# Biomolecules and Their Functions > Biomolecules and their functions for NEET Biology: the 20 amino acids, protein structure levels, the acid-insoluble pool and NCERT line traps. - Canonical URL: https://prepelephant.com/topics/neet-ug/biology/biomolecules-and-their-functions - Exam / course: NEET-UG · Subject: Biology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Biomolecules and Their Functions", PrepElephant, https://prepelephant.com/topics/neet-ug/biology/biomolecules-and-their-functions ## Direct answer Water makes up 70-90 per cent of cellular mass, but the chemistry of life runs on four classes of biomacromolecules — proteins, nucleic acids, polysaccharides and lipids — assembled from small organic molecules such as the twenty amino acids, sugars, fatty acids and nitrogenous bases. Proteins (peptide bonds), carbohydrates (glycosidic bonds) and nucleic acids (phosphodiester bonds) have molecular weights above roughly 1000 daltons and form the acid-insoluble fraction of the cell; lipids enter that fraction only because they cluster into membrane vesicles rather than being polymers. NCERT calls the living state a non-equilibrium steady state, with metabolites continuously made and broken down so the system never sits still. ## What you must remember - **Amino acid taxonomy:** twenty types serve all of life; classified by the number of amino and carboxyl groups (acidic — glutamic and aspartic acids; basic — lysine, arginine, histidine; the rest neutral) and as aromatic (tyrosine, phenylalanine, tryptophan) versus aliphatic. - **Bond map for matching questions:** peptide bonds join amino acids, glycosidic bonds join sugars, phosphodiester bonds join nucleotides — each formed with loss of a water molecule. - **Protein architecture:** primary (linear sequence), secondary (alpha helix and beta pleated sheet, hydrogen-bonded), tertiary (compact 3-D folding), quaternary (assembly of chains, as in the 2-alpha-2-beta globin units of haemoglobin). - **Carbohydrate division:** monosaccharides include glucose, fructose and the pentoses ribose and deoxyribose; storage polysaccharides are starch in plants and glycogen in animals; cellulose is the structural polysaccharide of plant cell walls. - **The lipid exception:** lipids have molecular weights below 1000 daltons — they are not true macromolecules — yet they appear in the acid-insoluble pool because cell membranes break into vesicles during extraction. This is the single most examined line of the chapter. - **Nucleic acid grammar:** a nucleotide equals nitrogenous base plus pentose sugar plus phosphate; in double-stranded DNA, adenine pairs thymine and guanine pairs cytosine (Chargaff's equivalence), with the two chains running antiparallel. - **Primary versus secondary metabolites:** primary metabolites (proteins, carbohydrates, lipids, nucleic acids) occur in all cells; secondary metabolites — alkaloids, flavonoids, terpenoids — have limited distribution and supply many drugs, such as quinine and codeine. ## How an acid extraction teaches macromolecule logic Grind any living tissue in trichloroacetic acid and the sludge sorts itself into two pools, and NCERT uses this experiment to define what "big" means biochemically. The acid-soluble pool holds the micromolecules — amino acids, sugars, nucleotide bases, the small stuff. The acid-insoluble pool holds proteins, nucleic acids and polysaccharides, which are genuine polymers with molecular weights from a thousand to over a million daltons. Then comes the twist worth a whole MCQ: lipids are chemically small, yet they sediment with the insoluble pool, because when the membrane is disrupted it curls into vesicles that do not dissolve. So "acid-insoluble" is an operational definition, not a synonym for macromolecule. Reason it through once and you never forget why the filter paper of the exam always catches lipids. Follow the logic a step further. The acid-soluble pool holds the working metabolites, whose simultaneous synthesis and degradation keeps the living state far from equilibrium — a steady state that must draw energy from outside to persist, a favourite assertion in assertion-reason questions. ## How NEET frames biomolecules The chapter is mined for match-the-following and statement sets rather than long case questions. Repeated templates include: bond-type matching (peptide, glycosidic, phosphodiester); classifying amino acids as acidic, basic or aromatic; and the classic trap pair about the acid-insoluble pool, where options try to make you deny that lipids belong there or claim they are polymers. A second trap calls the alpha helix a tertiary feature; NCERT assigns helices and sheets to secondary structure. Expect also the twenty-amino-acid alphabet line and haemoglobin illustrating quaternary structure. In previous-year papers these appear as one-mark facts, so precise recall of the NCERT wording beats elaborate understanding here. ## Frequently asked questions ### Why do lipids appear in the acid-insoluble fraction despite low molecular weight? During extraction with trichloroacetic acid, membrane lipids fragment into vesicles that fail to dissolve, so they sediment with true macromolecules even though their molecular weights are below about 1000 daltons. ### Which amino acids are classified as basic, and on what criterion? Lysine, arginine and histidine carry more amino groups than carboxyl groups; the same counting logic places glutamic and aspartic acids in the acidic group. ### What distinguishes quaternary protein structure from the other levels? Quaternary structure is the assembly of more than one polypeptide chain into a functional unit, as in haemoglobin's two alpha and two beta chains; the other levels describe a single chain's sequence, local folding and 3-D shape. ### What did Chargaff's data establish about DNA? In double-stranded DNA the molar ratio of adenine equals that of thymine and of guanine equals that of cytosine, the numerical base of Watson-Crick base pairing. ### How do secondary metabolites differ from primary metabolites? Secondary metabolites such as alkaloids, flavonoids and terpenoids occur only in specific taxa and stages, whereas primary metabolites are universal; many secondary metabolites serve as drugs, dyes and spices.