Carbohydrates - condensation of monosaccharides to form disaccharides and polysaccharides


CONDENSATION/DEHYDRATION Reactions:

A reaction in which monosaccharides join together and release water as a product. An -H is removed from one molecule, and an -OH group from another, resulting in a glycosidic bond forming between the two monosaccharides to form disaccharides and eventually polysaccharides.

Disaccharides:
Sucrose
Formed from the condensation of α-glucose and β-fructose. The link between the two sugars is known as a glycoside link. In the case of sucrose, the link is between the C-1 atom of glucose in the α-configuration and the C-2 atom of fructose. The link is known as a β-1, 2 bond.

Lactose
The β-galactose is linked at the C-1 atom to the C-4 atom of β-glucose. The link is known as a β-1, 4 bond.

ch2f10.gif

In lactose both galactose and glucose are the β isomer because the -OH groups are oriented in the following way in the plane: up, down, up, up.

Polysaccharides:

Starch
Exists in two forms: amylose and amylopectin.
Amylose is water soluble, its structure is composed of a straight chain polymer of α-glucose units with α-1, 4 bonds. The molecules coil into a helical structure. It forms a colloidal suspension in hot water.
external image amylose.gif
Amylopectin is insoluble in water. Its structure also consists of α-D-glucose unites but has a branched structure with both α-1, 4 and α-1, 6 bonds. Most plants use starch as a store of carbohydrates and thus energy.
external image amylopectin.gif