Storage Of Polysaccharides

Function as reserve food which can be hydrolysed to form sugars required for respiration and biosynthesis, e.g., starch, glycogen, inulin.

Starch

A polymer of glucose,  a major fuel store in plants, but is absent from animals where the equivalent is glycogen, can easily be converted back to glucose for use in respiration. In

germinating seeds the glucose may also be used to make cellulose and other materials needed for growth.

Starch has two components, amylose (20-30% of starch) and amylopectin (70-80% of starch). In wrinkle seeded pea, amylose fraction is upto 98%, while waxy starch of maize may be almost entirely made of amylopectin.

Amylose has a straight chain structure consisting of several thousand glucose residues joined by 1, 4 bonds. These bonds cause the chain to coil helically into a more compact shape.

Amylopectin is also compact as it has many branches, formed by 1, 6 glycosidic bonds. It has up to twice as many glucose residues as amylose. A suspension of amylose in water gives a blue-black colour with iodine-(potassium iodide solution), whereas a suspension of amylopectin gives a red-violet colour. This forms the basis of the test for starch.

Starch molecules accumulate to form starch grains. These are visible in many plant cells, notably in the chloroplasts of leaves, in storage organs such as the potato tuber, and in seeds of cereals and legumes. The grains appear to be made of layers of starch and are usually of a characteristic size and shape for a given plant species.

e.g., Oblong (e.g. Banana), rounded (e.g., Wheat), oval (e.g. Potato), polygonal (e.g. Rice), rod shaped, etc. An amyloplast may have a single grain when it is called simple (e.g., Wheat Banana, Potato) or two to many joined together when the complex is called compound starch grains (e.g. Rice, Oat, Potato).

Each starch grain has a growing point called hilum. Starch is laid around it in concentric (e.g. Wheat, Banana) or eccentric (e.g. Potato) layers or shells. The shells or layers contain starch molecules in parallel bundles of 50-100.

Glycogen

Glycogen is the animal equivalent of starch, being a storage polysaccharide made from  glucose, many fungi also store it. Liver and muscle stores large amount of glycogen.

In vertebrates, glycogen is stored chiefly in liver and muscles, where it provides a useful energy reserve.

Very similar in structure to amylopectin, but shows more branching.

Forms tiny granules inside cells which are usually associated with smooth endoplasmic reticulum.

Inulin

Small sized (25-35 residues) fructosan homopolysaccharide which is a reserve carbohydrate present in roots, tubers and rhizomes of many composites e.g. Dahlia, Dandelion, Jerusalem Artichoke.

Also called dahlia starch. A section of Dahlia root treated with alcohol is found to have fan-like crystals of inulin.

Formed by β (2 →1)  glycosidic linkages between fructofuranose units.

Soluble in water. Colour reaction to iodine is absent.

Human body does not have enzymes to metabolise inulin and is filtered out through kidneys. It is used for determining the rate of glomerular filtration.