NEET Biology Biomolecules Notes

Biomolecules are the building blocks of every living cell, and they form a core part of NEET Biology. These notes on Biomolecules cover carbohydrates, lipids, proteins, amino acids, nucleic acids (DNA and RNA), enzymes and their mechanism of action, minerals and vitamins, with the important tests, deficiency disorders, comparisons and check point questions with answers.

Terms and definitions

1. Cellobiose
2. Z-DNA
3. Benedict test
4. PUFA
5. Pyrimidines
6. Feulgen test
7. Blood sugar
8. β-oxidation
9. Kwashiorkor
10. Phosphodiester bond

The chapter also covers the tests for carbohydrates, lipids and proteins, the compounds (inulin, tryptophan, protoglycan, cholesterol, scleroprotein, glycine, arachidonic acid, chitin), and the differences between HDL and LDL, reducing and non-reducing sugars, saturated and unsaturated fatty acids, essential and non-essential amino acids, first and second class proteins, nucleosides and nucleotides, and DNA and RNA.

Tests: 1. Benedict test 2. Biuret test 3. Sudan III test 4. Feulgen test.

At a Glance

• Carbohydrates
  • Monosaccharides
  • Oligosaccharides
  • Polysaccharides
  • Conjugated or complex carbohydrates
• Lipids
  • Simple lipids (homolipids)
  • Compound lipids (heterolipids)
  • Derived lipids
• Fatty acids
• Fat metabolic disorders
• Proteins
• Amino acids
  • Standard amino acids
  • Non-standard amino acids
• Deoxyribonucleic acid (DNA)
  • Nucleosides
  • Nucleotides
• Ribonucleic acid (RNA)
• Enzymes
  • Properties of enzymes
  • Enzymes versus inorganic catalysts
  • Important terms
  • Structure of enzymes
  • Theories about enzymatic action
  • Enzymatic action
  • Inhibition of enzymatic action
  • Classification of enzymes
• Minerals
  • Macro elements
  • Micro elements
• Vitamins
  • Fat soluble vitamins
  • Vitamin A
  • Vitamin D
  • Vitamin E
  • Vitamin K
  • Water soluble vitamins
  • Vitamin B
  • Vitamin C

BIOMOLECULES: ELEMENTS AND COMPOUNDS OF THE CELL

The most abundant element in protoplasm is oxygen. Oxygen, carbon and hydrogen together form 93% of the cellular pool. About 98% of the mass of every living organism is composed of just six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur.

The maximum compound in the cellular pool is water, while the maximum (principal) organic compound is protein. The ratio of inorganic to organic compounds in the cell is 1 : 9.

CARBOHYDRATES

Carbohydrates are micromolecules (monosaccharides and oligosaccharides, including disaccharides) with a molecular weight below 1000 Daltons, or macromolecules (polysaccharides) with a molecular weight above 1000 Daltons.

The average carbohydrate requirement of an adult is about 400 g per day, which provides about 50 to 70% of the total energy. The essential elements in carbohydrates are C, H and O. The general formula is C_x(H₂O)_y, and they contain more than one - OH group. Polyhydroxy aldehydes or ketones can also be called saccharides. About 80% of the dry weight of a plant is carbohydrate.

Carbohydrates are divided into three classes: (1) monosaccharides, (2) oligosaccharides and (3) polysaccharides.

(1) Monosaccharides are the simplest carbohydrates, with 3 to 7 carbon atoms. All are reducing sugars with a free aldehyde (-CHO) or ketone (-CO) group.
(i) 3 carbons: trioses (C₃H₆O₃), e.g. glyceraldehyde (aldose) and dihydroxyacetone (ketose). Acetic acid (CH₃COOH) and lactic acid are not considered carbohydrates.
(ii) 4 carbons: tetroses (C₄H₈O₄), e.g. erythrose, an aldose, which is the raw material for lignin.
(iii) 5 carbons: pentoses (C₅H₁₀O₅), e.g. ribose (present in RNA, ATP and vitamin B₂), xylose and arabinose (all aldoses), and ribulose (a ketose).
(iv) 6 carbons: hexoses (C₆H₁₂O₆), e.g. glucose, galactose and mannose (all aldoses), and fructose (a ketose). The alcohol of mannose, called mannitol, is found in brown algae.
(v) 7 carbons: heptoses (C₇H₁₄O₇), e.g. glucoheptose.
Both pentoses and hexoses may occur in ring form and open-chain form.

Glucose is called blood sugar or grape sugar, and occurs in two forms, the D-form (dextro) and the L-form (levo). All naturally occurring sugars are in the D-form. Glucose is an aldose sugar.
Fructose is called fruit sugar. It is the most common sugar in plants and the sweetest among naturally occurring sugars. It is a ketose sugar.

(2) Oligosaccharides contain 2 to 10 monosaccharide molecules.
(A) Disaccharides contain two monosaccharides:
(i) Maltose: malt sugar, formed during the germination of starchy seeds. It is a reducing sugar. It contains two α-glucose units joined by an α-1,4 glycosidic bond.

(ii) Lactose: milk sugar, also a reducing sugar. It contains one α-glucose and one β-galactose, joined by a β-1,4 glycosidic bond. Lactose is maximum in human milk. The galactose produced from milk digestion is also a constituent of agar-agar.
(iii) Sucrose: the sugar of sugar cane and sugar beet. It is a non-reducing sugar, because it has no free aldehyde or ketone group. It contains one α-glucose and one β-fructose unit (α-1,2 glycosidic linkage). An equimolecular mixture of glucose and fructose is called invert sugar, which is sweeter than sucrose.
(iv) Trehalose: an α-1,1 linked disaccharide present in micro-organisms. It is also a non-reducing sugar.
(v) Cellobiose: contains two β-glucose units (β-1,4 linkage). It cannot be digested by mammalian enzymes.
(B) Trisaccharides contain three monosaccharides, e.g. raffinose (one glucose, one fructose and one galactose).
(C) Dextrin is also an oligosaccharide, formed during the digestion of starch.

(3) Polysaccharides are formed by joining monosaccharide monomers through glycosidic bonds between C-1 and C-4 carbon atoms. In a polysaccharide chain, the right end is called the reducing end and the left end is called the non-reducing end. They are non-reducing and mostly insoluble in water. There are two types:
1. Storage polysaccharides, e.g. glycogen and starch.
2. Structural polysaccharides, e.g. cellulose and chitin.

Glycogen is present in animals and is called animal starch. It is a branched-chain compound with about 30 α-glucose units. It gives a red colour with iodine solution.
Starch is present in plants. Natural starch is a mixture of amylose (10 to 20%) and amylopectin (80 to 90%). Amylopectin is branched and insoluble in water. Both contain only α-glucose. Amylose is responsible for the deep blue colour with iodine.
Cellulose contains only β-glucose. It is the most abundant organic compound in the biosphere. It is a fibrous polysaccharide that forms the cell wall of plants and is roughage in human food. It is a straight-chain (unbranched) compound. It forms 25 to 50% of wood and about 90% of cotton. Each molecule of cellulose contains about 6000 glucose units, so it is a homoglycan hexosan. Rayon, an artificial and regenerated fibre, is produced from cellulose.
Chitin forms the exoskeleton, mainly of arthropods, and is also present in the cell wall of fungi. Its unit is β-N-acetylglucosamine. It is a homopolymer.

