Chapter 10: Biomolecules | Summary

Disaccharides consist of two monosaccharide units joined by a glycosidic linkage — an oxide bond formed by the loss of water. Sucrose (glucose + fructose, C1–C2 linkage) is a non-reducing sugar since both reducing groups are involved in bonding. Maltose (two alpha-D-glucose units, C1–C4 linkage) and lactose (beta-D-galactose + beta-D-glucose, C1–C4 linkage) are reducing sugars. The hydrolysis of sucrose produces laevorotatory invert sugar because the laevorotation of fructose exceeds the dextrorotation of glucose.

Polysaccharides are the most common carbohydrates in nature and serve as food storage or structural materials. Starch (main plant storage polysaccharide) consists of amylose (water-soluble, unbranched, C1–C4 linkage) and amylopectin (water-insoluble, branched, C1–C4 chain and C1–C6 branching). Cellulose, composed only of beta-D-glucose units (C1–C4 linkage), is the most abundant organic substance in the plant kingdom and forms cell walls. Glycogen, the animal storage polysaccharide, is more highly branched than amylopectin and is stored in liver, muscles, and brain.

Proteins are the most abundant biomolecules in living systems and are polymers of alpha-amino acids linked by peptide bonds. Amino acids carry both an amino group (–NH2) and a carboxyl group (–COOH); only alpha-amino acids are obtained from protein hydrolysis. They are classified as acidic, basic, or neutral based on the relative number of these groups. Essential amino acids cannot be synthesised in the body and must be obtained through diet, while non-essential amino acids can be synthesised in the body. Amino acids exist as zwitter ions in aqueous solution, exhibiting amphoteric behavior.

Proteins are classified into fibrous proteins (polypeptide chains running parallel, held by hydrogen and disulphide bonds, generally insoluble in water; e.g., keratin, myosin) and globular proteins (coiled polypeptide chains with spherical shape, soluble in water; e.g., insulin, albumins). The structure of proteins is studied at four levels: primary (sequence of amino acids), secondary (alpha-helix or beta-pleated sheet arising from hydrogen bonding), tertiary (overall folding stabilised by hydrogen bonds, disulphide linkages, van der Waals forces), and quaternary (spatial arrangement of two or more polypeptide subunits).

Denaturation is the process by which a native protein loses its biological activity upon physical changes (temperature) or chemical changes (pH). During denaturation, the secondary and tertiary structures are disrupted while the primary structure remains intact. Common examples include the coagulation of egg white on boiling and the curdling of milk due to lactic acid produced by bacteria. Enzymes are biocatalysts that are almost all globular proteins. They are highly specific for particular substrates and reactions, are named after the substrate or reaction type, and reduce the activation energy of reactions.

Enzymes are required only in small quantities and their mechanism involves the reduction of activation energy. For example, sucrase reduces the activation energy for sucrose hydrolysis from 6.22 kJ/mol (acid hydrolysis) to 2.15 kJ/mol. Enzymes that catalyse simultaneous oxidation of one substrate and reduction of another are called oxidoreductases. The naming convention for enzymes uses the suffix '-ase' added to the substrate or reaction type.

Vitamins are organic compounds required in small amounts in the diet to perform specific biological functions for normal maintenance of optimum growth and health. They are termed essential food factors because most cannot be synthesised in the human body, though gut bacteria can produce some. Vitamins are classified as fat-soluble (A, D, E, K — stored in liver and adipose tissue) and water-soluble (B group and C — excreted in urine, cannot be stored except B12, must be supplied regularly in diet).

Key deficiency diseases include: Vitamin A — night blindness and xerophthalmia; Vitamin B1 — beriberi; Vitamin B2 — cheilosis; Vitamin B6 — convulsions; Vitamin B12 — pernicious anaemia; Vitamin C — scurvy; Vitamin D — rickets in children and osteomalacia in adults; Vitamin E — increased fragility of RBCs and muscular weakness; Vitamin K — increased blood clotting time. Excess of vitamins can also be harmful, and vitamin supplements should be taken only on medical advice.

Nucleic acids are long-chain polymers (polynucleotides) responsible for the transmission of hereditary characters. There are two types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Complete hydrolysis of nucleic acids yields a pentose sugar, phosphoric acid, and nitrogenous bases. DNA contains beta-D-2-deoxyribose sugar, while RNA contains beta-D-ribose. DNA has four bases: adenine (A), guanine (G), cytosine (C), and thymine (T); RNA replaces thymine with uracil (U).

A nucleoside is formed when a base attaches to the 1' position of sugar. When a nucleoside is linked to phosphoric acid at the 5' position, a nucleotide is formed. Nucleotides are joined by phosphodiester linkages between the 5' and 3' carbons. Watson and Crick proposed the double-strand helix structure of DNA, held together by hydrogen bonds between complementary base pairs (A with T, G with C). RNA is single-stranded and exists as three types — mRNA (messenger), rRNA (ribosomal), and tRNA (transfer) — which carry out protein synthesis in the cell.

Hormones are intercellular messengers produced by endocrine glands and transported through the bloodstream to their target sites. Chemically, they may be steroids (estrogens, androgens), polypeptides (insulin, endorphins), or amino acid derivatives (epinephrine, norepinephrine). Hormones help maintain the balance of biological activities: insulin lowers blood glucose, glucagon raises it, epinephrine and norepinephrine mediate responses to external stimuli, and growth and sex hormones regulate development. Thyroxine, produced in the thyroid gland, is an iodinated derivative of tyrosine; its deficiency leads to hypothyroidism, while excess causes hyperthyroidism.

Steroid hormones are produced by the adrenal cortex and gonads. Glucocorticoids control carbohydrate metabolism and stress response; mineralocorticoids regulate water and salt excretion. Dysfunction of the adrenal cortex can cause Addison's disease. Testosterone drives secondary male sex characteristics, estradiol controls secondary female characteristics and the menstrual cycle, and progesterone prepares the uterus for implantation of a fertilised egg.

Biomolecules — carbohydrates, proteins, nucleic acids, vitamins, and hormones — form the chemical foundation of all life processes. Carbohydrates serve as energy sources and structural materials; proteins perform structural, enzymatic, transport, and regulatory roles; nucleic acids carry and transmit genetic information; vitamins and hormones regulate metabolism and development. The interplay of these molecules, their structures, and their chemical reactions defines the molecular logic of life.

From the perspective of the CBSE board examination, this chapter is highly significant as it covers multiple important topics tested at both the recall and application level. Students must be thorough with the classification and structures of carbohydrates, the four levels of protein structure, the differences between DNA and RNA, the classification of vitamins and their deficiency diseases, and the chemical nature and functions of hormones. Understanding these concepts deeply, with attention to specific chemical structures and biological roles, is essential for scoring well in examinations.