UPSC Chemistry Biomolecules / जैव-अणु Biomolecules

Biomolecules

1.           Carbohydrates

• Carbohydrates are primarily produced by plants and form a very large group of naturally occurring organic compounds. Some common examples are cane sugar, glucose, starch, etc.
• Some of the carbohydrates, which are sweet in taste, are also called sugars. The most common sugar, used in our homes is named as sucrose whereas the sugar present in milk is known as lactose.
• Carbohydrates are classified on the basis of their behaviour on hydrolysis. They have been broadly divided into following three groups.
• Monosaccharides: A' carbohydrate that cannot be hydrolysed further to give simpler unit of polyhydroxy aldehyde or ketone is called a monosaccharide. About 20 monosaccharides are known to occur in nature. Some common examples are glucose, fructose, ribose, etc.
• Oligosaccharides: Carbohydrates that yield two to ten monosaccharide units, on hydrolysis, are called oligosaccharides. They are farther classified as disaccharides, trisaccharides, tetrasaccharides, etc., depending upon the number of monosaccharides, they provide on hydrolysis. Amongst these the most common are disaccharides. The two monosaccharide units obtained on hydrolysis of a disaccharide may be same or different. For example, sucrose on hydrolysis gives one molecule each of glucose and fructose whereas maltose gives two molecules of glucose only.
• Polysaccharides: Carbohydrates which yield a large number of monosaccharide units on hydrolysis are called polysaccharides. Some common examples are starch, cellulose, glycogen, gums, etc. Polysaccharides are not sweet in taste, hence they are also called non-sugars.
• Glucose occurs freely in nature as well as in the combined form. It is present in sweet fruits and honey. Ripe grapes also contain glucose in large amounts.
• Glucose is an aldohexose and is also known as dextrose. It is the monomer of many of the larger carbohydrates, namely starch, cellulose. It is probably the most abundant organic compound on earth.
• Fructose is an important ketohexose. It is obtained along with glucose by the hydrolysis of disaccharide, sucrose. Another disaccharide is maltose.
• Lactose is more commonly known as milk sugar since this disaccharide is found in milk.
• Starch is the main storage polysaccharide of plants. It is the most important dietary source for human beings. High content of starch is found in cereals, roots, tubers and some vegetables. It is a polymer of \[\alpha \]-glucose and consists of two components – Amylose and Amylopectin.
• Cellulose occurs exclusively in plants and it is the most abundant organic substance in plant kingdom. It is a predominant constituent of cell wall of plant cells.
• The carbohydrates are stored in animal body as glycogen. It is also known as animal starch because its structure is similar to amylopectin and is rather more highly branched. It is present in liver, muscles and brain. When the body needs glucose, enzymes break the glycogen down to glucose. Glycogen is also found in yeast and fungi,
• Carbohydrates are essential for life in both plants and animals. They form a major portion of our food. Honey has been used for a long time as an instant source of energy by 'Vaids’ in ayurvedic system of medicine. Carbohydrates are used as storage molecules as starch in plants and glycogen in animals. Cell wall of bacteria and plants is made up of cellulose. We build furniture, etc. from cellulose in the form of wood and cloth ourselves with cellulose in the form of cotton fibre. They provide raw materials for many important industries like textiles, paper, lacquers and breweries.
• Two aldopentoses viz. D-ribose and 2-deoxy-D-ribose are present in nucleic acids. Carbohydrates are found in biosystem in combination with many proteins and lipids.

2.           Proteins and Amino Acids

• Proteins are the most abundant biomolecules of the living system. Chief sources of proteins are milk, cheese, pulses, peanuts, fish, meat, etc. They occur in every part of the body and form the fundamental basis of structure and functions of life. They are also required for growth and maintenance of body.
• The word protein is derived from Greek word, "proteios" which means primary or of prime importance. All proteins are polymers of oc-amino acids.
• Amino acids contain amino (\[-N{{H}_{2}}\]) and carboxyl (\[-COOH\]) functional groups.
• Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule. The amino acids, which can be synthesized in the body, are known as non-essential amino acids. On the other hand, those which cannot be synthesised in the body and must be obtained through diet, are known as essential amino acids.
• Protein found in a biological system with a unique three-dimensional structure and biological activity is called a native protein. When a protein in its native form, is subjected to physical change like change in temperature or chemical change like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helix get uncoiled and protein loses its biological activity. This is called denaturation of protein. During denaturation 2° and 3° structures are destroyed but \[1{}^\circ \]structure remains intact. The coagulation of egg white on boiling is a common example of denaturation. Another example is curdling of milk which is caused due to the formation of lactic acid by the bacteria present in milk.

