11th Class Biology Biomolecules / जैव-अणु Macromolecules                                         

Macromolecules are polymerisation product of micromolecuels, have high molecular weight and low solubility. They include mainly polysaccharide, protein and nucleic acids.

(1) Polysaccharide : They are branched or unbranched polymers of monosaccharides jointed by glycosidic bond.  Their general formula is \[{{({{C}_{6}}{{H}_{10}}{{O}_{5}})}_{n}}.\] Polysaccharides are amorphous, tasteless and insoluble or only slightly soluble in water and can be easily hydrolysed to monosaccharide units.

Types of polysaccharides

(i) On the basis of structure

Homopolysaccharides : These are made by polymerisation of single kind of monosaccharides. e.g., starch, cellulose, glycogen, etc.

Heteropolysaccharide : These are made by condensation of two or more kinds of monosaccharides. e.g., chitin, pectin, etc.

(ii) On the basis of functions

Food storage polysaccharides : They serve as reserve food. e.g., starch and glycogen.

Structural polysaccharides : These take part in structural framework of cell wall e.g., chitin and cellulose.

Description of some polysaccharides

Glycogen : It is a branched polymer of glucose and contain 30,000 glucose units. It is also called animal starch. It is also found as storage product in blue green algae, slime moulds, fungi and bacteria. It is a non-reducing sugar and gives red colour with iodine. In glycogen, glucose molecule are linked by \[14\] glycosidic linkage in straight part and \[16\] linkage in the branching part glycogen has branch points about every \[8-10\] glucose units.

Starch : Starch is formed in photosynthesis and function as energy storing substance. It is found abundantly in rice, wheat, legumes, potato (oval and ecentric shaped), banana, etc. Starch is of two types. Straight chain polysaccharides known as amylose and branched chain as amylopectin. Both composed of \[D\]glucose units jointed by \[\alpha -1-4\] linkage and \[\alpha -1-6\] linkage. It is insoluble in water and gives blue colour when treated with iodine.

Inulin : Also called “dahlia starch”(found in roots). It has unbranched chain of 30 – 35 fructose units linked by \[\beta -2-1\] glycosidic linkage between 1 and 2 of carbon atom of D– fructose unit.

Cellulose : An important constituent of cell wall \[(2040%),\] made up of unbranched chain of \[6000\,\beta D\]glucose units linked by 1 – 4 glycosidic linkage. It is fibrous, rigid and insoluble in water. It doesn’t give any colour when treated with iodine. It is a most abundant polysaccharide.

Chitin : It is a polyglycol consisting of \[N-\]acetyl\[D\]glucosamine units connected with \[\beta -1,\,4\] glycosidic linkage. Mostly it is found in hard exoskeleton of insects and crustaceans and some times in fungal cell wall. Second most abundant carbohydrate. It is a most abundant heteropolysaccharide.

Agar-Agar : It is a galactan, consisting of both D and L galactose and it is used to prepare bacterial cultures. It is also used as luxative and obtained from cell wall of red algae e.g., Gracilaria, Gelidium etc.

Pectin : It is a cell wall material in collenchyma tissue may also be found in fruit pulps, rind of citrus fruits etc. It is water soluble and can undergo sol \[\rightleftharpoons \] gel transformation. It contain arabinose, galactose and galacturonic acid.

Neutral sugars : It is found associated with cellulose in cell wall. The common sugars in hemicellulose are \[D-\]xylose, \[L-\]arabinose, \[D-\]galactose, \[D-\]mannose and \[D-\]glucusonic acid. e.g., hemicellulose.

Gum : It secreted by higher plants after injury or pathogenic attacks. It is viscous and seals the wound. It involves sugars like L-arabinose, \[D-\]galactose, \[D-\]glucusonic acid. e.g., gum arabic.

(2) Mucopolysaccharides : These are gelatinous substance, containing amino sugars, uronic acid, etc. All slimy substances of plant are mucopolysaccharide. e.g., hyaluronic acid, vitreous humour, chondridine sulphate, heparin, husk of isabgol and mucilage also.

Glycoproteins : They include some plasmaprotein and blood group substances. They doesn’t contain uronic acid.

Murein : It is a peptidoglycan, linked to short chains of peptides. It is constituent of cell wall of bacteria and blue green algae.

Functions

(i) Cellulose pectin and chitin are constituents in cell wall of higher plants but peptidoglycan in the cell wall of prokaryotes.

(ii) They are reserve food material and form protective covering.

