NEET Chemistry Notes Biomolecules – Structure of Fructose

NEET Chemistry Notes Biomolecules – Structure of Fructose

Structure of Fructose

Structure of Fructose
(Laevulose: Fruit Sugar)

It is a functional isomer of glucose and has ketone group.

Oligosaccharides

These are the carbohydrates that yield two to ten monosaccharide units on hydrolysis. Oligosaccharides are further, classified as disaccharides, trisaccharides, tetrasaccharides etc., on the basis of number of monosaccharide units obtained on their hydrolysis, e.g. disaccharides : sucrose, maltose, lactose etc. trisaccharides : raffinose: tetrasaccharides : stachyrose.
Except sucrose, all other disaccharides are reducing in nature and hence, are called reducing sugars.
In disaccharides, the two monosaccharide units are joined together by an oxide linkage formed by the loss of a water molecule, this linkage is known as glycosidic linkage.

Sucrose

• It is non-reducing due to absence of free aldehyde or ketone group. It is cane sugar or table sugar.
• Sucrose is also known as invert sugar. It is due to the fact that on hydrolysis, (+)-sucrose gets inverted to give a mixture of D-(+)-glucose and D-(-)-fructose.
• In sucrose, free aldehyde or ketone group is absent. It is shown by the facts that it does not form osazone, does not exist in anomeric forms and also does not show mutarotation.

Structure of Sucrose

Maltose (C₁₂H₂₂O₁₁)

It is obtained by partial hydrolysis of starch by diastase enzyme present in malt, i.e. sprouted barley seeds (hence named maltose or malt sugar).

Maltose is’a white crystalline solid (with mp 160°-165°C), soluble in water and dextrorotatory. When it is hydrolysed with dilute acid or by enzyme maltase, maltose yields two molecules of D-(+)-glucose. Hence, maltose is a condensation product of two a-D-glucose units

Maltose is a reducing sugar. It reduces Fehling’s solution and Tollen’s reagent, it forms an oxime and an osazone and undergoes mutarotation. This indicates that atleast one aldehyde group is free in maltose.

Lactose (C₁₂H₂₂O₁₁)

It occurs in the milk of all animals [milk sugar]. It is a white crystalline solid (with mp 203°C), soluble in water and is dextrorotatory. It is hydrolysed by dilute acid or enzyme lactase, to an equimolar mixture oi D-(+)-glucose and D-(+)-galactose. Lactose is a reducing sugar, forms an oxime and osazone and also undergoes mutarotation.
It gets hydrolysed by emulsin also, an enzyme which specifically hydrolyses β -glycosidic linkage.

Polysaccharides

• Carbohydrates which yield a large number of monosaccharide units on hydrolysis are called polysaccharides, e.g. starch, cellulose, glycogen, gums arabic etc.
  Polysaccharides are not sweet in taste, hence they are also called non-sugars. Moreover, all polysaccharides are non-reducing due to absence of free —CHO or  group.
• Starch is the main storage polysaccharide of plants. It is a polymer of α – D-glucose units and consists of two components-amylose and amylopectin.
•  Cellulose is a predominant constituent of cell wall of plant cells. It is a straight chain polysaccharide composed only of β -D-glucose units which are joined together by β -1, 4-glycosidic linkage, i.e. the P-glycosidic linkages between C-l of one glucose and C-4 of the next glucose unit.
• Glycogen is the carbohydrate (a condensation polymer of α – D-glucose) which is stored in animal body. When the body needs glucose, enzymes break the glycogen down to glucose.

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