Carbohydrates are classified into monosaccharides, disaccharides, and polysaccharides based on the number of sugar units they contain. Glucose is an aldohexose and fructose is a ketohexose; both share the molecular formula $C_6H_{12}O_6$. Sucrose is the only common non-reducing disaccharide because the glycosidic bond involves the anomeric carbons of both glucose and fructose, leaving no free reducing group. Lactose and maltose are reducing disaccharides because each retains one free anomeric carbon. Cellulose contains $\beta$-1,4 glycosidic linkages that humans cannot digest, while starch uses $\alpha$-1,4 linkages that our amylase can cleave. Proteins are built from amino acids linked by peptide bonds, and their structure has four levels — primary, secondary, tertiary, and quaternary — with denaturation destroying all levels except primary. Fat-soluble vitamins are A, D, E, and K, while all B vitamins and vitamin C are water-soluble; each has a specific deficiency disease that NEET tests directly. DNA is a double helix with antiparallel strands, base-paired by A–T (2 hydrogen bonds) and G–C (3 hydrogen bonds), and higher G–C content raises the melting temperature. RNA differs from DNA by using ribose sugar, uracil instead of thymine, and being mostly single-stranded; the three types are mRNA, tRNA, and rRNA. Mastering the sucrose anomeric carbon concept, the A-T/G-C hydrogen bond numbers, and the vitamin–disease table accounts for the majority of NEET marks in this chapter.