The polysaccharide agar has more than one type of monosaccharide unit, so it is a heteroglycan. The polysaccharide inulin (dahlia starch) is a polymer of fructose (homoglycan) with β-1,2 linkage.

Connecting concepts: In ruminants (e.g. cows and sheep), cellulose is digested by bacteria and protozoans in the stomach. In other herbivores (e.g. rabbits), it is digested by bacteria in the large intestine. Termites digest the cellulose of wood with the help of Trichonympha, a symbiotic protozoan.

Conjugated or complex carbohydrates contain carbohydrates combined with non-carbohydrate units:
1. Glycoproteins, e.g. blood antigens, collagen, lens protein, blood proteins, and hormones such as FSH, LH, TSH and hCG, and cell membranes.
2. Glycolipids, e.g. blood antigens, nerve fibres and cell membranes.
3. Mucopolysaccharides, e.g. heparin, hyaluronic acid, synovial fluid, vitreous humour, the cell wall of bacteria, and mucilages (galactose and mannose). They are also present in bhindi (lady's finger) and isabgol.

Tests for carbohydrates: Benedict's test and Fehling's test (glucose test).

LIPIDS

The term 'lipid' was given by Bloor (1943) for fats and fat-like substances. Lipids are non-polymers and micromolecules. They provide about 30% of the calories in the daily diet. The requirement is about 50 g in a daily diet for an adult of 60 kg. Lipogenesis (synthesis of lipids) occurs in the liver and adipose tissue. Excess fat in the diet is harmful. Fat prevents the rapid emptying of the stomach. A rise of blood fat after a meal is called lipaemia. The essential constituents are C, H and O. Lipids are of three classes: (1) simple lipids, (2) compound lipids and (3) derived lipids.

(1) Simple lipids (homolipids) are esters of fatty acids with various alcohols.
a. Neutral fats (e.g. oil, butter, margarine, ghee) are esters of fatty acids with glycerol. Three fatty acids are attached to the three carbon atoms of glycerol (forming triglyceride) by ester linkage, which is a weak bond and is broken on heating. The specific gravity of solid fat is 0.86, which is why fatty people float easily in water. Fats that are liquid at room temperature (20 °C) are called oils, e.g. groundnut oil, mustard oil, til oil and safflower oil. Oils contain unsaturated fatty acids, which on hydrogenation form saturated fatty acids or ghee. Neutral fats are straight-chain compounds.
[Figure: Fig. 9.2, Triglyceride (neutral fat) molecule, with glycerol as the head and three fatty acids as the tails.]
[Figure: Fig. 9.3, Structure of a triglyceride formed from one glycerol (1-glycerol) and three fatty acids (3-fatty acids), joined by ester linkages to form 1 triglyceride.]

b. Waxes (e.g. paraffin, beeswax, sebum) are esters of fatty acids with alcohols higher than glycerol. The alcohol in waxes has only one hydroxyl group, compared with three in glycerol. All are straight-chain compounds. They form a waterproof coating on skin, hair, leaves and fruits. Bacteria that cause leprosy and tuberculosis also produce a wax (wax-D) that contributes to their pathogenicity. Beeswax (hexacosyl palmitate), produced by honey bees, is a combination of palmitic acid (a fatty acid) with hexacosanol (an alcohol). Sebum is a mixture of waxes and triacylglycerols. It prevents the growth of certain bacteria on the skin and prevents the loss of water from the skin.
c. Sterols (e.g. ergosterol and cholesterol) have a four-ring structure (three cyclohexane rings and one cyclopentane ring). Cholesterol is found only in animals, and ergosterol is found in plants. Cholesterol is the precursor of steroid hormones (e.g. testosterone, progesterone and cortisol) and bile salts (of cholic acid). It is abundant in nerve tissue and gallstones. Sterols are not straight-chain compounds.
[Figure: Fig. 9.4, Ring structure of a steroid.]

(2) Compound lipids (heterolipids) have a non-lipid part in addition to fatty acids and alcohol. They are also called conjugated lipids.
A. Phospholipids, e.g. lecithin (present in cell membranes and detergents). They are built like a neutral fat, except that in place of the third fatty acid there is a phosphate group. The phosphate group with glycerol forms the polar head (hydrophilic), while the hydrocarbon chains of fatty acids form the non-polar tails (hydrophobic). In water, phospholipid molecules form a double layer (bilayer) in which the polar heads face outward and the non-polar tails face each other. This property separates two solutions, and it is also the basis of the plasma membrane of the cell.
A monolayer of phospholipid reduces the surface tension of water. This is important to inflate the alveoli. Children who cannot produce phospholipid (lecithin, here called surfactant) in their alveoli at birth suffer from respiratory distress syndrome, and they need a lot of effort to overcome the surface tension of water on the alveoli. Such infants are kept in ICU until they start producing lecithin.
[Figure: Fig. 9.5, Phospholipid bilayer, with the hydrated heads on the outer surfaces of each layer and the hydrocarbon chains of one layer attracted to those of the other layer, between two layers of protein.]
B. Sphingolipids are present in cell membranes and are comparable to phospholipids, except that sphingosine is present instead of glycerol. Each sphingolipid has a hydrophilic (water-soluble) head and two hydrocarbon tails, one longer and one shorter.
C. Glycolipids are similar to sphingolipids, except that an oligosaccharide (of glucose or galactose) is bonded to sphingosine instead of phosphoric acid. The oligosaccharide head is water soluble, so their properties are similar to sphingolipids. Example: A and B antigens in the ABO blood group system. Galactocerebroside is the main glycolipid in the myelin sheath.
[Figure: Fig. (sphingolipid and glycolipid structures), showing sphingosine with a phosphorylated head for sphingolipids and an oligosaccharide of galactose or glucose as the head for glycolipids, with hydrocarbon chains in each.]

(3) Derived lipids are formed by the hydrolysis of fats, e.g. monoglycerides, diglycerides, steroids, terpenes and carotenoids.

FATTY ACIDS

Fatty acids usually contain an even number of carbon atoms (e.g. 14, 16 and 18 C).
a. Unsaturated fatty acids have one or more double bonds between carbon atoms. Examples: oleic acid (one double bond), linoleic acid (two), linolenic acid (three) and arachidonic acid (four). The first three are 18-carbon compounds, whereas arachidonic acid is a 20-carbon compound. Prostaglandins, which take part in allergy and inflammatory reactions, are derivatives of arachidonic acid. Unsaturated fatty acid chains bend at the double bonds and do not fit closely together. This lowers the melting point, so they are liquid at room temperature, e.g. oils.
b. Saturated fatty acids have no double bonds between carbon atoms, e.g. acetic acid, propionic acid, butyric acid, palmitic acid and stearic acid.
Essential fatty acids are not synthesised in the body. All are polyunsaturated (more than one double bond), so they are called PUFA (polyunsaturated fatty acids). Examples: linoleic acid (in corn, pea and cotton) and linolenic acid (in linseed oil and groundnut oil). Margarine has a high content of polyunsaturates. Mustard oil is one of the most unsaturated oils, sunflower oil is rich in linoleic acid, and coconut oil has the minimum fatty acid content. Arachidonic acid can, however, be synthesised in the body from linolenic acid. Human body fat contains about 50% oleic acid, 25% palmitic acid, 10% linoleic acid and 5% stearic acid.

