Question

What is the structural difference between starch and glycogen?

Answer 1

Hint: Starch and glycogen both are polysaccharides which on hydrolysis produce glucose molecules. These glucose molecules can be utilised by the organism if there is a spike in the demand for energy.

Complete step by step answer:
Polysaccharides are long chain polymeric carbohydrates that are composed of monosaccharide units bound together by glycosidic linkages. They can be linear or highly branched. There are two types of polysaccharides:
Homopolysaccharides: all the monosaccharides units are of the same type. They are also called homoglycans.
Heteropolysaccharides: more than one type of monosaccharide unit is present. They are also called heteroglycans.
The function of polysaccharides is either structure or storage related. Both starch and glycogen are used as storage polysaccharides.
Starch:
Starch is produced by most green plants as energy storage. It is present in rice, potatoes, corn etc. It is white in colour and is insoluble in cold water and alcohol. It contains 20-25% amylose (linear and helical in structure) and 75-80% amylopectin (branched in structure) by weight.
Some of the molecules of amylose contain a few branch points therefore it is not completely linear... About one quarter of the mass of starch granules in plants consist of amylose, although there are about 150 times more amylose than amylopectin molecules, this implies that amylose is a much smaller molecule than amylopectin. In plants, starch molecules are present as semi-crystalline granules. On heating, starch becomes soluble in water.

Glycogen:
The primary energy storage in animals and fungal cells is held in adipose tissue while the secondary long term energy storage is glycogen which is primarily made by the liver and the muscles, but can also be made by glycogenesis within the brain and stomach.
Glycogen is sometimes referred to as animal starch since it is analogous to starch since its structure is similar to amylopectin but it is more extensively branched and compact than starch. It consists of glucose units such that they are linked together linearly by α(1→4) glycosidic bonds from one glucose to the next while the branches are linked to the chains from which they are branching off by α(1→6) glycosidic bonds between the first glucose of the new branch and a glucose on the stem chain. “Glycogen is found in the form of granules in the cytosol/cytoplasm in many cell types, and plays an important role in the glucose cycle”. It is insoluble in water and gives a brown-red solution when mixed with iodine.
Note: If there is a sudden need of glucose, then glycogen can be quickly mobilized by the body since it is less compact and more immediately available as an energy reserve than triglycerides but only the glycogen stored in the liver can be made accessible to other organs. In a globular granule of glycogen, there is a core protein “ glycogenin” that is surrounded by branches of glucose units.