₆H₁₂O₆            CH₂OH (CHOH)₄ CHO             Glucose, Galactose, Mannose,                                                                                                            Allose, Talose, Gulose, Idose etc.

3          Ketotrioses: C₃H₆O₃               CH₂OH CO CH₂OH                            Dihydroxyacetone

4          Ketotetroses: C₄H₈O₄              CH₂OH CO CHOH CH₂OH                Erythrulose

5          Ketopentoses: C₅H₁₀O₅           CH₂OH CO (CHOH)₂ CH₂OH            Ribulose, Xylulose

6          Ketohexoses: C₆H₁₂O₆            CH₂OH CO (CHOH)₃ CH₂OH            Fructose, Sorbose,                                                                                                                              Tagatose, etc.

Reducing and non-reducing sugars:

Those saccharides which reduce Fehling’s reagent and Tollen’s reagent are called reducing sugars, and those which do not reduce, they are called non-reducing sugar.

Polysaccharides:

These are neutral polymeric compounds in which a large number of monosaccharide units are joined by glycosidic linkages. They have the general formula (C₆H₁₀O₅) _n, where n has very large value. They are colourless, tasteless and are insoluble in plant and animal life as food storage and structural role. They are usually made up of pentoses or hexoses. The important polysaccharides are cellulose, starch and glycogen.

• Starch: Starch is the main storage polysaccharide of plants. It is a polymer of α-glucose and consists of two components namely amylose and amylopectin.
• Amylose is a water soluble fraction. It is a linear polymer of α-glucose as shown below:
• Amylopectin is water insoluble fraction. It is consists of branched chains of α-glucose as shown below. For simplicity, it may represented as-Starch is used as the principal food storage of glucose energy. It comprises the major part of roots, seeds and corns.
• Cellulose: It is also major structural polysaccharide in higher plants where it constitutes the bulk of the cell wall. It is probably the most abundant organic substance found in nature. Over 50% of the total organic matter in the living world is cellulose. Structurally, its molecules are made up of a large number of β-glucose units. The chains are arranged to form bundles and are held together by hydrogen bonds between glucose of adjacent strands.