Biomolecules
Welcome to HSLC Guru. This chapter, prepared strictly according to the ASSEB (Assam State School Education Board) Class 12 Chemistry syllabus, presents complete English-medium notes and question-answers on Biomolecules. You will study the chemistry of carbohydrates, proteins, enzymes, vitamins and nucleic acids — the molecules that build and run every living cell. The material is designed for HS final examination preparation and includes detailed long answers, short answers, MCQs, fill-in-the-blanks, true/false statements and a glossary.
Summary
Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that yield such units on hydrolysis. They are classified by the number of sugar units released on hydrolysis: monosaccharides (glucose, fructose, ribose) cannot be hydrolysed further; oligosaccharides (sucrose, maltose, lactose) yield 2–10 monosaccharides; and polysaccharides (starch, cellulose, glycogen) yield many. Sugars containing an aldehyde group are aldoses (e.g. glucose) and those with a ketonic group are ketoses (e.g. fructose). Configuration about the carbon farthest from the carbonyl group decides the D- or L- designation; almost all naturally occurring sugars are D-sugars. Glucose exists as an open-chain Fischer projection and as cyclic α- and β-pyranose forms drawn as Haworth structures; fructose forms a furanose ring. Glucose undergoes oxidation (Tollens, Fehling, bromine water → gluconic acid; HNO₃ → saccharic acid), reduction (Na/Hg → sorbitol), reaction with HCN (cyanohydrin), HI (n-hexane) and acetylation (penta-acetate). The presence of a free anomeric carbon makes glucose, fructose and maltose reducing sugars; sucrose is non-reducing. Acid or invertase hydrolysis of sucrose gives an equimolar mixture of glucose and fructose called invert sugar. Starch (amylose + amylopectin, α-1,4 and α-1,6 links) is the storage carbohydrate in plants; glycogen is the animal counterpart with more branching; cellulose (β-1,4 linked glucose) is the main structural carbohydrate of plant cell walls.
Proteins are polymers of α-amino acids joined by peptide (–CO–NH–) bonds. Of the 20 standard amino acids, those the human body cannot synthesise and must obtain through diet are essential (e.g. valine, leucine, lysine, methionine), the remainder are non-essential. In aqueous solution an amino acid exists as a dipolar zwitterion (H₃N⁺–CHR–COO⁻); the pH at which the molecule carries no net charge is its isoelectric point (pI). Protein structure is described at four levels: primary — the sequence of amino acids; secondary — local folding stabilised by H-bonds, giving the right-handed α-helix and the pleated β-sheet; tertiary — overall three-dimensional folding stabilised by H-bonds, disulphide, ionic and hydrophobic interactions; and quaternary — assembly of two or more polypeptide subunits (e.g. haemoglobin). Proteins are classified as fibrous (keratin, collagen) or globular (insulin, albumin). Loss of biological activity due to disruption of secondary, tertiary and quaternary structures by heat, acid, base, alcohol or heavy metals — without breaking peptide bonds — is called denaturation (e.g. coagulation of egg-white on boiling).
Enzymes are globular proteins that act as highly specific biological catalysts; they typically end in -ase (e.g. maltase, urease, pepsin). They lower the activation energy of biochemical reactions and operate at optimum pH and temperature. Vitamins are organic micronutrients required in small amounts; they are divided into fat-soluble vitamins A, D, E and K (stored in the body) and water-soluble vitamins — the B-complex group (B₁, B₂, B₆, B₁₂ etc.) and vitamin C (not stored, must be supplied regularly). Each vitamin has a characteristic deficiency disease: vitamin A → night blindness/xerophthalmia, D → rickets and osteomalacia, E → muscular weakness/sterility, K → poor blood clotting, B₁ → beri-beri, B₂ → cheilosis, B₆ → convulsions, B₁₂ → pernicious anaemia, C → scurvy, niacin → pellagra.
Nucleic acids — DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) — are biopolymers that store and express genetic information. Their monomers are nucleotides, each made of a nitrogenous base (purine: adenine A, guanine G; pyrimidine: cytosine C, thymine T in DNA, uracil U in RNA), a pentose sugar (β-D-2-deoxyribose in DNA, β-D-ribose in RNA), and a phosphate group; a base + sugar without phosphate is a nucleoside. Watson and Crick (1953) proposed the DNA double helix: two antiparallel polynucleotide strands wound right-handedly about a common axis, with sugar-phosphate backbones outside and base pairs (A=T two H-bonds, G≡C three H-bonds) stacked inside. DNA undergoes replication (semi-conservative duplication of itself before cell division) and transcription (copying of a DNA segment into mRNA); the mRNA is then translated on ribosomes into protein. The three principal RNAs are messenger RNA (mRNA) — carries the genetic message from DNA to ribosomes; transfer RNA (tRNA) — brings specific amino acids to the ribosome; and ribosomal RNA (rRNA) — structural and catalytic component of ribosomes. Biological functions of nucleic acids include heredity, protein synthesis, control of metabolism and cellular regulation.