Check point: Differentiate between

1. Phospholipid and glycolipid
2. Oil and ghee
3. Sterols and waxes
4. Saturated and unsaturated fatty acids
5. Essential and non-essential fatty acids
6. Chitin and keratin
7. Aldose and ketose
8. First class and second class proteins
9. Melting and annealing of DNA
10. Kwashiorkar and marasmus
11. Achiral and chiral amino acids
12. Good and bad cholesterol

Connecting concepts (omega numbering): The carbon atoms of fatty acids are numbered from the carboxyl (-COOH) group. They may also be numbered from the methyl end, as omega (ω) 1, 2, 3 and so on.
Among unsaturated fatty acids there are three omega families:
1. ω3 family, e.g. linolenic acid.
2. ω6 family, e.g. linoleic acid and arachidonic acid. (The source lists this family as ω5.)
3. ω9 family, e.g. oleic acid.
Linolenic acid, an 18-carbon compound, belongs to the ω3 family because its first double bond begins at the ω3 carbon. Omega-3 unsaturated fatty acids, which are abundant in fish oil, have great nutritional value and are required for the proper mental growth, especially of the grey matter.

Tests for lipids: (1) Grease spot test; (2) Sudan III (black) or Sudan red test.

Functions of lipids: poor conductor of heat (for insulation), shock absorber, and food storage (its primary function).

Metabolism of fats: β-oxidation is the conversion of fatty acids into acetyl CoA. Fatty acids, through acetyl CoA, enter the Krebs cycle. Glycerol enters the Krebs cycle at PGAL. Oxidation of fatty acids requires more O₂ (R.Q. = 0.7). In a shortage of carbohydrates or oxygen, ketone bodies are formed.

Problem: How many ATPs are produced in β-oxidation and complete oxidation of palmitic acid (16 C)?

Answer:
1. Two ATPs are used in converting palmitic acid into palmitoyl CoA.
2. One mole of palmitoyl CoA (16 C) produces 8 moles of acetyl CoA (2 C), and 7 cleavages give 7 molecules each of FADH₂ and NADH.
3. Each FADH₂ produces 2 ATPs and each NADH produces 3 ATPs.
4. Total ATPs = (2 × 7) + (3 × 7) = 14 + 21 = 35 ATPs.
5. Net ATPs in β-oxidation = 35 − 2 = 33 ATPs.
6. Short cut: [(number of cleavages × 5) − 2], where the number of cleavages is one less than half the number of carbon atoms in the fatty acid.
7. On complete oxidation, each acetyl CoA produces 12 ATPs, so net production = 33 + (12 × 8) = 33 + 96 = 129 ATPs.

Fat metabolic disorders:
1. Gaucher's disease: autosomal recessive. The liver and spleen are enlarged. Erosion of long bones and mental retardation occur.
2. Tay-Sachs disease (infantile amaurotic idiocy): autosomal recessive. Mental retardation, blindness and death by the age of 3 years.

PROTEINS

The term 'protein' was given by Berzelius and Mulder. Proteins are heteropolymers (homopolymers have only one type of monomer). They provide about 15% of the calories in the daily diet, with a requirement of 1 g of protein per kg of body weight (60 g for a 60 kg person). Collagen is the most abundant protein of vertebrates (more than 30%). Proteins are the most diverse macromolecules in living organisms. They contain C, H, O and N, and some contain S and P. The structural unit of protein is the amino acid. Proteins are polymers of more than 100 amino acids, with one or more polypeptide chains. They are synthesised in the body from 20 types of amino acids.

Among proteins, collagen is the most abundant protein of the animal world, while RubisCO (ribulose bisphosphate carboxylase-oxygenase) is the most abundant protein of the biosphere.

First class proteins have high biological value and contain all essential amino acids, e.g. animal proteins such as milk, egg, fish and mutton.
Second class proteins have low biological value and are deficient in one or more essential amino acids, e.g. vegetable proteins such as gluten in wheat (lysine absent) and zein in maize (lysine and tryptophan absent). Soyabean and rice, however, have first class protein.

Structure of proteins: Proteins may have primary, secondary, tertiary or quaternary structures.
1. Primary structure: Only peptide bonds are present between amino acids. The first (left) amino acid is the N-terminal amino acid (-NH₂), and the last (right) amino acid is the C-terminal amino acid (-COOH group). Such proteins are not functional.
[Figure: Fig. 9.6, Peptide or amide bond formed by loss of water between the - COOH group of one amino acid and the - NH₂ group of the next.]
2. Secondary structure: Besides peptide bonds, hydrogen bonds are also present in the polypeptide chain. Such proteins may have helical or pleated structures.
(A) α-helix (right-handed screw): the most common form, also known as the 3.6¹³ helix, since each turn has about 3.6 amino acids and a 13-membered ring is formed by H-bonding. All amino acids in such a chain are in the L-configuration, so the helix is stable only if it is right-handed. Example: keratin in the exoskeleton of mammals.
(B) Pleated structure: polypeptide chains are held side by side by H-bonds, resembling the pleated folds of drapery. This is called a β-pleated sheet. The chains may be parallel or antiparallel. Example: keratin in birds (parallel β-sheets) and silk protein (fibroin), with antiparallel β-sheets.
3. Tertiary structure: Further folding and coiling, stabilised by S - S (disulphide) bonds, hydrophobic bonds and ionic bonds. These are three-dimensional structures with two major shapes. Fibrous proteins: silk, collagen, elastin and keratin. Globular proteins: albumin, globulin and protamine.
4. Quaternary structure: More than one polypeptide chain forms a large multiunit protein, e.g. haemoglobin.
Peptide bonds and disulphide bonds are strong bonds, whereas hydrogen bonds and hydrophobic bonds are weak bonds.

Types of proteins:

1. Simple proteins are composed of amino acids only. They are soluble or insoluble in water, and structurally they are of two types: globular and fibrous. Simple proteins include albumins, globulins, protamines, prolamines, lectins and scleroproteins. Albumins and protamines are water soluble. Albumins are coagulated by heat, while protamines are not. Protamines contain a large number of arginine and lysine amino acids, so they are strongly basic. Prolamines are rich in proline but lack lysine. Fibrous proteins (scleroproteins) include collagen and keratin.

2. Conjugated proteins are formed by the binding of a simple protein with a non-protein part, the prosthetic group. They are classified on the basis of the prosthetic group:
(i) Glycoproteins: proteins combined with carbohydrates. When the carbohydrate content is more than 10%, the viscosity increases and such proteins are called proteoglycans or mucoproteins. Examples: hormones FSH, LH, TSH and hCG, blood group antigens, and serum proteins.
(ii) Lipoproteins: proteins combined with lipids, e.g. cell membranes, chylomicrons, LDL (low density lipoprotein) and HDL (high density lipoprotein). Cholesterol is carried in soluble form as HDL and LDL. HDL is 'good cholesterol', with a normal value of 30 to 70 mg/100 ml in human plasma (below 30 mg/100 ml is risky for cardiovascular problems). LDL is 'bad cholesterol', with a normal value of 80 to 170 mg/100 ml (more than 160 mg/100 ml is risky).
(iii) Nucleoproteins: proteins attached to nucleic acids, e.g. histones in chromosomes. Histones are basic proteins, rich in lysine and arginine.
(iv) Phosphoproteins: proteins containing phosphorus as the prosthetic group, e.g. casein of milk and vitellin of egg yolk.
(v) Metalloproteins: proteins with metal ions, e.g. tyrosinase (copper), carbonic anhydrase (zinc), cytochrome (iron), insulin (zinc) and trypsin.
(vi) Chromoproteins: proteins with a pigment or coloured prosthetic group, e.g. flavoproteins (yellow), haemoglobin (red) and visual pigment (purple).

