Biomolecules

Biomolecules — carbohydrates, proteins, vitamins, and nucleic acids — form the chemical basis of life. NEET focuses on classification, structural features, and disease associations.

Carbohydrates are classified as monosaccharides (glucose C₆H₁₂O₆, fructose C₆H₁₂O₆), disaccharides (sucrose, lactose, maltose), and polysaccharides (starch, cellulose, glycogen).
Glucose () is an aldohexose (aldehyde + 6 carbons); fructose is a ketohexose (ketone + 6 carbons). In cyclic form, glucose adopts a pyranose ring (6-membered) with two anomers: $\alpha$-D-glucose (C₁-OH axial/down in Haworth) and $\beta$-D-glucose (C₁-OH equatorial/up). Mutarotation is the interconversion between $\alpha$ and $\beta$ forms through the open-chain intermediate.

Reducing vs Non-Reducing Sugars: Reducing sugars have a free anomeric carbon (hemiacetal/hemiketal that can open to expose a free aldehyde/ketone), giving positive Tollens' and Fehling's tests. Glucose, fructose, lactose, and maltose are all reducing sugars. Sucrose is the key non-reducing sugar: its glycosidic bond links the anomeric carbon of BOTH glucose (C₁) and fructose (C₂), leaving NO free anomeric carbon. Lactose (glucose + galactose, $\beta$-1,4 linkage) and maltose (glucose + glucose, $\alpha$-1,4 linkage) are reducing because one anomeric carbon remains free.

Polysaccharides: starch contains amylose (linear, $\alpha$-1,4 linkages) and amylopectin (branched, $\alpha$-1,4 + $\alpha$-1,6 at branch points). Cellulose has $\beta$-1,4 linkages (humans cannot digest — we lack the enzyme to cleave $\beta$ linkages). Glycogen (animal starch) is highly branched like amylopectin.

Proteins: Amino acids have the general structure H₂N-CHR-COOH ( for glycine). At the isoelectric point, amino acids exist as zwitterions (+H₃N-CHR-COO-). The peptide bond (-CO-NH-) forms by condensation between -COOH and -NH₂ with loss of H₂O. Protein structure hierarchy: primary (amino acid sequence, peptide bonds), secondary ($\alpha$-helix — intramolecular H-bonding between C=O and N-H of every 4th residue; $\beta$-pleated sheet — intermolecular H-bonding between adjacent chains), tertiary (overall 3D fold — disulfide bonds, hydrophobic interactions, ionic bonds), quaternary (multiple polypeptide subunits — hemoglobin has 4 subunits). Denaturation disrupts secondary/tertiary/quaternary structures by heat, pH change, or chemicals, but does NOT break peptide bonds (primary structure is preserved).

Vitamins: Water-soluble: B-complex (B₁-thiamine/beriberi, B₂-riboflavin/cheilosis, B₃-niacin/pellagra, B₆-pyridoxine/convulsions, B₇-biotin, B₉-folic acid/megaloblastic anemia, B₁₂-cyanocobalamin/pernicious anemia) and C (ascorbic acid/scurvy). Fat-soluble: A (retinol/night blindness, xerophthalmia), D (calciferol/rickets in children, osteomalacia in adults), E (tocopherol/infertility), K (phylloquinone/delayed blood clotting).

Nucleic Acids: DNA is a double helix with complementary base pairing: A-T (2 hydrogen bonds) and G-C (3 hydrogen bonds). More G-C content = higher melting temperature. DNA uses deoxyribose sugar; RNA uses ribose sugar and replaces thymine with uracil. Three RNA types: mRNA (carries genetic code from DNA to ribosome), tRNA (cloverleaf structure, transfers amino acids to ribosome), rRNA (structural component of ribosomes).

The key testable concept is that sucrose is non-reducing because both anomeric carbons are involved in the glycosidic bond, and that G-C base pairs have 3 hydrogen bonds (not 2) while A-T has 2.