Biomolecules notes

Biomolecules are carbon-based compounds present in living systems and are essential for structure, energy, growth, heredity and regulation. In NCERT Class 12 Chemistry, the focus is mainly on carbohydrates, proteins, enzymes, vitamins, nucleic acids and hormones. Carbohydrates act as energy sources and structural materials; proteins form body tissues and catalyse reactions as enzymes; vitamins are required in small amounts for normal metabolism; nucleic acids store and transfer genetic information; and hormones act as chemical messengers. For NEET, the chapter is highly memory-based but also concept-driven, especially in classification, structures, deficiency diseases, enzyme activity and DNA-RNA comparison.

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that produce such units on hydrolysis. They are classified by hydrolysis into monosaccharides, oligosaccharides and polysaccharides, and by functional group into aldoses and ketoses. Glucose is an aldohexose and fructose is a ketohexose. Disaccharides like sucrose, maltose and lactose contain two monosaccharide units joined by glycosidic linkage. Polysaccharides such as starch, cellulose and glycogen are large carbohydrate polymers. Important NEET concepts include reducing and non-reducing sugars, cyclic structures, glycosidic bonds, hydrolysis, oxidation reactions and biological roles such as energy storage, cell wall formation and dietary fibre.

Proteins are natural polymers of alpha-amino acids joined by peptide bonds. Amino acids contain both amino and carboxyl groups and often exist as zwitterions. The sequence of amino acids gives the primary structure, while hydrogen bonding forms secondary structures such as alpha-helix and beta-pleated sheet. Further folding produces tertiary structure, and association of multiple chains gives quaternary structure. Proteins may be fibrous or globular and perform structural, transport, protective, hormonal and catalytic functions. Enzymes are highly specific biological catalysts, mostly proteins, that lower activation energy. NEET frequently tests peptide bond formation, protein denaturation, enzyme specificity, optimum pH and temperature, and factors affecting enzyme activity.

Vitamins are organic compounds required in small amounts for normal growth, metabolism and health. Most vitamins cannot be synthesized in sufficient quantity by the body and must be obtained from diet. They are classified as fat-soluble vitamins, namely A, D, E and K, and water-soluble vitamins, namely B-complex and C. Fat-soluble vitamins are stored in the liver and adipose tissue, so excess intake may cause toxicity. Water-soluble vitamins are generally not stored significantly and must be supplied regularly. NEET questions commonly ask classification, sources, functions and deficiency diseases such as night blindness, rickets, scurvy, beriberi and pernicious anaemia.

Nucleic acids are biomolecules responsible for storage, transfer and expression of genetic information. Their monomeric units are nucleotides, each containing a nitrogenous base, pentose sugar and phosphate group. DNA contains deoxyribose sugar and bases adenine, guanine, cytosine and thymine, while RNA contains ribose sugar and uracil instead of thymine. DNA generally exists as a double helix with complementary base pairing: A-T and G-C. RNA is usually single-stranded and occurs as mRNA, tRNA and rRNA, each involved in protein synthesis. NEET questions focus on nucleotide composition, DNA-RNA differences, base pairing, types of RNA and functions of nucleic acids.

Hormones are chemical messengers secreted by endocrine glands directly into blood and transported to target organs where they regulate physiological and metabolic processes. Chemically, hormones may be steroids, proteins, peptides or amino acid derivatives. They act in very small quantities and help maintain homeostasis, growth, development, metabolism, reproduction and stress responses. Important examples include insulin and glucagon for blood glucose regulation, thyroxine for metabolic rate, adrenaline for emergency response, growth hormone for body growth and sex hormones for reproductive functions. NEET questions usually test source gland, chemical nature, biological function and basic disorder associations such as diabetes mellitus, goitre, dwarfism and gigantism.