1-Mark Questions
Q1. Define carbohydrates.
Answer: Carbohydrates are optically active polyhydroxy aldehydes or polyhydroxy ketones, or compounds that produce such units on hydrolysis.
Q2. What are reducing sugars?
Answer: Sugars that reduce Tollens’ reagent or Fehling’s solution because they contain a free aldehydic or ketonic group (free anomeric carbon) are called reducing sugars; e.g. glucose, fructose, maltose, lactose.
Q3. Why is sucrose called a non-reducing sugar?
Answer: In sucrose the anomeric carbons of glucose (C-1) and fructose (C-2) are joined by a glycosidic linkage, so no free aldehyde or ketone is available; hence sucrose does not reduce Fehling’s solution.
Q4. What is invert sugar?
Answer: The equimolar mixture of D-glucose and D-fructose obtained by hydrolysis of sucrose is called invert sugar, because the optical rotation changes (inverts) from dextro (+) to laevo (–) during hydrolysis.
Q5. Name one essential and one non-essential amino acid.
Answer: Essential — valine (or lysine, leucine); non-essential — glycine (or alanine, serine).
Q6. Define peptide bond.
Answer: The amide linkage –CO–NH– formed between the –COOH group of one amino acid and the –NH₂ group of another amino acid, with loss of a water molecule, is called a peptide bond.
Q7. What is denaturation of protein?
Answer: The process in which a protein loses its biological activity due to disruption of its secondary, tertiary and quaternary structures (without breaking peptide bonds), caused by heat, change in pH, alcohol or heavy metals, is called denaturation.
Q8. Name the disease caused by deficiency of vitamin C.
Answer: Scurvy (bleeding gums).
Q8a. Name two fat-soluble vitamins.
Answer: Vitamin A (retinol) and Vitamin D (calciferol). Vitamins E and K are also fat-soluble.
Q8b. Why are vitamins called accessory food factors?
Answer: Vitamins do not provide energy and are not building units of the body, but they are essential in small amounts for normal growth and metabolism. Hence they are called accessory food factors.
Q9. Which sugar is present in DNA and which in RNA?
Answer: DNA contains β-D-2-deoxyribose; RNA contains β-D-ribose.
Q10. What is a nucleotide?
Answer: A nucleotide is the monomer unit of a nucleic acid composed of a nitrogenous base, a pentose sugar and a phosphate group.
Q11. Name the four bases present in DNA.
Answer: Adenine (A), Guanine (G) — purines; Cytosine (C), Thymine (T) — pyrimidines.
Q12. Write the chemical name of vitamin B₁ and the disease caused by its deficiency.
Answer: Thiamine; deficiency causes beri-beri.
2–3 Marks Questions
Q1. Distinguish between aldoses and ketoses with one example each.
Answer:
| Feature | Aldoses | Ketoses |
|---|---|---|
| Functional group | Aldehyde (–CHO) at C-1 | Ketone (>C=O) usually at C-2 |
| Reducing nature | Strongly reducing | Reducing (via tautomerism) |
| Example | D-Glucose, D-galactose | D-Fructose, dihydroxyacetone |
Q2. Define D- and L-configuration in sugars. To which series does naturally occurring glucose belong?
Answer: A monosaccharide is given D-configuration when, in its Fischer projection, the –OH on the chiral carbon farthest from the carbonyl group is on the right; if it is on the left it is L. Naturally occurring glucose is D-(+)-glucose; the D refers to configuration and (+) to the dextrorotatory optical rotation.
Q3. Write any three reactions of glucose that establish its open-chain structure.
Answer: (i) Reaction with HI / red P gives n-hexane → six carbons in straight chain. (ii) Reaction with hydroxylamine forms an oxime, with HCN forms cyanohydrin → presence of carbonyl group. (iii) Acetylation with acetic anhydride gives glucose pentaacetate → five –OH groups. Mild oxidation with bromine water gives gluconic acid (–CHO group); strong oxidation with HNO₃ gives saccharic acid.
Q4. Differentiate between starch and cellulose.
