Educational Galaxy

Chapter 10: Biomolecules — EG's own summary, in simple words

Book: Chemistry — Part II (CBSE, Class 12)  |  Chapter 10  |  Medium: English

Living things are built from a small set of molecules: carbohydrates for energy and structure, proteins for work, nucleic acids for information, and vitamins and hormones as regulators. This chapter walks through each family — their structures, their characteristic reactions, and the tests and properties the board asks about — ending with DNA, the double helix that carries heredity.

Diagram (labels on the image): Carbohydrate, Protein, Nucleic acid (labels as drawn). Labels added by Educational Galaxy on an originally generated illustration.

EG schematic (drawn and labelled by Educational Galaxy): DNA double helix at a glance: sugar–phosphate backbones outside, base pairs inside — A with T (two hydrogen bonds), G with C (three) — with the 5′→3′ strands running antiparallel.

10.1 Carbohydrates — monosaccharides, disaccharides, polysaccharides

Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that hydrolyse to them. Monosaccharides cannot be hydrolysed further: glucose and fructose (both C₆H₁₂O₆), ribose (C₅H₁₀O₅). Glucose is an aldohexose; fructose is a ketohexose. In solution, glucose exists mainly as a six-membered ring (pyranose): the open chain’s C5–OH attacks C1, forming a cyclic hemiacetal. The new C1 carbon is the anomeric carbon; α-D-glucose (OH down) and β-D-glucose (OH up) are anomers.

Tests for glucose: it reduces Tollens’ reagent and Fehling’s solution (the aldehyde group), gives a pentaacetate with acetic anhydride (five OH groups), and does not restore the pink colour of Schiff’s reagent quickly — evidence used to argue for the ring form (the free aldehyde is present only in traces). Glucose also forms an oxime with hydroxylamine and adds HCN, proving a carbonyl group exists in the open form.

Disaccharides: sucrose (glucose + fructose, α-1,2-glycosidic link) is non-reducing because both anomeric carbons are tied up in the link; maltose (glucose + glucose, α-1,4 link) and lactose (galactose + glucose, β-1,4 link) are reducing because one anomeric carbon is free. Sucrose is dextrorotatory but hydrolyses to laevorotatory invert sugar (glucose + fructose, fructose’s strong laevorotation wins) — hence the name ‘invert sugar’.

Polysaccharides: starch (amylose, α-1,4 links, helical and soluble; amylopectin, α-1,4 with α-1,6 branches) is the plant energy store and gives a blue colour with iodine (amylose helix traps iodine). Cellulose (β-1,4 links, straight chains hydrogen-bonded into fibres) is the plant structural material; humans cannot digest it because we lack the enzyme for β-links. Glycogen (like amylopectin but more branched) is the animal store.

10.2 Proteins — amino acids, peptide bond, structure

Proteins are polymers of α-amino acids: each has NH₂, COOH, H and a side chain R on one carbon. Twenty amino acids occur in proteins. They are classified by side chain: neutral (glycine, alanine), acidic (aspartic acid, glutamic acid — extra COOH), basic (lysine, arginine — extra NH₂), and sulphur-containing (cysteine, methionine). Essential amino acids (about ten, including lysine, leucine, tryptophan) cannot be made by the body and must come from food.

Amino acids are zwitterions in water: the COOH donates its proton to NH₂, giving —COO⁻ and —NH₃⁺ in the same molecule. They are therefore amphoteric, have high melting points, and migrate in an electric field according to pH: at the isoelectric point the net charge is zero and they do not move.

The peptide bond forms when the COOH of one amino acid condenses with the NH₂ of the next, losing water: —CO–NH—. The link is planar and partly double-bonded (resonance), so rotation is restricted — this stiffness shapes proteins.

Levels of protein structure: primary (the amino-acid sequence); secondary (α-helix held by hydrogen bonds along the chain, and β-pleated sheet held by hydrogen bonds between chains); tertiary (overall 3D folding from hydrogen bonds, disulphide bridges, ionic bonds and hydrophobic packing); quaternary (several polypeptide subunits together, e.g. haemoglobin’s four chains). Denaturation (heat, acid, heavy metals) destroys secondary and higher structures without breaking peptide bonds — the protein loses its function, as when egg white sets on cooking. Fibrous proteins (keratin, collagen) are tough and insoluble; globular proteins (enzymes, haemoglobin, insulin) are folded, soluble and functional.

