Biomolecules
3.0 Disaccharides
3.0 Disaccharides
Sucrose:
- $C_{12}H_{12}O_{11}$.
- It is a white crystalline solid, soluble in water.
- It is dextrorotatory (specific rotation $ + {66.5^ \circ }$).
- On hydrolysis gives equimolar mixture of $D$-(+)-glucose [dextrorotatory $ + {52.5^ \circ }$] and $D$-(-)fructose [laevorotatory $ - {92.4^ \circ }$]. Since the laevorotation of fructose is more than dextrorotation of glucose, the mixture is laevorotatory. Due to the hydrolysis of sucrose there is a change in the sign of rotation and the product is called as invert sugar.
- Sucrose is a non reducing sugar as the aldehyde group of glucose and the ketonic group of fructose are involved in glycosidic bond formation and are not free.
Maltose: 
- $C_{12}H_{22}O_{11}$.
- On hydrolysis with dilute acids or by the enzymes yields two molecules of $\alpha$-$D$-Glucose.
- It is a reducing sugar which indicates that atleast one aldehyde group is free in maltose.
Lactose: 
- $C_{12}H_{22}O_{11}$.
- Commonly known as milk sugar since it is found in milk.
- Hydrolysed by dilute acids or by the enzyme lactase to an equimolar mixture of $\beta$-$D$-galactose and $\beta$-$D$-glucose.
- It is a reducing sugar.
