Class 12 Biomolecules — The Structures Worth Knowing

Biomolecules looks like a reading chapter, and that is exactly why students lose marks in it. The questions are precise: which sugar is reducing, which linkage joins starch, what survives denaturation, the difference between a nucleoside and a nucleotide. This guide takes the structures that actually get asked, states each one carefully in words and tables, and works the numerical checks that prove you have the formula right.

Carbohydrates — the classification you are graded on

Glucose — an aldohexose, described carbon by carbon

Glucose is C₆H₁₂O₆. Its open-chain structure is a straight six-carbon chain:

Why the open chain is not the whole story. Glucose fails some standard aldehyde tests: it gives no reaction with sodium hydrogensulphite or ammonia, it does not form the expected Schiff's test result, and its pentaacetate does not react with hydroxylamine. These observations show that the aldehyde group is not freely available — glucose exists mostly as a six-membered pyranose ring, formed when the –OH on C5 adds across the C1 carbonyl.

Ring closure creates a new stereocentre at C1, giving two forms called α-D-glucose and β-D-glucose. In solution the two interconvert through the open-chain form, so the optical rotation of a freshly prepared solution changes until it settles at an equilibrium value. This is mutarotation: pure α-D-glucose starts at about +111°, pure β-D-glucose at about +19.2°, and both drift to the equilibrium value of about +52.5°.

Fructose, also C₆H₁₂O₆, is a ketohexose — the carbonyl is at C2, not C1 — and it closes into a five-membered furanose ring. Even though it is a ketone, fructose still reduces Tollens' and Fehling's reagents, because in alkaline solution it rearranges to an aldose. That single sentence answers a very common two-mark question.

Disaccharides — the linkage decides everything

Invert sugar. Sucrose is dextrorotatory, about +66.5°. Hydrolysis gives an equimolar mixture of glucose (+52.5°) and fructose (−92.4°). The sign of the mixture flips to negative, which is why hydrolysed sucrose is called invert sugar.

Polysaccharides — same monomer, different linkage

Starch and cellulose are built from the same sugar. The only difference is α versus β at the linking carbon — and that one difference is why humans can digest starch but not cellulose.

Worked example 1 — glucose, sucrose, and a hydrolysis cross-check

Proteins — from one amino acid to four levels of structure

An α-amino acid carries both an amino group and a carboxyl group on the same carbon:

There are 20 α-amino acids in proteins; about 10 of them cannot be made by the human body and must come from the diet, so they are called essential. In the solid state and in neutral solution an amino acid exists as a zwitterion, ⁺H₃N–CH(R)–COO⁻, which is why amino acids are high-melting, water-soluble solids rather than ordinary organic liquids. At the isoelectric point the zwitterion carries no net charge and does not migrate in an electric field.

Two amino acids join by losing a water molecule between the –COOH of one and the –NH₂ of the other. The resulting –CO–NH– link is the peptide bond.

Denaturation — heating, or adding acid — destroys the secondary and tertiary structure while leaving the primary structure intact, so the protein loses its biological activity. Boiling an egg and curdling milk are the two standard examples.

Worked example 2 — a dipeptide, checked two ways

Enzymes and vitamins — the exam-relevant facts

Enzymes are biocatalysts, and almost all of them are proteins. They are extremely specific — one enzyme typically catalyses one reaction — and they work by lowering the activation energy of that reaction, so it proceeds far faster at body temperature than it otherwise would. Because they are proteins, they are destroyed by the same conditions that denature any protein.

Vitamins divide into two groups by solubility, and the grouping explains the storage behaviour:

These deficiency links are syllabus content for a chemistry paper, not medical guidance. Nutrition and any supplement decision belongs to a qualified doctor, never to a textbook list.

Nucleic acids — get these two definitions exactly right

Adenine and guanine are purines (two fused rings); cytosine, thymine and uracil are pyrimidines (one ring). In the double helix the bases pair specifically: A with T through two hydrogen bonds, G with C through three. Because G–C pairs have an extra hydrogen bond, a region rich in G and C is harder to separate — a fact worth remembering as the reason the pairing is not arbitrary.

Mistakes that lose marks in this chapter

Where biomolecules is examined