10.3 Enzymes and vitamins

Enzymes are biological catalysts — mostly proteins, some RNA (ribozymes). They work by the lock-and-key or induced-fit model: the substrate fits the active site, the reaction runs, and the enzyme is unchanged. They are specific, work best at an optimum temperature (about 37 °C) and pH, and are slowed by inhibitors. Cofactors help some enzymes: metal ions (Zn²⁺ in carboxypeptidase) or coenzymes (NAD, derived from vitamins).

Vitamins are organic compounds needed in small amounts, not made (or not enough) by the body. Fat-soluble vitamins (A, D, E, K) are stored in the liver and fat; excess can be toxic. Water-soluble vitamins (B group, C) are not stored and must come regularly in food; excess is excreted. Deficiency diseases: A — night blindness (xerophthalmia); D — rickets; E — muscle weakness and anaemia; K — delayed blood clotting; B₁ — beri-beri; B₂ — cheilosis; B₆ — convulsions; B₁₂ — pernicious anaemia; C — scurvy.

10.4 Nucleic acids — DNA and RNA

A nucleotide has three parts: a pentose sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base. Bases: adenine (A) and guanine (G) are purines (double ring); cytosine (C), thymine (T) and uracil (U) are pyrimidines (single ring). DNA uses A, G, C, T; RNA uses A, G, C, U (uracil replaces thymine).

Nucleotides join by phosphodiester bonds between the 3′ carbon of one sugar and the 5′ carbon of the next, forming a polynucleotide chain with direction 5′ → 3′. A nucleoside is base + sugar only (no phosphate); a nucleotide is nucleoside + phosphate.

Watson–Crick DNA: two antiparallel strands wind into a double helix; the sugar–phosphate backbones are outside and the bases point inward, pairing A with T (two hydrogen bonds) and G with C (three hydrogen bonds). Chargaff’s rule follows: amount of A equals T, and G equals C. The sequence of bases carries genetic information; replication copies each strand into a new double helix.

RNA is usually single-stranded and comes in three kinds: messenger RNA (carries the code from DNA), transfer RNA (brings amino acids, with an anticodon), and ribosomal RNA (builds ribosomes). The central dogma: DNA → RNA → protein. Differences between DNA and RNA: DNA is double-stranded, has deoxyribose and thymine, and is chemically stable (the store); RNA is single-stranded, has ribose and uracil, and is reactive (the worker).

DNA vs RNA

Feature	DNA	RNA
Strands	Double helix, antiparallel	Usually single-stranded
Sugar	Deoxyribose	Ribose
Bases	A, G, C, T	A, G, C, U
Base pairing	A–T (2 H-bonds), G–C (3 H-bonds)	Pairs with itself in folds
Role	Genetic store	Messenger, adapter, ribosome builder
Stability	Stable	Less stable, short-lived

Key Terms / Important Terms

• Monosaccharide: A simple sugar that cannot be hydrolysed further (glucose, fructose).
• Anomer: Isomers differing only at the anomeric (C1) carbon: α vs β glucose.
• Glycosidic linkage: The oxygen bridge between sugar units (α-1,4 in starch, β-1,4 in cellulose).
• Zwitterion: An amino acid with both —COO⁻ and —NH₃⁺; net charge zero at the isoelectric point.
• Peptide bond: The —CO–NH— link between amino acids; planar, partly double-bonded.
• Denaturation: Loss of secondary/tertiary structure (and function) without breaking peptide bonds.
• Nucleoside / nucleotide: Base + sugar / nucleoside + phosphate.
• Chargaff’s rule: In DNA, A = T and G = C (base pairing).

Practice set — try it yourself

1. Why is sucrose non-reducing while maltose is reducing, even though both are disaccharides?
2. What is an anomer? Draw α- and β-D-glucose in words (position of the C1–OH).
3. Why can humans digest starch but not cellulose?
4. What is a zwitterion? Why do amino acids have high melting points?
5. State Chargaff’s rule and explain it from base pairing.