(iii) Fibres are obtained used in making cloth and rope.

(iv) Nitrocellulose and trinitrate cellulose (gun-cotton) used as explosive.

(3) Protein : The word protein was coined by Berzelius in 1838 and was used by G. J. Mulder first time 1840.  15% of protoplasm is made up of protein.

Average proteins contain 16% nitrogen, \[5055%\] carbon, oxygen \[2024%,\] hydrogen 7% and sulphur \[0.30.5%.\] Iron, phosphorous, copper, calcium, and iodine are also present in small quantity.

Structure of proteins : It is due to different rearrangement of amino acids. When carboxyl group \[(-COOH)\] of one amino acid binded with amino group \[(N{{H}_{2}})\] of another amino acid the bond is called peptide bond.

(i) Primary structure : The primary structure is the covalent connections of a protein. It refers to linear sequence, number and nature of amino acids bonded together with peptide bonds only. e.g., ribonuclease, insulin, myoglobin and lysozyme.

(ii) Secondary structure : The folding of a linear polypeptide chain into specific coiled structure \[(\alpha -\]helix) is called secondary structure. e.g., fur, keratin of hair claws, and feathers.

(iii) Tertiary structure : The arrangement and interconnection of proteins into specific loops and bends is called tertiary structure of proteins. It is found in e.g., globular proteins.

(iv) Quarternary structure : It is shown by protein containing more than one peptide chain. The protein consists of identical units. It is known as homologous quarternary structure e.g., lactic dehydrogenase. If the units are dissimilar, it is called as heterogeneous quarternary structure e.g., haemoglobin.

Classification of proteins : Proteins are classified on the basis of their shape, constitution and function.

On the basis of shape

Fibrous protein/Scleroprotein : Insoluble in water. Animal protein resistant to proteolytic enzyme is spirally coiled thread like structure form fibres. e.g., collagen (in connective tissue), actin and myosin, keratin in hairs, claws, feathers, etc.

Globular proteins : Soluble in water. Polypeptides coiled about themselves to form oval or spherical molecules e.g., albumin insulin hormones like ACTH, oxytosin, etc.

On the basis of constituents

Simple proteins : The proteins which are made up of amino acids only. e.g., albumins, globulins, prolamines, glutelins, histones, etc.

Conjugated proteins : These are complex proteins combined with characterstic non–amino acid substance called as prosthetic group. These are of following types :

(i) Nucleoproteins : Combination of protein and nucleic acids, found in chromosomes and ribosomes. e.g., deoxyribonucleoproteins, ribonucleoproteins, etc.

(ii) Mucoproteins : These are combined with large amount (more than 4%) of carbohydrates e.g., mucin.

(iii) Glycoproteins : In this, carbohydrate content is less (about 2 – 3%) e.g., immunoglobulins or antibiotics.

(iv) Chromoproteins : These are compounds of protein and coloured pigments. e.g., haemoglobin, cytochrome, etc.

(v) Lipoproteins : These are water soluble proteins and contain lipids. e.g., cholesterol and serum lipoproteins.

(vi) Metalloprotein : These are metal binding proteins, AB₁–globin known as transferring is capable of combining with iron, zinc and copper e.g., chlorophyll.

(vii) Phosphoprotein : They composed of protein and phosphate e.g., casein (milk) and vitellin (egg).

Derived proteins : When proteins are hydrolysed by acids, alkalies or enzymes, the degredation products obtained from them are called derived proteins.

On the basis of nature of molecules

Acidic proteins : They exist as anion and include acidic amino acids. e.g., blood groups.

Basic proteins : They exist as cations and rich in basic amino acids e.g., lysine, arginine etc. 

Function of Proteins

(i) Proteins occur as food reserves as glutelin, globulin casein in milk.  

(ii) Proteins are coagulated in solutions, alkaline to the isoelectric pH by positive ions such as \[Zn_{{}}^{2+},\,C{{d}^{2+}},\,\,H{{g}^{2+}}\] etc. Casein \[pH\,\,4.6,\text{ }\,cyt.\,\,\,C9.8,\] resum globulin 5.4, pepsin 2.7, lysozyme 11.0 etc.

(iii) Proteins are the most diverse molecule on the earth.

(iv) They are biological buffers.

(v) Monelin is the sweetest substance obtained from African berry (2000 time sweeter than sucrose).

(vi) Most abundant protein on earth is RUBP.

(vii) Myosin is structural as well as enzymatic protein (ATPase).