Polysaccharides

• They are large molecules formed by condensation of a large no. of monosaccharides. They may be folded or branched.
• The general formula is:
• They are either insoluble in water or form colloidal solutions only.
• Unsweet and cannot be crystallized.
• They cannot diffuse out of the cell and exert no osmotic action within the sell, but they can be hydrolyzed readily into free monosaccharides.
• Polysaccharide is a macromolecular substance that can be hydrolyzed to yield many monosaccharide units:
• Polysaccharides are important as structural supports (particularly in plants) and also serve as a storage depot for monosaccharides (which cells use for energy).

1. Starch

Starch is found in plants, mainly in the seeds, roots, or tubers. Corn, wheat, potatoes, rice, and cassava are the chief sources of dietary starch. The 2 main components of starch are amylose and amylopectin.

Structure

• Starch = n (a-glucose)
• It is not a pure substance but is instead a mixture consisting of the unbranched chain molecules of a polysaccharide called amylose and the branched chain molecules of a second polysaccharide, called amylopectin. Amylose and amylopectin fit together to form a complex 3D structure. Amylose helices are entangled in the branches of amylopectin molecules.
• Amylose molecule is a polymer of 250-300 a-D-glucose rings joined by a(1 à 4) glycosidic linkages. It is unbranched and wound into helix because hydroxyl groups project inward with no cross-linkage between each helix. The bore of the helix can trap iodine molecules to produce blue-colored complex.

• Amylopectin differs from amylose only by having a branching structure. Its backbone is held together by a(1 à 4) glycosidic linkage but branches arise about once every 24-30 glucose units, where a(1 à 6) glycosidic bonds occur it reacts with iodine to form red-violet color.

Function

Starch is the main storage material of green plants. It is the most important energy storage carbohydrate of the plant kingdom. In turn, humans and other animals consume huge quantities of starch. This polymer is such an important food source because it has the appropriate structure to be readily broken down to d-glucose. The reduced carbons from starch provide much of our daily energy needs as they are oxidized to carbon dioxide.

Physical Properties

Starch granules swell when exposed to water, but without applied heat, the swelling may not be immediately noticeable. Though swollen, the granules will settle to the bottom if not agitated. Heating breaks enough of the intermolecular hydrogen bonds to allow the granule to swell noticeably. Generally, starches with small granules swell at higher temperatures than do those with larger granules. Continued heating results in further swelling and in the starch-water mixture becoming translucent.

As the starch granules swell, they result in an increase in viscosity or thickness. The viscosity is due to the swollen starch molecules colliding. It is possible to tear swollen starch molecules in which case the viscosity would decrease. As the starch granules are swelling it is easy for lumps to form. To prevent lumps during the gelatinization stage, starch granules must be completely separated and surrounded by water before the temperature of the starch mixture reaches the gelatinization range.

Types of starches

General categories of starches are as follows:

cereal starches-form viscous pastes and cool to be opaque gels root & tuber starches- form highly viscous pastes and cool to be only weak gels.

waxy starches- form heavy, stringy pastes and do not gel well at all.

Modified starch

Starches can be chemically modified by various means to produce desired characteristics. For example, while starches normally require heat to thicken, pregelatinized starches can disperse and thicken in cold water. While the product will not be as thick as if non-modified starch had been used, at least it can be dispersed in cold water. Other modifications to starch are made for a multitude of reasons; reducing stringiness, improving clarity, reducing viscosity, etc.

2.Glycogen

• The backbone is held by a(1 à 4) glycosdic linkages of 2 a-D-glucose molecules. Branches arise about every 10^th glucose ring where a(1 à 6) glycosidic bonds occur. This side chains occur more frquently and are longer thanthat of amylopectin.

• It is the storage polysaccharides in muscles and liver tissues of animals. (stored as granules)
• Insoluble in water
• Reacts with iodine to form red-violet color.

3. Cellulose

Cellulose is the most abundant organic substance found in nature. It is the chief structural component of plants and wood. Cotton fibres are almost pure cellulose; wood, after removal of moisture, consists of about 50% cellose. It is an important substance in the textile and paper industry.

Structure

Cellulose, like starch and glycogen, is a polymer of glucose. But cellulose differs from starch and glycogen because the glucose units are joined by b-1,4-glycosidic linkages instead of a-1,4-glycosidic linkages. The stereochemistry of the b-anomer allows the polymer to form an extended chain that can H-bond to adjacent cellulose molecules. The large no. of H-bonds so formed partially accounts for the strength of the resulting plant cell walls.

A partial hydrolysis of cellulose of cellulose produces the disaccharide cellobiose, b-D-glucopyranosyl-(1-4)-b-Dglucopyranose. However, cellulose has greater resistance to hydrolysis than either starch or glycogen.

Function

Cellulose acetate is also used as a clear, transparent pachaging film.in another process, cellulose reacts with carbon dislfide in the presence of NaOH to form a soluble cellulose derivative called cellulose xanthate, from which cellulose can be regenerated, viscose rayon textiles and cellophane packaging materials are made of regenerated cellulose prepared by this process.