Carbohydrates- disaccharides

Disaccharides and glycosidic bonds

These are formed when two monosaccharides are condensed together:

• One monosaccharide loses an H atom from carbon atom number 1 and the other loses an OH group from carbon 4 to form the bond.
• This condensation reaction involves the loss of water and the formation of an 1,4-glycosidic bond.
• Depending on the monosaccharides used, this can be an α-1,4-glycosidic bond or a β-1,4-glycosidic bond.
• The reverse of this reaction is called a hydrolysis reaction and requires one water molecule to supply the H and OH to the sugars formed.

Examples of Disaccharides

Sucrose: glucose + fructose, used in many plants for transporting food reserves, often from the leaves to other parts of the plant.

Lactose: glucose + galactose, the sugar found in the milk of mammals.

Maltose: glucose + glucose, the first product of starch digestion and is further broken down to glucose before absorption in the human gut.

Biochemical tests

All monosaccharides and some disaccharides including maltose and lactose are reducing sugars.

These can be tested fo, by adding Benedict's reagent to the sugar and heating in a water bath.

If a reducing sugar is present, the solution turns green, then yellow and finally produces a brick red precipitate.

Non-reducing sugars can also be tested for using Benedict's reagent but first require addition of an acid and heating to hydrolyse (break apart) the sugar.

The acid must then be neutralised using an alkali like sodium hydroxide before carrying out the test as described above.