Questions with Answers — full answers

1. Why does glucose give a pentaacetate with acetic anhydride but not a hexaacetate? Answer: Glucose has five OH groups in its structure (four on the chain plus the ring OH in the cyclic form is the same carbon count — the open chain has five OH groups). Acetic anhydride acetylates each OH, giving the pentaacetate; there is no sixth OH.
2. What is invert sugar? Why is it called ‘invert’? Answer: Hydrolysed sucrose gives glucose + fructose. Sucrose rotates light to the right (dextrorotatory), but the mixture rotates to the left because fructose’s strong laevorotation outweighs glucose’s dextrorotation — the sign of rotation inverts.
3. How do amylose and amylopectin differ? Which gives the blue colour with iodine, and why? Answer: Amylose is unbranched (α-1,4 links) and coils into a helix; amylopectin is branched (α-1,4 with α-1,6 branch points). Amylose gives the blue colour because iodine molecules fit inside its helix.
4. Why does egg white become solid and white on heating? Name the process. Answer: Denaturation: heat breaks the hydrogen bonds and other weak forces holding the protein’s secondary and tertiary structure, so the protein unfolds, aggregates and loses solubility — but no peptide bonds break.
5. Distinguish fibrous from globular proteins with one example each. Answer: Fibrous proteins (keratin in hair, collagen in tendons) are long, tough, water-insoluble structural fibres. Globular proteins (haemoglobin, insulin, enzymes) are folded into compact soluble shapes with specific functions.
6. Why are water-soluble vitamins needed regularly in food, while fat-soluble vitamins are not? Answer: Water-soluble vitamins (B, C) are not stored and are excreted in urine, so they must be replaced daily. Fat-soluble vitamins (A, D, E, K) are stored in liver and fat and drawn on over time.
7. A DNA sample has 20% adenine. What percentage of guanine does it have? Answer: A = T = 20%, so A + T = 40%. The rest is G + C = 60%, split equally: G = 30%, C = 30%.
8. What is the difference between a nucleoside and a nucleotide? Give one example of each. Answer: A nucleoside is base + sugar (e.g. adenosine). A nucleotide is nucleoside + phosphate (e.g. adenosine monophosphate, AMP).

Extension Questions with Answers

1. Glucose reacts with hydroxylamine to give an oxime and with HCN to give a cyanohydrin, yet it does not give a positive Schiff’s test quickly. Explain. Answer: These reactions prove a carbonyl group exists in the open-chain form, but in solution glucose is almost entirely in the cyclic hemiacetal form with only traces of open chain. Slow opening limits the Schiff’s reaction, while oxime/cyanohydrin formation pulls the equilibrium open over time.
2. Why is the peptide bond planar? What does this mean for protein shape? Answer: Resonance gives the C–N bond partial double-bond character, preventing rotation. The chain can therefore only bend at the α-carbon angles, which constrains folding into helices and sheets.
3. A protein is treated with heavy metal ions and loses activity but its amino-acid sequence is unchanged. Explain. Answer: Heavy metals denature the protein: they disrupt tertiary structure (binding to sulphur groups, for example) without breaking peptide bonds, so the primary sequence survives but the functional shape is lost.
4. Why is RNA more reactive and shorter-lived than DNA? Answer: RNA has a 2′-OH group on ribose that can attack its own phosphate backbone (self-cleavage), and it is single-stranded, so its bases are exposed. DNA lacks the 2′-OH and hides its bases inside the helix.
5. Name the vitamin whose deficiency causes (a) scurvy, (b) rickets, (c) beri-beri, (d) pernicious anaemia. Answer: (a) Vitamin C. (b) Vitamin D. (c) Vitamin B₁ (thiamine). (d) Vitamin B₁₂.

Board-pattern Questions with Answers

1. Define essential amino acids. Name two. Answer: Amino acids the body cannot synthesise, so they must come from food. Examples: lysine, leucine, tryptophan, valine, isoleucine.
2. Why do amino acids show amphoteric behaviour? Answer: They carry both an acidic —COOH and a basic —NH₂ group (as a zwitterion), so they react with both acids and bases.
3. What are the two components of starch? Which one is water-soluble? Answer: Amylose (unbranched, water-soluble, helical) and amylopectin (branched, water-insoluble).
4. Name the sugar present in (a) DNA, (b) RNA. Which base is unique to each? Answer: (a) Deoxyribose; thymine is unique to DNA. (b) Ribose; uracil is unique to RNA.
5. What is the role of the 5′ → 3′ direction in a polynucleotide chain? Answer: It defines the chain’s direction: nucleotides link the 3′ carbon of one sugar to the 5′ carbon of the next via phosphodiester bonds, and replication and transcription read and build in this direction.