Polysaccharides, which are also known as glycans composed of number of monosccharide units. They represent condensation products of several molecules of simple sugars or monosaccharides. Monosaccharides are linked together by glycosidic bonds in polysaccharides. They form linear chain or branched chain molecules. They contain only one type of monosaccharide units or many types of monosaccharide units. According to this nature polysaccharides are classified into two groups, homopolysaccharides and heteropolysaccharides.
Homopolysaccharides are composed of only one type of monosaccharides. On hydrolysis they yield only one type of monosaccharides Eg. starch, glycogen, cellulose etc. which yield only glucose on hydrolysis.
Heteropolysaccharides are composed of a mixture of monosaccharides. On hydrolysis, they yield a mixture of monosaccharides. Eg. Hyaluronic acid, Heparin, Mucopolysaccharides.
This is the storage form of carbohydrate present in plants. They are abundantly found in root, stem, vegetables, fruits and cereals. The bulk of our diet which consists mainly of rice, wheat and vegetables is good source of starch.
Starch occurs in the forms of grain which may be spherical or oval in shape. Microscopically, the starch grains are found to differ in size and shape according to their sources (Fig.4.7).
Starch is made up of two structurally different homopolysaccharide units. They are amylose and amylopectin.
In amylose, glucose molecules are arranged in a linear form. Glucose molecules are arranged in a highly branched form in amylopectin.
Amylose has 1,4 glycosidic linkages. The glycosidic - OH group of C1 in one glucose unit is joined to that of C4 of the next unit (Fig 4.8).
The glucose unit present at each point of branching has subtituents at carbon atoms 1,4 and 6 i.e it has three points of attachment (Fig.4.9).
A solution of starch react with iodine to give blue colour. The blue colour formation is mainly due to the presence of amylose in starch.
Digestion of starch, the main carbohydrate source in the human diet, begins in the mouth. Saliva contains α amylase, which randomly hydrolyses all the a, 1-4 glycosidic bonds of starch except its outer most bonds and those next to branches.By the time thoroughly chewed food reaches the stomach, where the acidity inactivates a-amylase, the average chain length of starch has been reduced from several thousand to fewer than eight glucose units.
Starch digestion continues in the small intestine under the influence of pancreatic amylase which is similar to salivary amylase. This enzyme degrades starch to a mixture of disaccharide maltose and the trisaccharide maltotriose. These oligosaccharides are hydrolysed to their component monosaccharides by specific enzymes persent in the brush border membranes of the intestinal mucosa. The resulting monosaccharides are absorbed in the intestine and transported to the blood stream.
Glycogen is a homopolysaccharide since it gives only glucose units on hydrolysis. It is the major reserve carbohydrate in animals. Glycogen is present in all cells of skeletal muscle and liver and occur as cytoplasmic granules. Among plants, it is found in fungi and yeast. Primary structure of glycogen resembles that of amylopectin but glycogen is more highly branched, with branch points occurring every 8 to 12 glucose residues (Fig. 4.10). Glycogen is readily available as immediate source of energy. During starvation glycogen is mobilised from the storage tissue and converted to glucose by the enzyme glycogen phosphorylase. Glucose formed like this is oxidised to produce energy. The degree of branching is considerably more in glycogen when compared to starch.
Formation of glycogen from glucose is called as Glycogenesis and breakdown of glycogen to form glucose is called as Glycogenolysis.