CBSE Class 12 Chemistry Biomolecules Notes Set 05

These Class 12 CBSE Chemistry notes for Unit 10 Biomolecules cover the classification of carbohydrates, the structure and reactions of glucose and fructose, disaccharides and polysaccharides, amino acids and the four levels of protein structure, enzymes, vitamins and the chemistry of DNA and RNA. The tables bring together the facts that board examinations ask most often.

Important Notes and Formulas

Carbohydrates and Their Classification

1. Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis. Many have the general formula Cx(H₂O)y, but not all (rhamnose, C₆H₁₂O₅, is a carbohydrate that does not fit).
2. Sweet carbohydrates are called sugars, for example sucrose (cane sugar) and lactose (milk sugar).
3. Monosaccharides cannot be hydrolysed further (glucose, fructose, ribose). An aldose has an aldehyde group and a ketose has a keto group; the prefix tri-, tetr-, pent-, hex- gives the number of carbon atoms, as in aldohexose or ketopentose.
4. Oligosaccharides give 2 to 10 monosaccharide units on hydrolysis. Disaccharides are the commonest: sucrose gives one glucose and one fructose, while maltose gives two glucose units.
5. Polysaccharides (starch, cellulose, glycogen) give a large number of monosaccharide units on hydrolysis. They are not sweet and are called non-sugars.
6. Reducing sugars reduce Fehling's solution and Tollens' reagent. All monosaccharides are reducing. A disaccharide is non-reducing if the reducing groups of both units are involved in the glycosidic bond (sucrose); maltose and lactose have a free group and are reducing.

Glucose

1. Preparation: (i) from sucrose, by boiling with dilute HCl or H₂SO₄ in alcoholic solution, which gives equal amounts of glucose and fructose (C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆); (ii) from starch, by boiling with dilute H₂SO₄ at 393 K under 2 to 3 atm pressure ((C₆H₁₀O₅)n + nH₂O → nC₆H₁₂O₆).
2. Glucose, C₆H₁₂O₆, is an aldohexose. Its open-chain structure is HOCH₂–(CHOH)₄–CHO, with the aldehyde at C-1 and a primary alcohol at C-6. It is D-(+)-glucose (dextrorotatory), also called dextrose or grape sugar.

Cyclic Structure, Anomers and Mutarotation

1. Glucose does not give Schiff's test or the NaHSO₃ addition product, and its pentaacetate does not react with hydroxylamine. These facts show that the aldehyde group is not completely free; the open chain closes into a cyclic hemiacetal.
2. The –OH on C-5 adds to the aldehyde carbon C-1 to give a six-membered ring containing five carbon atoms and one oxygen atom. This ring is called the pyranose ring (like pyran), and the Haworth structure represents it.
3. The new chiral centre at C-1 is the anomeric carbon. The two cyclic forms, α-D-(+)-glucose and β-D-(+)-glucose, differ only in the configuration at C-1 and are called anomers.
4. A freshly prepared solution of either anomer slowly changes its specific rotation until it reaches an equilibrium value of +52.5°. This is mutarotation; it occurs through the open-chain form.
5. In the α-anomer the –OH on C-1 lies on the opposite side of the ring to the –CH₂OH group at C-5 in the usual drawing; in the β-anomer it lies on the same side.

Fructose

1. Fructose, C₆H₁₂O₆, is a ketohexose with the keto group at C-2. It is D-(–)-fructose, laevorotatory, so it is also called laevulose.
2. In its cyclic form the –OH on C-5 adds to the C=O group, giving a five-membered ring (one oxygen, four carbons) called the furanose ring. The two anomers are α-D-(–)-fructofuranose and β-D-(–)-fructofuranose.
3. Fructose reduces Tollens' reagent and Fehling's solution even though it is a ketose, because the alkaline reagent isomerises it (through an enediol) into glucose and mannose, both of which are aldoses.
4. Glucose and fructose are compared in the table below.

Disaccharides

1. Two monosaccharide units are joined through an oxygen atom with loss of a water molecule. This C–O–C link is called a glycosidic linkage.
2. Sucrose: on hydrolysis it gives an equimolar mixture of D-(+)-glucose and D-(–)-fructose. The two units are joined by a glycosidic linkage between C-1 of α-D-glucose and C-2 of β-D-fructose. Both reducing groups are used in the bond, so sucrose is a non-reducing sugar.
3. Sucrose is dextrorotatory (+66.5°), but the hydrolysis mixture is laevorotatory because fructose (–92.4°) is more strongly laevorotatory than glucose (+52.5°) is dextrorotatory. This change of sign is called inversion, and the mixture is invert sugar.
4. Maltose: two α-D-glucose units joined from C-1 of one unit to C-4 of the other. The second glucose unit has a free hemiacetal group, so maltose is a reducing sugar.
5. Lactose (milk sugar): β-D-galactose and β-D-glucose joined from C-1 of galactose to C-4 of glucose. It is a reducing sugar.