Answer:
| Property | Starch | Cellulose |
|---|---|---|
| Glycosidic linkage | α-1,4 (and α-1,6 branches) | β-1,4 |
| Components | Amylose + amylopectin | Linear, unbranched |
| Function | Energy store in plants | Structural in cell walls |
| Digestibility (humans) | Digestible | Indigestible (no cellulase) |
Q5. Explain the term zwitterion and isoelectric point.
Answer: An α-amino acid contains both –NH₂ (basic) and –COOH (acidic) groups. In aqueous solution the –COOH transfers a proton to the –NH₂ to form a dipolar internal salt, H₃N⁺–CHR–COO⁻, called a zwitterion. The pH at which the zwitterion is exactly balanced and the amino acid carries zero net charge (does not migrate in an electric field) is its isoelectric point (pI); at this pH solubility is minimum.
Q6. Differentiate fat-soluble and water-soluble vitamins with examples.
Answer:
| Property | Fat-soluble | Water-soluble |
|---|---|---|
| Solubility | Soluble in fats/oils | Soluble in water |
| Storage | Stored in liver and adipose tissue | Not stored (excreted in urine) |
| Daily intake | Not required daily | Required regularly |
| Examples | A, D, E, K | B-complex (B₁, B₂, B₆, B₁₂), C |
5–7 Marks Questions
Q1. Classify carbohydrates with examples and state the Haworth structures of α-D-glucopyranose and β-D-fructofuranose.
Answer: Carbohydrates are classified on the basis of behaviour towards hydrolysis:
- Monosaccharides — cannot be hydrolysed further; e.g. glucose, fructose, ribose, galactose.
- Oligosaccharides — yield 2–10 monosaccharides on hydrolysis; the most common are disaccharides such as sucrose (glucose + fructose), maltose (glucose + glucose) and lactose (glucose + galactose).
- Polysaccharides — yield a large number of monosaccharide units; e.g. starch, cellulose, glycogen.
They are also grouped on the basis of taste and reducing nature: sugars (sweet, crystalline — glucose, sucrose) and non-sugars (tasteless, amorphous — starch, cellulose); reducing sugars (glucose, maltose, lactose) and non-reducing sugars (sucrose, all polysaccharides). In the Haworth projection of α-D-glucopyranose, –OH on C-1 is below the ring (α) and on the same side as C-6 –CH₂OH up; in β-D-fructofuranose the five-membered ring contains C-2 to C-5, with –OH at C-2 above the ring (β) and –CH₂OH groups at C-1 and C-6.
Q2. Describe the four levels of structure of proteins.
Answer:
- Primary structure — The specific sequence in which amino acids are joined by peptide bonds in a polypeptide chain. Any change in the sequence produces a different protein (e.g. sickle-cell haemoglobin differs from normal haemoglobin in one amino acid).
- Secondary structure — The local regular folding of the polypeptide chain stabilised by hydrogen bonds between –C=O and –N–H of peptide bonds. Two common forms are: (a) right-handed α-helix — chain coiled like a spring with H-bonds within the same chain (e.g. keratin); (b) β-pleated sheet — chains lie side by side with H-bonds between adjacent chains (e.g. fibroin in silk).
- Tertiary structure — The overall three-dimensional folding of a single polypeptide into a compact globular or fibrous shape, stabilised by H-bonds, disulphide (–S–S–) bridges, ionic salt bridges and hydrophobic interactions. It determines the biological activity (e.g. myoglobin).
- Quaternary structure — The arrangement of two or more polypeptide subunits (each with its own tertiary structure) into a functional protein. Example — haemoglobin has four subunits (two α and two β chains).
Q3. Discuss the structure of DNA double helix as proposed by Watson and Crick.
Answer: In 1953 J. D. Watson and F. H. C. Crick proposed the following model of DNA:
- DNA consists of two right-handed polynucleotide strands wound around a common axis to form a double helix.
- The two strands are antiparallel — one runs 5′→3′ and the other 3′→5′.
- The sugar-phosphate backbones lie on the outside of the helix; the nitrogenous bases project into the interior and are stacked perpendicular to the axis.
- The two strands are held together by hydrogen bonding between complementary bases — Adenine pairs with Thymine through two H-bonds (A=T) and Guanine pairs with Cytosine through three H-bonds (G≡C). This is called Chargaff’s rule of base pairing: A = T and G = C.
- The diameter of the helix is 20 Å (2 nm); each turn (10 base pairs) measures 34 Å along the axis, so the rise per base pair is 3.4 Å.