3.           Enzymes and Mechanism of Enzyme Action

• Life is possible due to the coordination of various chemical reactions in living organisms. An example is the digestion of food, absorption of appropriate molecules and ultimately production of energy. This process involves a sequence of reactions and all these reactions occur in the body under very mild conditions. This occurs with the help of certain biocatalysts called enzymes. Almost all the enzymes are globular proteins. Enzymes are very specific for a particular reaction and for a particular substrate.
• Enzymes are needed only in small quantities for the progress of a reaction. Similar to the action of chemical catalysts, enzymes are said to reduce the magnitude of activation energy. For example, activation energy for acid hydrolysis of sucrose is \[6.22\]kJ\[mo{{l}^{-1}}\], while the activation energy is only \[2.15\]kJ \[mo{{l}^{-1}}\]when hydrolysed by the enzyme, sucrase.

4.           Vitamins

• It has been observed that certain organic compounds are required in small amounts in our diet but their deficiency causes specific diseases. These compounds are called vitamins.
• Most of the vitamins cannot be synthesised in our body but plants can synthesise almost all of them, so they are considered as essential food factors. However, the bacteria of the gut can produce some of the vitamins required by us. All the vitamins are generally available in our diet.
• Different vitamins belong to various chemical classes and it is difficult to define them on the basis of structure. They are generally regarded as organic compounds required in the diet in small amounts to perform specific biological functions for normal maintenance of optimum growth and health of the organism.
• Vitamins are designated by alphabets A, B, C, D, etc. Some of them are further named as sub-groups e.g. \[{{B}_{1}},\,{{B}_{2}},{{B}_{6}},\,{{B}_{12}},\] etc. Excess of vitamins is also harmful and vitamin pills should not be taken without the advice of doctor.
• The term 'Vitamine" was coined from the word vital + amine since the earlier identified compounds had amino groups. Later work showed that most of them did not contain amino groups, so the letter 'e' was dropped and the term vitamin is used these days.
• Vitamins are classified into two groups depending upon their solubility in water or fat.
• Fat soluble vitamins: Vitamins which are soluble in fat and oils but insoluble in water are kept in this group. These are vitamins A, D, E and K. They are stored in liver and adipose (fat storing) tissues.
• Water soluble vitamins: B group vitamins and vitamin C are soluble in water so they are grouped together. Water soluble vitamins must be supplied regularly in diet because they are readily excreted in urine and cannot be stored (except vitamin \[{{B}_{12}}\]) in our body.
• Vitamin A source is Fish liver oil, carrots, butter and milk deficiency causes Xerophthalmia (hardening of cornea of eye) Night blindness.
• Vitamin \[{{B}_{1}}\] (Thiamine) source is Yeast, milk, green vegetables and cereals Milk, eggwhite, liver, kidney deficiency causes Beri beri (loss of appetite, retarded growth).
• Vitamin \[{{B}_{2}}\] (Riboflavin) source is Milk, egg white, liver, kidney deficiency causes Cheilosis (fissuring at comers of mouth and lips), digestive disorders and burning sensation of the skin.
• Vitamin \[{{B}_{6}}\](Pyridoxine) source is Yeast, milk, egg yolk, cereals and grams deficiency causes Convulsions.
• Vitamin \[{{B}_{12}}\]source is Meat, fish, egg and curd deficiency causes Pernicious anaemia (RBC deficient in haemoglobin).
• Vitamin C (Ascorbic acid) source is Citrus fruits, amia and green leafy vegetables deficiency causes Scurvy (bleeding gums).
• Vitamin D source is Exposure to sunlight, fish and egg yolk deficiency causes Rickets (bone deformities in children) and osteomalacia (soft bones and joint pain in adults).
• Vitamin E source is Vegetable oils like wheat germ oil, sunflower oil, etc. deficiency causes Increased fragility of RBCs and muscular weakness.
• Vitamin K source is Green leafy vegetables deficiency causes Increased blood clotting time.

