Biomolecules
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Direct answer
Glucose fails the Schiff test and forms no bisulphite adduct despite containing an aldehyde — its open chain closes into a cyclic hemiacetal (pyranose) that hides the carbonyl, and this one fact anchors carbohydrate chemistry for NEET. Proteins are polypeptides of twenty α-amino acids whose levels of structure (primary sequence, secondary α-helix and β-sheet, tertiary, quaternary) collapse on denaturation — egg white coagulates while the primary sequence survives. DNA carries the hereditary code in a Watson–Crick double helix where adenine pairs with thymine through two hydrogen bonds and guanine with cytosine through three.
What you must remember
- Monosaccharide evidence: Br2 water oxidises glucose (aldehyde present) to gluconic acid; acetic anhydride gives the pentaacetate (five OH groups); HI reduces it to n-hexane (a six-carbon straight chain).
- Anomers α and β differ at the new carbon created by ring closure; equilibrium in solution is mutarotation.
- Sucrose is non-reducing (the glycosidic bond consumes both anomeric carbons); maltose joins glucose–glucose α-1,4; lactose is galactose–glucose β-1,4.
- Starch is about 20 per cent amylose (linear, blue with iodine) and 80 per cent amylopectin (branched through α-1,6 links); cellulose is β-1,4 glucose, undigestible by humans.
- Ten amino acids are essential (valine, leucine, isoleucine, phenylalanine, tryptophan, methionine, threonine, lysine, arginine, histidine); amino acids exist as zwitterions at their isoelectric point.
- Vitamin deficiency map: B1 beri-beri, C scurvy, D rickets and osteomalacia, K poor coagulation; vitamins B and C are water-soluble (not stored), A, D, E and K fat-soluble.
- Enzymes are globular protein colloidal catalysts, highly specific, each with an optimum temperature and pH.
- Nucleotides join through phosphodiester links; RNA substitutes uracil for thymine and ribose for deoxyribose.
Chargaff arithmetic, step by step
If a DNA sample contains 20 per cent adenine, what is its guanine percentage? Reason from base-pairing, not memory tables. Chargaff's rule follows directly from Watson–Crick pairing: adenine pairs only with thymine, so the two occur in equal amounts. Twenty per cent adenine therefore means 20 per cent thymine, consuming 40 per cent of the bases. The remaining 60 per cent splits equally between guanine and cytosine, since G pairs only with C — so guanine is 30 per cent. The template generalises to every such question: set A = T, G = C, and let the four sum to 100. Note that this applies to double-stranded DNA only; in single-stranded RNA or DNA, no such equality is forced, and a distracter option will usually assume it anyway. The same hydrogen-bond arithmetic explains melting behaviour too — a GC-rich DNA strand needs more heat to separate than an AT-rich one, because every GC pair carries three hydrogen bonds against adenine-thymine's two.
One Greek letter, one lost mark
Reducing versus non-reducing is the carbohydrate trap of the decade: sucrose's glycosidic linkage ties up both anomeric carbons, so no free aldehyde can open — sucrose answers no Tollens test, while maltose and lactose do. Second, linkage identity: α-1,4 for starch and maltose, α-1,6 at amylopectin branch points, β-1,4 for cellulose — one Greek letter flips the entire digestion question, because humans lack cellulase. Third, denaturation destroys secondary and tertiary structure while the primary sequence survives; calling it "hydrolysis of the protein" is wrong and is offered as an option. Fourth, nucleoside versus nucleotide: the nucleotide adds a phosphate ester bond to the nucleoside's sugar — the distinction is one ester bond, and the exam states it exactly that way.
Frequently asked questions
Why is sucrose a non-reducing sugar?
Its glucose–fructose glycosidic bond involves both anomeric carbons, leaving no free hemiacetal that can reopen to an aldehyde.
If DNA has 20 per cent adenine, what is its guanine content?
30 per cent — A = T (20 per cent each), so G = C splits the remaining 60 per cent.
Which vitamins are water-soluble and what follows from that?
B-complex and C; the body cannot store them, so excess is excreted and a regular dietary supply is needed — the reason sailors' scurvy preceded vitamin C knowledge.
What exactly is lost when a protein denatures?
Secondary and tertiary (and quaternary) structure, with loss of biological activity; the primary peptide sequence remains chemically intact.
Which bond joins the glucose units of cellulose?
A β-1,4-glycosidic linkage, which no human enzyme can hydrolyse — ruminants manage only through symbiotic microorganisms.