Carbohydrates

Carbohydrates are organic compounds containing carbon, hydrogen, and oxygen. Since the proportion of hydrogen and oxygen in carbohydrates is the same as that in water they were thought to represent 'hydrated carbon', hence the name 'carbohydrate' was given to these compounds.

Carbohydrates are called saccharides or compounds containing sugar. Based on the number of sugar molecules present in carbohydrates, they can be classified into various categories specified below:

· Monosaccharides

· Disaccharides

· Oligosaccharides

· Polysaccharides

Monosaccharides:

Monosaccharides are simple sugars containing many numbers of carbon atoms.

Glucose is an example of a carbohydrate monomer or monosaccharide. Other examples of monosaccharides include mannose, galactose, fructose, etc.

Monosaccharides can be further classified depending on the number of carbon atoms it contains:

i. Trioses (C₃H₆O₃)

ii. Tetroses (C₄H₆O₄)

iii. Pentoses (C₅H₁₀O₅)

iv. Hexoses (C₅H₁₂O₆)

v. Heptoses (C₇H₁₄O₇)

Examples of monosaccharides are glyceraldehyde (triose), erythrose (tetrose), ribose (pentose) and glucose (hexose). Hexoses and pentoses exist in both open chain and ring forms.

Disaccharides:

Two monosaccharides combine to form a disaccharide. Examples of carbohydrates having two monomers include- Sucrose, Lactose, Maltose, etc.

Oligosaccharides:

Carbohydrates formed by the condensation of 2-9 monomers are called oligosaccharides. By this convention, trioses, pentoses, hexoses are all oligosaccharides.

Polysaccharides:

The acid insoluble pellet also has polysaccharides (carbohydrates) as another class of macromolecules. Polysaccharides are long chains of sugars, they are threads (literally a cotton thread) containing different monosaccharides as building blocks.

For example, cellulose is a polymeric polysaccharide consisting of only one type of monosaccharide i.e., glucose, it is also a homopolymer. Starch is a variant of cellulose but present as a store house of energy in plant tissues and animals belong to another variant called glycogen, whereas Insulin is a polymer of fructose. In a polysaccharide chain (say glycogen), the right end is called the reducing end and the left end is called the non-reducing end. Starch forms helical secondary structures. In fact, starch can hold I₂ molecules in the helical portion and starch-I₂ is blue in colour. Cellulose does not contain complex helices and hence cannot hold I₂.

Plant cell walls are made of cellulose. Paper made from plant pulp and cotton fibre is cellulosic. There are more complex polysaccharides in nature. They have as building blocks, amino-sugars and chemically modified sugars (e.g., glucosamine, N-acetyl galactosamine, etc.). Exoskeletons of arthropods, for example, have a complex polysaccharide called chitin. These complex polysaccharides are mostly homopolymers.