3. Derived proteins are produced during the hydrolysis of proteins, e.g. proteoses and peptones.

Tests for proteins:
(1) Biuret test: NaOH and CuSO₄ solutions give a reddish-violet colour with proteins. It is a test for the peptide link (CO - NH) and is the test for all proteins, peptones and peptides.
(2) Xanthoproteic test: concentrated HNO₃ gives a yellow colour with proteins. It is a test for proteins having at least one aromatic amino acid.
(3) Millon's test is also used for proteins.

Functions of proteins: formation of cells and tissues for growth; repair of tissues; formation of hormones; muscle contraction (actin and myosin); formation of enzymes; blood clotting; transport (haemoglobin, transferrin); defence against infection (antibodies); formation of hereditary material (nucleoproteins); storage (myoglobin and ferritin); and support (collagen and elastin).

Protein deficiency disorders:
(a) Kwashiorkor: occurs in children below 5 years of age, due to protein deficiency. Symptoms include tissue oedema (pot-bellied), dark skin, reddish hair and mental retardation.
(b) Marasmus: due to protein and calorie deficiency. There is no oedema and no dark pigmentation, but muscle wasting occurs. It develops in children aged 1 to 2 years, and it is primarily a starvation disease.
Both kwashiorkor and marasmus are PEM (protein energy malnutrition) diseases.

AMINO ACIDS

Amino acids are the structural units of proteins and the digestive end products of proteins. Chemically they are substituted methanes. All contain at least one amino group and one carboxylic group. Most amino acids are amphoteric in nature. The ionisable nature of the - NH₂ and - COOH groups into NH₃⁺ and COO⁻ makes the amino acid a zwitterion.

Amino acids are of two types:
Standard amino acids are the α-amino acids that participate in protein formation, and they are of the L-form. Non-standard amino acids do not form proteins, but they take part in many important metabolic reactions. There are 20 standard amino acids. Except glycine, all are levorotatory.

A. Essential amino acids cannot be synthesised in the body and must be present in the daily diet. There are ten, of which two are semi-essential: arginine (essential only in infants and children) and histidine. The other eight are very essential: isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Tryptophan is the most complex amino acid and has an indole ring.

B. Non-essential amino acids can be synthesised in the body. There are ten: alanine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine and tyrosine. Aspartic acid and glutamic acid are acidic amino acids, with two carboxylic groups and one amino group. Asparagine and glutamine are their amides. Arginine, lysine and histidine are basic amino acids, with two amino groups and one carboxylic group.

The 20 amino acids can also be classified on the basis of the side chain R:
(i) Hydrophilic or polar amino acids: asparagine, cysteine, glutamine, glycine, serine, threonine and tyrosine.
(ii) Hydrophobic or non-polar amino acids: alanine, isoleucine, leucine, methionine, tryptophan, proline, phenylalanine and valine.
(iii) Acidic amino acids: aspartate (aspartic acid) and glutamate (glutamic acid).
(iv) Basic amino acids: arginine, histidine and lysine. Arginine is the most basic, and histidine is the least basic.

Glycine is the simplest amino acid. It is sweet in taste and also acts as a neurotransmitter. Its side chain is a hydrogen atom, not an alkyl group. Because of its two identical hydrogen atoms, it is symmetrical and achiral. All other amino acids are asymmetrical (chiral) and can exist as D- and L-stereoisomers. Phenylalanine, tyrosine and tryptophan contain a benzene ring and are aromatic amino acids (tyrosine is formed in the body from phenylalanine). Proline is a cyclic amino acid, also called an imino acid. Serine and threonine are hydroxy amino acids. Cysteine and methionine contain sulphur.

Amino acids are joined by a - CO - NH - bond, called a peptide or amide bond. More than 200 amino acids are known in nature.

Important peptides:
- Glutathione: a tripeptide of glycine, glutamic acid and cysteine.
- Oxytocin and vasopressin: each has nine amino acids.
- Bradykinin: has nine amino acids. It is the most potent pain-producing substance and mediates prostaglandins.
- Angiotensin: angiotensin-I has 10 amino acids and angiotensin-II has 8 amino acids. Both are vasoconstrictors.
Non-standard amino acids include ornithine, histamine, citrulline and thyroxine.

Check point 1: Name the following

(i) Most abundant protein in the body.
Answer: Collagen.

(ii) Blood protein with quaternary structure.
Answer: Haemoglobin.

(iii) Semi-essential amino acids.
Answer: Arginine and histidine.

(iv) Acidic amino acids.
Answer: Aspartic acid (aspartate) and glutamic acid (glutamate).

(v) Metal ion in tyrosinase.
Answer: Copper.

(vi) Imino acid.
Answer: Proline.

(vii) Two aromatic amino acids.
Answer: Phenylalanine and tyrosine (tryptophan is also aromatic).

(viii) High energy bond in ATP.
Answer: The second and third phosphate bonds, which are high-energy phosphate (phosphoanhydride) bonds.

(ix) RNA with clover leaf structure.
Answer: t-RNA (transfer RNA).

(x) Left-handed DNA.
Answer: Z-DNA.

NUCLEIC ACIDS (DNA AND RNA)

Nucleic acids are informational molecules of universal occurrence. They are present in all cells and viruses. They are polymers of nucleotides, and hence macromolecules. On hydrolysis, a nucleic acid yields one pentose sugar, two types of heterocyclic nitrogenous bases (purines and pyrimidines), and phosphoric acid.

Purines: adenine and guanine (in both DNA and RNA).
Pyrimidines: cytosine and thymine (in DNA); cytosine and uracil (in RNA).

[Figure: Fig. 9.7, Purine ring (with adenine and guanine structures) and pyrimidine ring (with cytosine, thymine and uracil structures), with the carbon and nitrogen positions numbered.]

Differences between DNA and RNA:

[Figure: Fig. 9.8, Sugars in nucleic acids: ribose (with a 2'-OH group) and deoxyribose (with a 2'-H group).]

Deoxyribonucleic acid (DNA)

Mitochondrial DNA (mtDNA) is circular and contains about 16,500 nucleotides. It codes for 2 rRNA and 22 tRNA, and it is inherited from the mother through the female gamete. It has a low mutation rate but shows no genetic recombination. Most DNA is localised in the nucleus. A small amount of DNA is also present in mitochondria and chloroplasts.

Nucleosides: A combination of a pentose sugar with a nitrogenous base (purine or pyrimidine) is called a nucleoside.
- Purine nucleosides: adenosine (adenine), guanosine (guanine).
- Pyrimidine nucleosides: cytidine (cytosine), thymidine (thymine, in DNA) and uridine (uracil, in RNA).

