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How many marks is Biomolecules in Class 12 Chemistry 2026-27?
Biomolecules carries 7 of the 70 theory marks in the CBSE Class XII Chemistry curriculum for 2026-27. The curriculum also notes no chapter-wise weightage is provided, so 7 is a syllabus share, not a promise.
Your text is NCERT Chemistry Part II, Unit 10, Reprint 2026-27, covering carbohydrates, proteins, enzymes, vitamins, nucleic acids and hormones. For entrance-exam practice, see our Biomolecules guide for JEE Main; this post is about board answers.
Which Biomolecules topics come up in board answers?
Learn these as NCERT states them:
Glucose structure evidence. Five reactions fix the open chain (see the chart above). Three facts it cannot explain point to a six-membered cyclic hemiacetal.
Anomers. The α and β forms differ only in the –OH at C1, the anomeric carbon.
Reducing vs non-reducing sugars. Reducing sugars reduce Fehling's and Tollens' reagents; every monosaccharide does. Sucrose is non-reducing because its bond joins C1 of α-D-glucose to C2 of β-D-fructose, using both reducing groups. Maltose and lactose keep a free group.
Glycosidic linkages. Maltose: α-D-glucose C1 to C4. Lactose: β-D-galactose C1 to glucose C4. Amylose: unbranched C1–C4. Amylopectin: C1–C4 with C1–C6 branches. Cellulose: β-D-glucose, C1–C4. See why some rice is sticky for these in your kitchen.
Protein structure and denaturation. Primary is the amino-acid sequence; secondary is the α-helix or β-pleated sheet, held by hydrogen bonds between C=O and –NH–; tertiary is overall folding (fibrous or globular); quaternary is the arrangement of sub-units. Heat or a pH change breaks hydrogen bonds, destroying secondary and tertiary structure but not primary (boiled egg white, curdled milk).
Vitamins and deficiencies. A, D, E, K are fat-soluble and stored; B-group and C are water-soluble and excreted, so needed regularly (B12 is stored). Diseases: A, night blindness and xerophthalmia; B1, beri beri; B12, pernicious anaemia; C, scurvy; D, rickets and osteomalacia; K, longer blood clotting time.
DNA vs RNA. DNA has β-D-2-deoxyribose and bases A, G, C, T; RNA has β-D-ribose and A, G, C, U. DNA is a double helix (A pairs with T, C with G); RNA is single-stranded, as mRNA, rRNA and tRNA.
How do you set out 2-, 3- and 5-mark answers?
The 2026-27 sample paper puts 2-mark questions in Section B, 3-mark in Section C, 4-mark case studies in Section D and 5-mark long answers in Section E. Write one point per mark: a fact with its reason. The marking scheme for Question 19 gives 1 mark for completing the bases in a DNA strand and 1 for saying lemon juice lowers pH, breaks hydrogen bonds and denatures casein. For 3 marks, give three labelled parts, each naming reagent and product. For 5 marks, add structures and a closing inference.
The same question, twice:
2-mark version: give two facts the open-chain structure of glucose cannot explain.
Despite its –CHO group, glucose gives no Schiff's test and no addition product with NaHSO₃.
Glucose pentaacetate does not react with hydroxylamine, so no free –CHO is present.
5-mark version: which reactions show glucose's open-chain nature, and which show its cyclic nature?
Open chain, as reagent, product, inference: HI on prolonged heating gives n-hexane (straight six-carbon chain); hydroxylamine gives an oxime and HCN a cyanohydrin (carbonyl group); bromine water gives gluconic acid (aldehyde); acetic anhydride gives a pentaacetate (five –OH groups); nitric acid gives saccharic acid (primary –OH).
Cyclic: the Schiff's and NaHSO₃ results, the unreactive pentaacetate, and two crystalline forms, α (m.p. 419 K) and β (m.p. 423 K).
Conclusion: the C5 –OH adds to –CHO, forming a six-membered cyclic hemiacetal. α and β are anomers at C1, in equilibrium with the open chain. Draw both Haworth structures.
Part 2 reuses the 2-mark answer; the conclusion is what answers "show".
What are the assertion-reason and case-study traps?
Assertion-reason items in the sample paper use four fixed options, from "both true, R explains A" to "A false, R true". Traps here:
"Sucrose is reducing because it contains glucose": the assertion is false.
"Denaturation destroys primary structure": false; only secondary and tertiary go.
D and L give configuration relative to glyceraldehyde, not rotation: D-(–)-fructose is laevorotatory.
Case study Question 30 gives a water-soluble plant sugar that turns Benedict's reagent brick-red and gives only one kind of monosaccharide on hydrolysis. The marking scheme answer is maltose: reducing, so not sucrose; it hydrolyses, so not glucose or fructose; one kind of monosaccharide only, so not lactose. The trap is writing "glucose" after the first clue; let each clue eliminate a candidate.
A six-day Biomolecules revision plan
Reducing sugars and glucose evidence; redraw the chart from memory.
Cyclic glucose, anomers, Haworth structures, D/L.
Sugars table: units, linkage, reducing or not.
Peptide bond, structure levels, denaturation.
Vitamins, enzymes, hormones, DNA vs RNA.
Sample paper Questions 7, 19 and 30, timed, marked with the scheme; then NCERT exercises.
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