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Home » Chemistry Homework Help » Biochemistry » Carbohydrates
Carbohydrates
Carbohydrates constitute an important class of compounds like glucose, fructose, sucrose, starch, cellulose, etc which play a vital role in our everyday life. Carbohydrates are also much known as saccharides. Originally, the name carbohydrates was given to the compounds pertaining to general formula Cx(H2O)y, and they were considered to be hydrates of carbon. However, this definition could not hold ground for long due to following reasons:

(i) Many compounds like formaldehyde (CH2O), acetic acid (C2H4O2), confirm to formula Cx(H2O)y but they do not exhibit the characteristic properties of carbohydrates.

(ii) Some carbohydrates like rhamnose (C6H12O5), deoxyribose (C5H10O4) do not conform to the formula Cx(H2O)y, and

(iii) Carbon is not known to form hydrates.

The above facts ultimately gave way to the modern definition of carbohydrates. Carbohydrates are now defined as polyhydroxy aldehydes or polyhydroxy ketones or the compounds which can be hydrolysed to them.

Classification of carbohydrates

Carbohydrates are classified into three major categories depending upon their behavior towards hydrolysis:

1. Monosaccharides: These are simple carbohydrates which cannot be hydrolysed to still simpler carbohydrates. About 20 monosaccharides are known to occur in nature. Glucose and fructose are common examples.

2. Oligosaccharides: These are the carbohydrates which on hydrolysis give two to ten units of monosaccharides. Accordingly, they may be further divided into di, tri or tetrasaccharides depending upon the actual number of monosaccharide units formed by the hydrolysis of a particular oligosaccharide.

Disaccharides give tow units of monosaccharides on hydrolysis. The two monosaccharide units obtained on hydrolysis of a disaccharides may be same or different. Common examples are sucrose and maltose. Both have molecular formula C12H22O11.

Sucrose on hydrolysis gives one molecule of glucose and one molecule of fructose whereas maltose on hydrolysis gives two molecules of glucose only.

Trisaccharides give three units of monosaccharides on hydrolysis. Raffinose, C15H22O16 is a common example.

Tetrasaccharides give four units of monosaccharides on hydrolysis. Stachyose, C24H42O21 is a common example.

3. Polysaccharides: These are the carbohydrates which are polymers molecules and can be hydrolysed to give large number of monosaccharide units. The commonly occurring polysaccharides have the general formula (C6H10O6). The common examples are starch, glycogen and cellulose.

It may be noted that the carbohydrates which are sweet in taste are collectively called sugars while those which are not sweet are called non-sugars. Monosaccharides and disaccharides are sugars but polysaccharides are non-sugars.

The relative degree of sweetness of various sugars is given below in tabular form:

Sugar Lactose Maltose Galactose Glucose Sucrose Fructose
Relative sweetness 16 32 32 74 100 173

The carbohydrates may also be classified as reducing and non-reducing sugars. The carbohydrates which can reduce Tollen’s reagent of Fehling’s solution are classified as reducing sugars, while which do not reduce these reagents are called non-reducing sugars.

Reducing sugars contain free aldehydes or ketones group. All monosaccharides are reducing sugars. Disaccharides may be reducing or non-reducing. If the carbonyl groups of both the monosaccharides are involved in linkage, the disaccharide is non-reducing. On the other hand, of one of the carbonyl groups is free, the disaccharides is reducing. Sucrose is a non-reducing sugar while maltose is a reducing sugar.

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