Nucleotides: A phosphate ester of a nucleoside is called a nucleotide. Each nucleotide has a nitrogenous base, a pentose sugar and one or more phosphate groups.
- Purine nucleotides: adenosine + 1 phosphate = AMP (adenylic acid); adenosine + 2 phosphates = ADP; adenosine + 3 phosphates = ATP (adenosine triphosphate); guanosine + 1 phosphate = GMP (guanylic acid).
- Pyrimidine nucleotides: cytidine + 1 phosphate = CMP (cytidylic acid); cytidine + 2 phosphates = CDP; cytidine + 3 phosphates = CTP; thymidine + 1 phosphate = thymidine monophosphate (thymidylic acid); uridine + 1 phosphate = uridine monophosphate (uridylic acid).
With deoxyribose sugar (in DNA), the names are preceded by 'd-', for example dAMP (deoxyadenosine monophosphate) and dTDP (deoxythymidine diphosphate).
Nucleic acids (DNA and RNA) are polymers of nucleotides (nucleoside monophosphates). Nucleotides are acids and are negatively charged at neutral pH. The carbons of the pentose sugar are primed (1', 2', 3' ...) to distinguish them from the carbons of the nitrogenous bases.

Connecting concepts (ATP): ATP (adenosine triphosphate) is also a nucleotide. It contains one adenine base, one ribose sugar and three phosphate groups. It is an energy-rich compound and is called the 'energy currency'. Its second and third phosphate bonds are energy-rich bonds, each releasing 31 kJ of energy per mole.
[Figure: Fig. 9.9, ATP molecule showing adenine, ribose and the three phosphate groups (I, II and III), with bonds II and III marked as energy-rich.]

Nucleotides are joined together by phosphodiester linkages between the 5' and 3' carbon atoms of the pentose sugars. The chain of a nucleic acid is abbreviated from the 5' end to the 3' end, in left-to-right order.

[Figure: Fig. 9.10, DNA double helical structure. Two antiparallel strands with sugar-phosphate backbones joined by phosphodiester bonds, with nitrogenous bases (A, T, G, C) in the middle linked by hydrogen bonds, one complete turn with 10 base pairs = 3.4 nm, a diameter of 2 nm, the major groove and the minor groove marked, and the 5' and 3' ends labelled on each strand.]

Edwin Chargaff reported that the amount of adenine was equal to thymine (A = T), and the amount of guanine was equal to cytosine (G = C). This means that the total number of purines equals the total number of pyrimidines (A + G = T + C). As the base composition of DNA varies between species, the AT/CG ratio also varies from species to species. The AT/CG ratio is 1.52 in humans and 0.93 in E. coli.

Double helical structure of DNA: To explain base equivalence (A/T, G/C) and other properties of DNA, Watson and Crick (1953), based on X-ray diffraction studies, proposed the double helical structure of DNA. It has two right-handed helical polynucleotide chains (strands) around a central axis, resembling a spiral staircase. The two strands are antiparallel, meaning that the 5' to 3' phosphodiester bonds run in opposite directions, so the 5' end of one strand is opposite the 3' end of the other. The bases are held like the steps (rungs) of a spiral staircase by hydrogen bonds. The sugar-phosphate groups form the rails, or sides, of the ladder.
- Between A and T there are 2 hydrogen bonds (A = T).
- Between C and G there are 3 hydrogen bonds (C ≡ G). The C ≡ G pair is more stable than the A = T pair.
- The diameter of the double helix is 2 nm (20 Å).
- The distance from one base pair to the next (step or rise) is 0.34 nm (3.4 Å).
- The number of base pairs in one complete turn (pitch) is 10 (in B-type DNA).
- The distance covered in 10 steps, or one complete turn (pitch), is 3.4 nm (34 Å).

Types of DNA: On the basis of the number of base pairs per turn and the handedness of the helix, DNA has more than 12 forms. The main forms are:

Melting and annealing of DNA: On heating, the two strands of DNA separate from each other (melting). On cooling, they hybridise again (annealing). The temperature at which the two strands separate completely is called the melting temperature (T_m), which is specific for each sequence. If a sample of DNA has a higher T_m, it has more C ≡ G pairs, which have stronger bonds (three hydrogen bonds) and are more difficult to break.

Ribonucleic acid (RNA) has a single helical structure and is of three main types.

m-RNA (messenger RNA): about 5% of total RNA. Its base sequence is complementary to one DNA strand. Its length varies with the length of the polynucleotide used. It generally exists for a short time, with a life span of 1 to 4 minutes (the shortest). It has more than 1500 nucleotides, so it is the longest RNA.
r-RNA (ribosomal RNA): about 80% of total RNA. Its base sequence is similar in all organisms. It is synthesised in the nucleolar organiser region (NOR) of the DNA. Its life span is several days (the longest), and it is the most stable RNA. The enzyme ribozyme is also an r-RNA.
t-RNA (transfer RNA): about 15% of total RNA. It has an average of 80 nucleotides per molecule, and it is the smallest of all RNAs. More than 20 different t-RNAs may be present in a given cell. It is easily soluble, so it is also called soluble RNA (s-RNA). It transfers amino acids from the cytoplasm to the ribosomal machinery. It acquires a clover leaf structure by folding upon itself. Unusual bases present in tRNA are dihydrouracil, pseudouridine and hypoxanthine.

Feulgen, aceto-carmine and aceto-orcein are nuclear stains. The Feulgen stain (reaction) is specific for DNA, as it stains the deoxyribose sugar.

ENZYMES

While going through the chapter pay special attention to the following:
Definitions: 1. Holoenzyme 2. Km value 3. Immobilized enzyme 4. Co-factor
Differences: 1. Competitive and non-competitive inhibition 2. Hydrolases and lyases 3. Exoenzymes and Endoenzymes
Terms: 1. Feed back inhibition 2. Turn over number 3. Apoenzyme
Buchner, a German biochemist, discovered enzymes in yeast and stated that the living nature of cells is not essential for enzymatic activity. Kühne coined the term 'enzyme' (meaning 'in yeast'). J. Sumner isolated the first enzyme, urease, in pure crystalline form from jack beans. Northrop discovered that all enzymes are proteins. However, RNA can also act as an enzyme, and it is called a ribozyme.

Properties of enzymes:
- Enzymes are oligodynamic, i.e. they are required in small amounts.
- They are metabolic regulators.
- They act as catalysts in biological reactions and are therefore called biocatalysts.
- They are pH specific, substrate specific and temperature specific. The optimum temperature for enzymatic action is about 37 °C. At 0 °C, enzymes are inactivated, while at 60 °C or above most enzymes are denatured and lose their activity. The optimum pH for most enzymes is 7, but for pepsin/rennin it is 2, and for alkaline phosphatase (in the kidney) it is 10. pH is a logarithmic value, so a change of 1 unit on the pH scale causes a 10-fold change in H⁺, and a change of 2 units causes a 100-fold change.
- They cannot start a chemical reaction, but they can regulate the velocity of a reaction.
- They are reusable.
- They cannot change the equilibrium point of a reversible reaction.

[Figure: Fig. 9.11, Effect of temperature (0 °C, 37 °C, 50 °C) and of pH (6 to 8) on enzyme velocity, with the optimum points marked.]

Enzymes versus inorganic catalysts
Similarities: both cannot start a chemical reaction; both are oligodynamic; both are reusable; both cannot change the equilibrium point of a reaction.
Dissimilarities: enzymes are complex protein compounds, while catalysts are simple inorganic compounds. Enzymes are pH, temperature and substrate specific, while catalysts are not so specific. The activity of enzymes can be regulated, but the activity of catalysts cannot.

