Homopolysaccharides or homoglycans are those complex carbohydrates which are formed by polymerisation of only one
type of monosaccharide monomers. Heteropolysaccharides or heteroglycans are those complex carbohydrates which are
produced by condensation of two or more types of monosaccharide derivatives.
A. Food storage polysaccharides
They serve as reserve food. At the time of need, storage polysaccharides are hydrolysed. Sugars thus released become available to the
living cells for production of energy and biosynthetic activity. There are two main storage polysaccharides - starch and glycogen.
(i) Starch : It is storage polysaccharide of plants, and humans obtain it from cereal grains, legumes, potato, etc. It is homopolysaccharide
and is an end product of photosynthesis.
(ii) Glycogen is the carbohydrate reserve in animals, hence often referred to as animal starch. It is present in high concentration in
liver, followed by muscle, brain, etc. Glycogen is also found in plants that do not possess chlorophyll (e.g., yeast, fungi). Glucose is
the repeating unit in glycogen joined together by α(1 → 4) glycosidic bonds and α(1 → 6) glycosidic bonds at branching
points.
(iii) Inulin- It is a fructan storage polysaccharide of roots and tubers of Dahlia and related plants. Inulin is not metabolised in
human body and is readily filtered through the kidney. It is, therefore, used in testing of kidney functions, especially glomerular
filtration.
B. Structural polysaccharides
(i) Cellulose - It is a homopolysaccharide and is a predominant constituent of cell wall in plants, some fungi and protists. It is
totally absent in animal body. Cellulose is composed of β-D glucose units linked by β (1 → 4) glycosidic bonds. Cellulose
molecules have unbranched and linear chains unlike the branched and helical chains of starch and glycogen.
(ii) Chitin - It is a complex carbohydrate of heteropolysaccharide type which is found as the structural component of fungal walls
and exoskeleton of arthropods. In fungal walls, chitin is often known as fungus cellulose. Chitin is soft and leathery, therefore, it
provides both strength and elasticity. It becomes hard when impregnated with certain proteins and calcium carbonate. In chitin,
basic unit is not glucose but a nitrogen containing glucose derivative known as N-acetyl glucosamine. Chitin has an unbranched
configuration. Monomers are joined together by 1-4 β-linkages.
C. Mucosubstances
Mucopolysaccharides or glycosaminoglycans are complex carbohydrates containing amino sugars and uronic acids. They may
be attached to a protein molecule to form a proteoglycan. Proteoglycans provide the ground or packing substance of connective
tissue. E.g., hyaluronic acid, chondroitin sulfate, and heparin.
Starch and glycogen are the major storage
foods of organic world.
Mucilage present as a protective coating around
aquatic plants, bacteria, blue-green algae and
some animals is derived from polysaccharides.
Heparin prevents blood clotting inside blood
vessels of animals.Cellulose is the structural substance of cell walls in most of the plants.
It has a roughage value for human beings but a food for ruminants,
snails and termites.
Mucopolysaccharides in human body have several functions such as
lubrication of ligaments and tendons, formation of some types of body
fluids like cerebrospinal fluid, synovial fluid, vitreous humor, providing
strength and flexibility to skin, connective tissue and cartilage, binding
of proteins in cell walls and holding water in interstitial spaces.Functions of PolysaccharidesLIPIDS
Lipids are fatty acid esters of alcohols and related substances which are insoluble in water but get dissolved in a number
of nonpolar organic solvents like ether, benzene, chloroform, acetone, etc. However, they can form colloidal complex and get
dispersed uniformly in water in the form of minute droplets. The complex is called emulsion.
Lipids are basically made of carbon, hydrogen and sometimes oxygen in which the content of oxygen is always small as compared
to hydrogen and carbon. Many lipids contain small amounts of additional elements like phosphorus, nitrogen and sulphur.
Lipids consist of a polar or hydrophilic (attracted to water) head and one to three nonpolar or hydrophobic (repelled by
water) tails. Since lipids have both functions, they are called amphipathic.
Fatty acids: Basic components of fats
Fatty acids are organic acids having hydrocarbon chains that end in a carboxylic groups (–COOH). Hydrocarbon chains may be
straight or have ring structure. Some fatty acids also possess hydroxyl groups.