- The double helix has alternating major and minor grooves.
The complementary base pairing allows DNA to undergo accurate semi-conservative replication: the two strands separate and each acts as a template on which a new complementary strand is built; the two daughter molecules each contain one parent and one new strand.
Q4. Distinguish between DNA and RNA. List the three main types of RNA and their functions.
Answer:
| Feature | DNA | RNA |
|---|---|---|
| Sugar | β-D-2-deoxyribose | β-D-ribose |
| Pyrimidine bases | Cytosine, Thymine | Cytosine, Uracil |
| Purine bases | Adenine, Guanine | Adenine, Guanine |
| Strands | Double-stranded helix | Single-stranded |
| Location | Mostly nucleus (chromosomes) | Mainly cytoplasm |
| Function | Stores and transmits genetic information; replication | Reads genetic information and synthesises proteins |
| Replication | Self-replicating | Synthesised from DNA (transcription) |
Three types of RNA:
- Messenger RNA (mRNA) — carries the genetic message (codons) copied from DNA to the ribosomes where the message is translated into a protein.
- Transfer RNA (tRNA) — clover-leaf shaped, smallest RNA; brings specific amino acids to the ribosome by recognising mRNA codons through its anticodon.
- Ribosomal RNA (rRNA) — most abundant; together with proteins it forms the ribosome, the site of protein synthesis, and provides catalytic activity.
Q5. Write notes on (a) enzymes and their characteristics, (b) deficiency diseases of vitamins.
Answer: (a) Enzymes are globular proteins that act as biological catalysts in the chemical reactions of living systems. Their main characteristics are: (i) high specificity — each enzyme catalyses only one reaction or one type of reaction (lock-and-key model); (ii) efficiency — they speed up reactions 10⁶–10¹² times by lowering the activation energy; (iii) work at optimum temperature (~37 °C) and optimum pH (mostly near neutral, except pepsin ~2 and trypsin ~8); (iv) needed in very small amounts and not consumed; (v) most are named by adding -ase to the substrate name (e.g. maltase hydrolyses maltose, urease hydrolyses urea, lipase acts on lipids, sucrase on sucrose). They can be denatured by heat, acids, alkalis and heavy metals.
(b) Deficiency diseases of vitamins:
| Vitamin | Chemical name | Deficiency disease |
|---|---|---|
| A | Retinol | Night blindness, xerophthalmia |
| B₁ | Thiamine | Beri-beri |
| B₂ | Riboflavin | Cheilosis, glossitis |
| B₆ | Pyridoxine | Convulsions, anaemia |
| B₁₂ | Cyanocobalamin | Pernicious anaemia |
| Niacin (B₃) | Nicotinic acid | Pellagra |
| C | Ascorbic acid | Scurvy |
| D | Calciferol | Rickets (children), osteomalacia (adults) |
| E | Tocopherol | Muscular weakness, sterility |
| K | Phylloquinone | Increased clotting time, haemorrhage |
Q6. Explain the process of replication of DNA in brief.
Answer: DNA replication is the biological process by which a parent DNA molecule produces two identical daughter DNA molecules before cell division. It is described as semi-conservative: each daughter DNA contains one strand of the parent DNA and one newly synthesised strand.
- The enzyme helicase unwinds the double helix and breaks the hydrogen bonds between the complementary base pairs, producing a Y-shaped replication fork.
- Each separated single strand acts as a template.
- The enzyme DNA polymerase reads each template in 3′→5′ direction and adds free deoxyribonucleotides to the growing daughter strand in 5′→3′ direction, following the base-pairing rule (A with T, G with C).
- One strand (leading) is synthesised continuously while the other (lagging) is built in short Okazaki fragments later joined by DNA ligase.
- Two new identical double-helix DNA molecules are obtained, each containing one old and one new strand.
This accurate copying mechanism ensures that genetic information is transmitted unchanged from one cell or generation to the next.
Multiple Choice Questions (MCQ)
1. The sugar present in DNA is —
(a) ribose (b) glucose (c) 2-deoxyribose (d) fructose
Answer: (c) 2-deoxyribose.
2. Which of the following is a non-reducing sugar?
(a) Glucose (b) Maltose (c) Lactose (d) Sucrose
Answer: (d) Sucrose.
3. The carbohydrate that gives only β-D-glucose on hydrolysis is —
(a) starch (b) glycogen (c) cellulose (d) sucrose
Answer: (c) cellulose.