Important terms
- Proenzymes: inactive forms of enzymes, e.g. pepsinogen and trypsinogen.
- Isoenzymes: two enzymes with similar structure and catalytic activity but differing in some chemical properties, e.g. LDH (lactic acid dehydrogenase, or lactate dehydrogenase) and aldolases. LDH has two types: the M type (in muscles) and the H type (in the heart). LDH converts lactic acid into pyruvic acid.
- Lysozymes: germicidal enzymes present in sebum, tears, milk and saliva. They destroy bacteria by hydrolysing their polysaccharide cell walls.
- Exo-enzymes and endo-enzymes: enzymes that work outside the cells (e.g. digestive enzymes) are exo-enzymes, while those working inside the cells (e.g. respiratory enzymes) are endo-enzymes.
- Holoenzymes: conjugated proteins. Their non-protein part is called the co-factor. The co-factor can be inorganic or organic. An organic co-factor that is permanently attached to the enzyme is called a prosthetic group, and one that is temporarily attached (only during the reaction) is called a co-enzyme. Haem is a prosthetic group in catalases and peroxidases. NAD and NADP are co-enzymes. FAD is a co-enzyme in flavoproteins (yellow enzymes). Most co-enzymes are derivatives of vitamins B and C. The protein part of a holoenzyme is called the apo-enzyme.
- Flavoproteins (yellow enzymes): holoenzymes with riboflavin (vitamin B₂) as the co-factor.
- Immobilised enzymes: used in industries. They are protected from the action of proteases by attaching them to a solid support or a jelly, by covalent bonding, or by incorporating them into artificial cells.
- Turnover number: the number of substrate molecules catalysed by a single enzyme molecule in a unit time. The maximum turnover number (36 million mol/minute) is that of carbonic anhydrase. The turnover number depends on the number of active sites.

Structure of enzymes: Enzymes are tertiary proteins. Primary proteins have only peptide bonds. Secondary proteins have helical or pleated structures with hydrogen bonds. Tertiary proteins are formed by further coiling or folding of the polypeptide chain, stabilised by disulphide (S - S) bonds.
Active site: a specific region in the enzyme to which a substrate (reactant) binds. It has a specific type and number of amino acids. It is a small site and covers nearly 5% of the total area of most enzymes. In some cases, a metal (co-factor) also takes part in forming the active site.

Theories about enzymatic action:
(i) Emil Fischer's lock and key hypothesis: the active site of the enzyme acts as a lock and the substrate as a key. Fischer believed that the active site is rigid or non-flexible.
(ii) Koshland's induced fit theory: the active site is flexible, and the substrate can induce a change in its configuration.

Enzymatic action: The energy required for a chemical reaction to proceed is called activation energy. Enzymes lower the activation energy. (Enzymes cannot start a chemical reaction, but they make it easier.)

[Figure: Fig. 9.12, Lowering of activation energy by an enzyme. The energy level of the substrate (C) and of the product (D) are shown. C to A is the activation energy without enzyme, C to B is the activation energy with enzyme, and A to B is the lowering of activation energy due to the enzyme.]

Effect of substrate concentration on enzyme velocity: As the substrate concentration increases, the enzymatic velocity also increases. At a certain value, all active sites of the enzyme molecules are saturated, and further increase in substrate concentration does not increase the velocity. The substrate concentration at which the velocity of the enzymatic reaction reaches half of its maximum value is called the K_m value, or Michaelis constant. The higher the affinity of an enzyme for a substrate, the lower its K_m value; that is, K_m ∝ 1/affinity.

[Figure: Graph of substrate concentration (m.mol/litre) on the x-axis against velocity on the y-axis, showing the curve reaching V_max, with K_m marked where the velocity is ½ V_max.]

Inhibition of enzymatic action:
(1) By denaturation: if the enzyme is heated to 60 °C or above, its tertiary conformation (active site) changes and the enzyme loses its property.
(2) By competitive inhibition: a chemical similar in configuration to the substrate competes for the active site of the enzyme. Example: malonate competes with succinate for the active site of succinate dehydrogenase, so it is a competitive inhibitor of the conversion of succinic acid to fumaric acid. Methanol toxicity is similarly averted by ethanol, a competitive inhibitor of the enzyme alcohol dehydrogenase. Sulpha drugs compete with PABA (para-aminobenzoic acid) for the active site of an enzyme present in bacteria. Because of this, bacteria cannot synthesise folic acid (a component of vitamin B complex), and the drug acts as an antibacterial agent, like an antibiotic.
(3) By non-competitive inhibition: the chemical does not compete with the substrate for the active site. It binds to the enzyme at a site other than the active site, and it changes the configuration or 3-D shape of the active site. Example: cyanides stop the functioning of respiratory enzymes by binding with the iron of the prosthetic group. Ions of heavy metals (Hg, Ag and Cu) combine with the disulphide or thiol group and break it, changing the 3-D shape of the enzyme or denaturing it.
(4) Allosteric (feedback) inhibition: allosteric enzymes have an allosteric site separate from the active site. The chemical binds to this well-defined allosteric site and inhibits the reaction. The inhibitor is usually one of the products of a long chain of enzymatic reactions, so this inhibition is also known as feedback inhibition. Example: hexokinase (glucose + ATP → glucose 6-phosphate + ADP), in which the inhibitor binds to the allosteric site.
[Figure: Fig. 9.13, Hexokinase reaction: glucose + ATP → glucose 6-phosphate + ADP, with the inhibitor binding to the allosteric site.]

Classification of enzymes
(a) On the basis of the substrate:
(1) Proteases (proteolytic enzymes) act on proteins and peptides, e.g. pepsin, trypsin and polypeptidase.
(2) Carbohydrases (sucrolytic enzymes) act on carbohydrates, e.g. amylase, maltase and sucrase.
(3) Lipases (lipolytic enzymes) act on lipids or fats, e.g. steapsin.
(b) On the basis of the type of reaction:
(1) Oxidoreductases, e.g. oxidase (cytochrome oxidase) and dehydrogenase (alcohol dehydrogenase).
(2) Transferases, e.g. phosphorylase, transaminase and hexokinase.
(3) Hydrolases cause enzymatic breakdown with the addition of water, e.g. amylase, urease and lipase.
(4) Lyases cause enzymatic breakdown without the addition of water, e.g. decarboxylases and deaminases.
(5) Isomerases, e.g. aldolases.
(6) Ligases (synthetases) join two molecules to synthesise a complex molecule from simpler ones, e.g. DNA and RNA ligases.
Of about 2,500 different enzymes named so far, the most abundant group is oxidoreductases. The correct order of abundance is 1 > 2 > 4 > 3 > 6 > 5.
Enzymes are named according to the classification designed by the Enzyme Commission (EC) of the IUPAC (International Union of Pure and Applied Chemistry). This classification is based on the type of reaction they catalyse. Each enzyme has a specific EC number with four integers. The first integer designates one of the six major classes of enzymes, the second indicates a sub-class, the third indicates a classification of the bond or group transferred, and the fourth is a specific serial number. For example, the EC number of arginase is 3.5.3.1, which means it is a hydrolase (3), and the last two integers are beyond the scope of this book.

Check point 2: Differentiate between

(i) Lock and key theory and induced fit theory of enzyme action.
Answer: In the lock and key theory the active site is rigid and fits only one substrate. In the induced fit theory the active site is flexible and changes its shape to fit the substrate.

(ii) Co-enzyme and co-factor.
Answer: A co-factor is any non-protein part of an enzyme, which may be inorganic or organic. A co-enzyme is an organic co-factor that is temporarily attached to the enzyme during the reaction, e.g. NAD.