5.           Nucleic Acids

• Every generation of each and every species resembles its ancestors in many ways. How are these characteristics transmitted from one generation to the next? It has been observed that nucleus of a living cell is responsible for this transmission of inherent characters, also called heredity.
• The particles in nucleus of the cell, responsible for heredity, are called chromosomes which are made up of proteins and another type of biomolecules called nucleic acids.
• These are mainly of two types, the deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Since nucleic acids are long chain polymers of nucleotides, so they are also called polynucleotides.
• Complete hydrolysis of DNA (or RNA) yields a pentose sugar, phosphoric acid and nitrogen containing heterocyclic compounds (called bases). In DNA molecules, the sugar moiety is \[\beta \]-D-2-deoxyribose whereas in RNA molecule, it is \[\beta \]-D-ribose.
• DNA contains four bases viz. adenine (A), guanine (G), cytosine (C) and thymine (T). RNA also contains four bases, the first three bases are same as in DNA but the fourth one is uracil (U).
• James Dewey Watson is best known for his discovery of the structure of DNA for which he shared with Francis Crick and Maurice Wilkins the 1962 Nobel prize in Physiology and Medicine. They proposed that DNA molecule takes the shape of a double helix, an elegantly simple structure that resembles a gently twisted ladder. The rails of the ladder are made of alternating units of phosphate and the sugar deoxyribose; the rungs are each composed of a pair of purine/pyrimidine bases. This research laid the foundation for the emerging field of molecular biology. The complementary pairing of nucleotide bases explains how identical copies of parental DNA pass on to two daughter cells. This research launched a revolution in biology that led to modem recombinant DNA techniques.
• Information regarding the sequence of nucleotides in the chain of a nucleic acid is called its primary structure. Nucleic acids have a secondary structure also. James Watson and Francis Crick gave a double strand helix structure for DNA. Two nucleic acid chains are wound about each other and held together by hydrogen bonds between pairs of bases. The two strands are complementary to each other because the hydrogen bonds are formed between specific pairs of bases. Adenine forms hydrogen bonds with thymine whereas cytosine forms hydrogen bonds with guanine.
• In secondary structure of RNA, helices are present which are only single stranded. Sometimes they fold back on themselves to form a double helix structure. RNA molecules are of three types and they perform different functions. They are named as messenger RNA (m-RNA), ribosomal RNA (r-RNA) and transfer RNA (t-RNA).
• Har Gobind Khorana, was born in 1922. He obtained his M.Sc. degree from Punjab University in Lahore. He worked with Professor Vladimir Prelog, who moulded Khorana's thought and philosophy towards science, work and effort. After a brief stay in India in 1949, Khorana went back to England and worked with Professor G.W. Kenner and Professor A.R.Todd. It was at Cambridge, U.K. that he got interested in both proteins and nucleic acids. Dr Khorana shared the Nobel Prize for Medicine and Physiology in 1968 with Marshall Nirenberg and Robert Holley for cracking the genetic code.
• DNA is the chemical basis of heredity and may be regarded as the reserve of genetic information. DNA is exclusively responsible for maintaining the identity of different species of organisms over millions of years. A DNA molecule is capable of self-duplication during cell division and identical DNA strands are transferred to daughter cells.
• Another important function of nucleic acids is the protein synthesis in the cell. Actually, the proteins are synthesised by various RNA molecules in the cell but the message for the synthesis of a particular protein is present in DNA.

6.           DNA Fingerprinting

• It is known that every individual has unique fingerprints. These occur at the tips of the fingers and have been used for identification for a long time but these can be altered by surgery. A sequence of bases on DNA is also unique for a person and information regarding this is called DNA fingerprinting. It is same for every cell and cannot be altered by any known treatment. DNA fingerprinting is now used in forensic laboratories for identification of criminals, to determine paternity of an individual, to identify the dead bodies in any accident by comparing the DNA's of parents or children, to identify racial groups to rewrite biological evolution.