Dissaccharides
Disaccharides are the dimers of monosaccharides. The two monosaccharides in a disaccharide can be different. The common disaccharides are maltose, sucrose and lactose.
A maltose is composed of 2 glucose molecules.
A sucrose is made of one glucose and one fructose.
A lactose is made of one glucose and one galactose.
| 2 glucose | -----> | maltose
|
| glucose | + | fructose | -----> | sucrose
|
| glucose | + | galactose | -----> | lactose
|
Disaccharides cannot pass through the membrane of the cell. But they are very soluble in water and give the sweet taste. Taken the sweetness of sucrose as 1, the sweetness of other sugars are shown in the following table.
| Monosaccharide | degree of sweetness
|
| (1) | Glucose | 0.7
|
| (2) | Fructose | 1.7
|
| (3) | Galactose | 0.3
|
| Disaccharide |
|
| (4) | Sucrose | 1.0
|
| (5) | Maltose | 0.3
|
| (6) | Lactose | 0.2
|
Most disaccharides, except sucrose, are reducing sugars and give positive result in Benedict's Test or Fehling's Test.
Sucrose are non-reducing sugar, because the reducible group, the aldehyde group is used in the formation of the glycosidic bond. So, it has not enough reducing power to reduce Cu2+ to Cu22+.
(27.08.2005)