(iii) Biocatalyst and inorganic catalyst.
Answer: A biocatalyst is an enzyme, a complex protein that is specific for the substrate, the pH and the temperature. An inorganic catalyst is a simple inorganic substance that is not so specific.

(iv) Exo-enzymes and endo-enzymes.
Answer: Exo-enzymes work outside the cells (e.g. digestive enzymes). Endo-enzymes work inside the cells (e.g. respiratory enzymes).

(v) Competitive and non-competitive inhibition of enzymatic action.
Answer: In competitive inhibition, the inhibitor competes with the substrate for the active site. In non-competitive inhibition, the inhibitor binds to a site other than the active site and changes the shape of the enzyme.

(vi) Active site and allosteric site.
Answer: The active site is where the substrate binds to the enzyme. The allosteric site is a separate site where a regulator (inhibitor) binds to change the enzyme's activity.

(vii) Isoenzymes and lysozymes.
Answer: Isoenzymes are enzymes with similar structure and catalytic activity but different chemical properties (e.g. LDH). Lysozymes are germicidal enzymes that destroy bacterial cell walls.

(viii) Lyases and ligases.
Answer: Lyases break a molecule without adding water (e.g. decarboxylases). Ligases join two molecules to make a complex molecule (e.g. DNA ligase).

MINERALS

While going through the chapter pay special attention to the following:
Vitamin derivatives: 1. FAD 2. NADP 3. Co-enzyme A 4. T.P.P.
Deficiency disorders: 1. Nyctalopia 2. Pernicious anaemia 3. Pellagra 4. Beri-Beri 5. Scurvy 6. Rickets
Common names: 1. Anti-haemorrhagic vitamin 2. Vitamin PP 3. Extrinsic factor of Castle
Vitamins, minerals and water do not provide energy, but they help in energy production and maintain the proper metabolism of the body. They are called micronutrients or protective principles of food. Their deficiency causes various disorders or diseases.

The essential elements (minerals) required by animals and humans are 21. On the basis of the amount required, they are classified as:
(a) Macro or major elements (7): sodium, potassium, calcium, magnesium, phosphorus, sulphur and chlorine. The daily requirement of each is more than 100 mg.
(b) Micro elements or trace elements (14): iron, manganese, iodine, zinc, cobalt, copper, selenium, molybdenum, fluorine, chromium, nickel, barium, lithium and bromine. The functions of the last five are not fully understood.
In plants, the essential elements are 20, of which 10 are required in macro doses and 10 in micro doses. Sodium, iodine, fluorine and selenium, which are essential for humans and animals, are not essential for plants.

Disorders caused by the excess of certain minerals:
I. Siderosis: due to deposition or excess of iron (as oxide).
II. Wilson's disease: due to excess copper, and causes brain damage.
III. Fluorosis: due to excess fluorine, and produces yellow patches on teeth.

VITAMINS

The term 'vitamin' was given by Casimir Funk. Vitamins are organic compounds that are neither carbohydrates, proteins nor fats. They are also known as accessory food factors, and they are required in small amounts (in micrograms or IU, where 1 IU = 0.3 micrograms). Vitamin C, however, is required in macro doses. Vitamins are not pH specific, have no calorific value, and are also called growth or metabolic regulators. They are classified into (i) fat-soluble vitamins and (ii) water-soluble vitamins. (Vitamins are not included in the NEET syllabus.)

(1) Fat-soluble vitamins: A, D, E and K
They are taken as precursors or pro-vitamins. Fat in the diet is essential for their absorption. They can be stored in the body. Their deficiency disorders are not easily corrected.

Vitamin A
- Chemical name: retinol (A₁) and dehydroretinol (A₂).
- Common name: bright eye vitamin, anti-infection vitamin, anti-xerophthalmic factor.
- Sources: cod and shark liver oil, yellow fruits (papaya, mango) and vegetables (carrot), butter, ghee and egg. It is formed in the liver from the yellow and red carotenoid pigments, β-carotene.
- Functions: maintains the secretory nature of the epithelial lining; helps in the synthesis of rhodopsin (visual pigment of rod cells).
- Deficiency disorders: (a) phrynoderma, rough and keratinised epithelium of skin, respiratory tract and urinogenital tract; (b) xerophthalmia, dry eye condition, later causing keratomalacia (deficiency of vitamin A₁); (c) night blindness, or nyctalopia, the inability to see in dim light (deficiency of vitamin A₂).

Vitamin D
- Chemical name: calciferol (a steroid). D₂ (ergocalciferol) is of plant origin, and D₃ (cholecalciferol) is of animal origin and is the most abundant.
- Common name: anti-rachitic vitamin, poor man's vitamin, sunshine vitamin.
- Sources: fish liver oil, liver, egg, milk and ghee. Commercially, it is obtained from the fungus ergot. In human skin, it is synthesised from 7-dehydrocholesterol, an intermediate of cholesterol biosynthesis.
- Functions: helps in the absorption of calcium ions from the intestine, and is important for the growth and maintenance of bones and teeth. Calciferol is a prohormone, and it is synergistic to parathormone, increasing the calcium level in blood, while calcitonin decreases it. (Osteoporosis, also a bone disorder, is caused by hypersecretion of parathormone, PTH.)
- Deficiency disorders: (1) rickets (bow-legged) in children; (2) osteomalacia in adults (bones become weak and soft).

Vitamin E
- Chemical name: tocopherol.
- Common name: anti-sterility vitamin, vitamin of beauty.
- Sources: vegetable oils, such as wheat germ oil, sunflower oil and cottonseed oil. Fish liver oil is devoid of vitamin E.
- Functions: it is an antioxidant and prevents the oxidation of unsaturated fatty acids. It boosts the immune system and reduces the risk of myocardial infarction by reducing the oxidation of LDL.
- Deficiency disorders: (1) sterility in rats; (2) anaemia, as RBCs become fragile; (3) muscle degeneration.

Vitamin K
- Chemical name: K₁ is phylloquinone (a naphthoquinone derivative with a long isoprene chain), and K₂ is menaquinone.
- Common name: anti-haemorrhagic vitamin, or coagulation vitamin (from the German word Koagulation).
- Sources: leafy vegetables (for vitamin K₁). Vitamin K₂ is synthesised in the intestine by microflora.
- Functions: helps in the synthesis of prothrombin and blood clotting factors VII, IX and X. A synthetic, water-soluble form of this vitamin used commercially is menadione.
- Deficiency disorders: haemorrhage and delayed blood clotting.
Connecting concept: ubiquinone (coenzyme Q) is also a quinone derivative with a long isoprene chain. It takes part in the electron transport chain (ETC), where it is reduced to semiquinone (QH) and finally to quinol (QH₂). It accepts electrons from NADH and FADH₂. Its deficiency is associated with several mental disorders.

(2) Water-soluble vitamins: B and C
They are not taken as pro-vitamins. Fat in the diet is not essential for their absorption. They are not stored in the body, except vitamin B₁₂. Their deficiency disorders are easily corrected.