Polysaccharides

1. Starch is the main storage polysaccharide of plants and the most important dietary source of carbohydrates for humans (cereals, roots, tubers).
2. Glycogen is called animal starch. When the body needs glucose, enzymes break glycogen down.

Amino Acids

1. Amino acids contain both an amino group (–NH₂) and a carboxyl group (–COOH). In an α-amino acid, H₂N–CH(R)–COOH, the amino group is on the carbon next to the carboxyl group. Only α-amino acids are obtained on hydrolysis of proteins.
2. All naturally occurring α-amino acids except glycine are optically active, since the α-carbon is asymmetric. Most have the L-configuration (the –NH₂ group is on the left in the Fischer projection).
3. Classification by the number of functional groups: acidic (more –COOH, e.g. aspartic acid and glutamic acid), basic (more –NH₂, e.g. lysine) and neutral (equal numbers, e.g. glycine and alanine).
4. Essential amino acids cannot be made in the body and must come from the diet (for example valine, leucine, isoleucine, lysine, threonine, methionine, phenylalanine and tryptophan). Non-essential amino acids can be synthesised in the body (glycine, alanine, serine, and so on).
5. In aqueous solution the –COOH group gives a proton to the –NH₂ group, giving a dipolar ion called a zwitter ion, H₃N⁺–CH(R)–COO⁻. It is neutral overall and amphoteric, reacting with both acids and bases. This is why amino acids are crystalline, water-soluble solids that melt at high temperatures and behave like salts.
6. In acidic solution an amino acid exists as a cation and in alkaline solution as an anion. At the isoelectric point it is a neutral zwitter ion that does not migrate to either electrode and has minimum solubility.

Proteins

1. Proteins are polymers of α-amino acids joined by peptide bonds, –CO–NH–, an amide link formed by loss of water between the –COOH of one amino acid and the –NH₂ of the next. Gly–Ala (glycylalanine) is a dipeptide.
2. Fibrous proteins have parallel polypeptide chains held by hydrogen and disulphide bonds; they are insoluble in water (keratin in hair and wool, myosin in muscle). Globular proteins have chains coiled into a spherical shape and are usually soluble in water (insulin, albumins).

1. Denaturation: a physical change (temperature) or chemical change (pH) disturbs the hydrogen bonds, the globules unfold, the helix uncoils and the protein loses its biological activity. The secondary and tertiary structures are destroyed but the primary structure is unchanged. Examples are the coagulation of egg white on boiling and the curdling of milk.
2. Biuret test for proteins: an alkaline protein solution with a few drops of dilute CuSO₄ gives a violet colour, due to a Cu²⁺ complex with the peptide groups.

Enzymes

1. Enzymes are biological catalysts. Chemically almost all enzymes are globular proteins. They lower the activation energy of biological reactions and are highly specific for a particular substrate and reaction.
2. Some enzymes need a non-protein part called a cofactor. A coenzyme is a cofactor that attaches to the enzyme during the reaction; many vitamins act as coenzymes.
3. Sucrose is hydrolysed by invertase.

Vitamins

1. Vitamins are organic compounds required in small amounts for normal health and growth; lack of a vitamin causes a deficiency disease. Fat-soluble vitamins are A, D, E and K and are stored in the body. Water-soluble vitamins are B-complex and C and are excreted in urine, so they must be supplied regularly (vitamin B₁₂ is stored in the liver).

Nucleic Acids

1. Chromosomes in the nucleus are made of proteins and nucleic acids and are responsible for heredity. The two types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). They are long-chain polymers of nucleotides, so they are polynucleotides.
2. Complete hydrolysis of DNA or RNA gives a pentose sugar, phosphoric acid and nitrogen-containing heterocyclic bases. The sugar is β-D-2-deoxyribose in DNA and β-D-ribose in RNA.
3. DNA bases: adenine (A), guanine (G), cytosine (C) and thymine (T). RNA bases: A, G, C and uracil (U), which replaces thymine. Adenine and guanine are purines; cytosine, thymine and uracil are pyrimidines.
4. A nucleoside is a base attached to C-1' of the sugar (sugar carbons are numbered 1', 2', 3', and so on). A nucleotide is a nucleoside with a phosphate group on C-5' of the sugar.
5. Nucleotides are joined by phosphodiester linkages between the 5' carbon of one sugar and the 3' carbon of the next. The sequence of nucleotides is the primary structure of a nucleic acid.
6. Secondary structure of DNA (Watson and Crick): two polynucleotide strands wound about each other in a double helix, held by hydrogen bonds between complementary bases: A pairs with T by two hydrogen bonds and G pairs with C by three. The two strands are complementary.
7. RNA is single stranded, and its helices form only where the strand folds back on itself. The three types are messenger RNA (m-RNA), ribosomal RNA (r-RNA) and transfer RNA (t-RNA).
8. Biological functions: DNA is the chemical basis of heredity. It replicates (makes copies of itself) and carries the genetic information, in segments called genes, that is transcribed into RNA. RNA then directs protein synthesis, by translation of the m-RNA sequence into the sequence of amino acids.