4. Sucrose on hydrolysis with dilute acid gives —
(a) glucose only (b) fructose only (c) glucose + galactose (d) glucose + fructose
Answer: (d) glucose + fructose (invert sugar).
5. The amino acid that does not show optical activity is —
(a) glycine (b) alanine (c) valine (d) leucine
Answer: (a) glycine (no chiral C).
6. The structure of a protein at which biological activity is destroyed by heat is —
(a) primary (b) secondary and tertiary (c) only primary (d) none
Answer: (b) secondary and tertiary structures (denaturation).
7. Which of the following vitamins is water-soluble?
(a) Vitamin A (b) Vitamin D (c) Vitamin C (d) Vitamin K
Answer: (c) Vitamin C.
8. Beri-beri is caused by deficiency of —
(a) Vitamin B₂ (b) Vitamin B₁ (c) Vitamin C (d) Vitamin D
Answer: (b) Vitamin B₁ (thiamine).
9. In DNA, adenine is paired with —
(a) cytosine (b) guanine (c) uracil (d) thymine
Answer: (d) thymine (two H-bonds).
10. The biocatalysts of biological reactions are —
(a) hormones (b) enzymes (c) vitamins (d) lipids
Answer: (b) enzymes.
Fill in the Blanks
1. The simplest amino acid is __________.
Answer: glycine.
2. Storage carbohydrate of animals is __________.
Answer: glycogen.
3. The pH at which an amino acid carries no net charge is called its __________.
Answer: isoelectric point.
4. Pernicious anaemia is caused by deficiency of vitamin __________.
Answer: B₁₂.
5. The base present in RNA but not in DNA is __________.
Answer: uracil.
True / False
1. Sucrose is a reducing sugar. — False (its anomeric carbons are blocked).
2. All naturally occurring α-amino acids (except glycine) are L-amino acids. — True.
3. Vitamin K is required for blood clotting. — True.
4. Cellulose can be digested by humans. — False (we lack cellulase).
5. In DNA, guanine pairs with cytosine through three hydrogen bonds. — True.
6. Enzymes are inorganic catalysts. — False (they are globular proteins).
7. Glucose and fructose are constitutional isomers (C₆H₁₂O₆). — True.
Glossary
| Term | Meaning |
|---|---|
| Carbohydrate | Polyhydroxy aldehyde/ketone or substance giving such on hydrolysis |
| Monosaccharide | Simplest sugar, cannot be hydrolysed (e.g. glucose) |
| Disaccharide | Two monosaccharide units linked by glycosidic bond (e.g. sucrose) |
| Polysaccharide | Polymer of many monosaccharide units (starch, cellulose, glycogen) |
| Aldose / Ketose | Sugar with –CHO group / sugar with >C=O group |
| D / L sugar | Configuration based on –OH on chiral carbon farthest from C=O |
| Anomers | α and β cyclic forms differing only at the hemiacetal C-1 |
| Invert sugar | 1:1 mixture of glucose + fructose from sucrose hydrolysis |
| Glycosidic bond | Ether linkage joining two sugar units (e.g. C-1–O–C-4) |
| Essential amino acid | Amino acid which the body cannot synthesise; obtained from diet |
| Zwitterion | Dipolar form of amino acid: H₃N⁺–CHR–COO⁻ |
| Isoelectric point (pI) | pH at which amino acid carries no net charge |
| Peptide bond | –CO–NH– link between two amino acids |
| α-Helix / β-sheet | Right-handed coil / pleated sheet — secondary structures of proteins |
| Denaturation | Loss of biological activity by disruption of 2°/3°/4° structures |
| Enzyme | Globular protein acting as a biological catalyst |
| Vitamin | Organic micronutrient required in small amounts for health |
| Fat-soluble vitamins | A, D, E, K — stored in liver/adipose tissue |
| Water-soluble vitamins | B-complex and C — not stored, must be supplied regularly |
| Nucleoside | Base + pentose sugar |
| Nucleotide | Base + pentose sugar + phosphate |
| DNA | Deoxyribonucleic acid — double-stranded helix; stores genetic info |
| RNA | Ribonucleic acid — single-stranded; expresses genetic info |
| Replication | Semi-conservative duplication of DNA |
| Transcription | Synthesis of mRNA from DNA template |
| Translation | Synthesis of protein from mRNA on ribosomes |
| mRNA / tRNA / rRNA | Messenger / Transfer / Ribosomal RNA |
| Chargaff’s rule | In DNA: A = T and G = C |
| Watson–Crick model | 1953 antiparallel double-helix model of DNA |