Vitamin B complex

(i) Thiamine (vitamin B₁)
- Common name: anti-beri-beri vitamin, aneurine vitamin.
- Source: husk of cereals (whole wheat flour or brown bread). Polished rice lacks thiamine.
- Function: acts as a co-enzyme (TPP, thiamine pyrophosphate) in decarboxylation reactions in carbohydrate metabolism.
- Deficiency disorders: beri-beri (a Singhalese word meaning weakness), with anorexia (loss of appetite), dyspepsia (indigestion with abdominal discomfort) and weakness. Beri-beri has two forms: dry beri-beri, with extreme emaciation and no oedema; and wet beri-beri, with oedema (accumulation of tissue fluid). Polyneuritis (swelling at nerve endings) also occurs. Deficiency of thiamine also causes fatigue, headache, mental depression and heart enlargement. Chronic alcoholics generally develop thiamine deficiency, and hence polyneuritis and other disorders.

(ii) Riboflavin (vitamin B₂ or vitamin G)
- Sources: milk (lactoflavin), liver (hepatoflavin) and eggs (ovoflavin).
- Functions: acts as a co-enzyme (FAD or FMN) in flavoproteins for oxidative phosphorylation.
- Deficiency disorders: cheilosis (cracks at the ends of the lips), photophobia (hypersensitivity to light) and glossitis (fissures on the tongue and atrophy of papillae).

(iii) Niacin or nicotinic acid (vitamin B₃)
The term nicotinic acid has nothing in common with nicotine of tobacco, so this term is not widely used. Some amount of vitamin B₃ is synthesised from the amino acid tryptophan. People using maize as a staple food are deficient in niacin, because maize does not have this amino acid in usable form.
- Common name: vitamin PP (pellagra preventive).
- Functions: acts as a co-enzyme in hydrogen transport as (i) NAD (DPN, co-enzyme I) and (ii) NADP (TPN, co-enzyme II).
- Deficiency disorders: pellagra (rough and scaly skin with dark pigmentation), diarrhoea, nausea and vomiting. (Black tongue in dogs.)

(iv) Pantothenic acid (vitamin B₅)
- Functions: 'Pantos' is a Greek word meaning everywhere, which shows that the vitamin is widely distributed in nature. It is a component of co-enzyme A.
- Deficiency disorders: burning foot syndrome, inhibition of fatty acid synthesis, and anaemia due to reduced haem synthesis.

(v) Pyridoxine (vitamin B₆)
- Functions: acts as a co-enzyme (pyridoxal phosphate) in protein or amino acid metabolism.
- Deficiency disorder: convulsions (fits) or epilepsy, due to non-synthesis of GABA (an inhibitory neurotransmitter).

(vi) Biotin (vitamin B₇ or vitamin H)
- Common name: anti-egg-white injury factor.
- Functions: acts as a co-enzyme in carboxylation reactions. The affinity of avidin (a protein of egg white) for biotin is greater than an antigen-antibody reaction, so avidin is used in the ELISA test. Biotin is good for healthy hair.
- Deficiency disorders: hair fall and allergy from egg white.

(vii) Folic acid or pteroglutamic acid (vitamin B₉)
- Common name: 'folium' is a Latin word meaning leaves of vegetables.
- Source: green leafy vegetables. Milk is a poor source of this vitamin.
- Functions: takes part in the synthesis of nitrogenous bases of nucleic acids, and so stimulates cell division.
- Deficiency disorders: megaloblastic or macrocytic anaemia, in which few but large RBCs appear in the blood; and impairment of antibody synthesis.

(viii) Cyanocobalamine (vitamin B₁₂)
- Common name: anti-pernicious anaemia factor, or extrinsic factor of Castle.
- Sources: absent in plant products, but present in animal products such as milk, egg, fish, liver and meat. In curd, Lactobacillus can synthesise this vitamin. It is also synthesised by the microflora of the gut (colon). The blue-green alga Spirulina is also known to contain it.
- Functions: reduction of deoxyribose from ribose sugar for the formation of DNA from RNA; synthesis of the myelin sheath. It is the most complex vitamin and contains cobalt.
- Deficiency disorder: megaloblastic or macrocytic anaemia, better called pernicious (injurious) anaemia. This anaemia is associated with mental disorders, since the myelin sheath breaks down and interrupts the conduction of nerve impulses.

(ix) Choline
- Sources: milk, egg, liver and vegetables.
- Functions: a lipotropic factor that forms part of phospholipids, an essential constituent of cell membranes. It prevents fat deposition (neutral fat and cholesterol) in the liver. It helps in the synthesis of acetylcholine, the most common neurotransmitter. (B₄, adenine, and B₈, adenylic acid, are not considered vitamins.)

Vitamin C (ascorbic acid)
- Common name: anti-scurvy vitamin, anti-ageing vitamin, anti-rabies vitamin, anti-cancer vitamin and anti-viral vitamin. The anti-viral nature of vitamin C was discovered by Linus Pauling, who received a Nobel prize for this discovery.
- Sources: amla, citrus fruits, gooseberries, guava and green chillies. James Lind discovered that citrus fruits can cure scurvy.
- Functions: helps in the synthesis of collagen fibres for healing. It is a strong reducing agent (antioxidant). It prevents the blackening of banana and the colour change of cut fruits. It promotes the synthesis of hormones of the adrenal cortex. St. Gyorgyi received a Nobel prize for establishing the relationship between the hormones of the adrenal cortex and vitamin C.
- Deficiency disorders: scurvy (spongy and bleeding gums). Capillaries rupture due to the lack of cementing material, and healing is delayed.

Check point 3: Match the items of column I with column II

Answer: 1-D, 2-F, 3-A, 4-E, 5-C.

Check point 4: Fill in the blanks

(i) ................... is an antiviral vitamin.
Answer: Vitamin C (ascorbic acid).

(ii) ................. prevents blackening of cut fruits.
Answer: Vitamin C.

(iii) The term 'vitamin' was coined by ................. .
Answer: Casimir Funk.

(iv) Anorexia, dyspepsia and muscular weakness are the symptoms of vitamin ................... deficiency.
Answer: B₁ (thiamine).

(v) Deficiency of vitamin ................... causes nyctalopia.
Answer: A.

(vi) The vitamin ................... is a tryptophan derivative.
Answer: Niacin (vitamin B₃).

(vii) ................... is a cobalt-containing vitamin.
Answer: Vitamin B₁₂ (cyanocobalamine).

(viii) The commercial name of ................... is menadione.
Answer: Vitamin K.

(ix) Chemical name of vitamin C is ................... .
Answer: Ascorbic acid.

(x) Tocopherol is the chemical name of vitamin ................... .
Answer: E.

Check point 5: Fill in the blanks (carbohydrates)

(i) ................... elements constitute 93% of protoplasm.
Answer: Oxygen, carbon and hydrogen (C, H and O).

(ii) The maximum organic compound in nature is ................... .
Answer: Cellulose (the most abundant organic compound in the biosphere). Note: in the cell, the principal organic compound is protein.

(iii) ................... is a common ketose sugar.
Answer: Fructose.

(iv) In maltose the glycosidic bond is formed between ................... and ................... carbons.
Answer: C-1 and C-4 (α-1,4 linkage).

(v) Amongst the naturally occurring sugars ................... is the sweetest one.
Answer: Fructose.

(vi) ................... is the storage polysaccharide.
Answer: Starch (in plants) and glycogen (in animals).

(vii) ................... is a disaccharide with 2 β-glucose units.
Answer: Cellobiose.

(viii) The polymer of fructose is ................... .
Answer: Inulin.

(ix) ................... is the pentose sugar of nucleic acid.
Answer: Ribose (in RNA) or deoxyribose (in DNA).

(x) A straight chain polysaccharide ................... .
Answer: Amylose (the straight-chain fraction of starch), or cellulose.