Class 11 Bio-chemistry — Exercise Solutions
Key Concepts Quick Recap
| Biomolecule | Monomer | Important Examples | Key Bond |
|---|
| Carbohydrate | Monosaccharide | Glucose, sucrose, starch, cellulose, glycogen | Glycosidic bond |
| Protein | Amino acid | Haemoglobin, collagen, enzymes, antibodies | Peptide bond |
| Lipid | Fatty acids + glycerol | Triglycerides, phospholipids, cholesterol | Ester bond |
| Nucleic acid | Nucleotide | DNA, RNA (mRNA, tRNA, rRNA) | Phosphodiester bond |
I. Choose the Best Answer (MCQs)
1. The monomers of polysaccharides are: (a) Amino acids (b) Nucleotides (c) Fatty acids (d) Monosaccharides
Answer(d) Monosaccharides Monosaccharides (e.g., glucose, fructose, galactose) link via glycosidic bonds to form disaccharides (maltose, sucrose, lactose) and polysaccharides (starch, cellulose, glycogen). C, H, O in ratio Cₙ(H₂O)ₙ.
2. The bond that links amino acids in a protein chain is called: (a) Hydrogen bond (b) Ionic bond (c) Peptide bond (d) Ester bond
Answer(c) Peptide bond Peptide bond: covalent bond between the carboxyl group (−COOH) of one amino acid and the amino group (−NH₂) of the next, with loss of water (condensation). The backbone of polypeptides. Multiple peptide bonds form the primary structure of proteins.
3. Which of the following is a reducing sugar? (a) Sucrose (b) Starch (c) Glucose (d) Cellulose
Answer(c) Glucose Reducing sugars: have free aldehyde or ketone groups; reduce Fehling's solution (brick-red precipitate) and Benedict's reagent. Examples: glucose, fructose, maltose, lactose. Sucrose: non-reducing (no free aldehyde/ketone; glycosidic bond between both anomeric carbons).
4. Which level of protein structure is destroyed first by denaturation? (a) Primary structure (b) Secondary structure (c) Tertiary structure (d) Quaternary structure
Answer(b) Secondary structure Denaturation: disrupts non-covalent interactions (H-bonds, hydrophobic, ionic) that maintain secondary and tertiary structure. Primary structure (peptide bonds) remains intact under mild denaturation. Severe denaturation (strong acid/alkali) may break primary structure. Secondary (α-helix, β-sheet) is lost first.
5. The active site of an enzyme: (a) Changes permanently after each reaction (b) Is complementary in shape to the substrate (lock-and-key model) (c) Occupies the entire enzyme surface (d) Contains no amino acids
Answer(b) Is complementary in shape to the substrate (lock-and-key model) Lock and key model (Emil Fischer, 1894): active site has specific shape complementary to substrate. Induced fit model (Koshland, 1958): active site is flexible, changes shape slightly upon substrate binding. Most enzymes work by induced fit.
6. Deficiency of which vitamin causes night blindness? (a) Vitamin B₁ (b) Vitamin C (c) Vitamin D (d) Vitamin A
Answer(d) Vitamin A Vitamin A (retinol): fat-soluble. Essential for synthesis of rhodopsin (visual purple) in rod cells of retina → needed for vision in dim light. Deficiency: night blindness (nyctalopia) → prolonged deficiency: xerophthalmia (dry cornea) → blindness.
7. The sugar present in DNA is: (a) Ribose (b) Deoxyribose (c) Fructose (d) Galactose
Answer(b) Deoxyribose DNA: deoxyribonucleic acid — sugar is 2-deoxyribose (lacks −OH at position 2). RNA: ribonucleic acid — sugar is ribose (has −OH at position 2). This difference makes DNA more stable (less susceptible to hydrolysis).
II. Short Answer Questions
Q1. Describe the four levels of protein structure.
Answer
Proteins have four structural levels:
1. Primary structure: Linear sequence of amino acids linked by peptide bonds. Determined by gene (DNA sequence). Most fundamental level; changes in primary structure = completely different protein.
2. Secondary structure: Local folding patterns maintained by hydrogen bonds between backbone −NH and −C=O groups.
- α-helix: right-handed coil; H-bonds within same polypeptide chain every 4th residue.
- β-pleated sheet: H-bonds between neighbouring strands running parallel or antiparallel.
3. Tertiary structure: Overall 3D shape of single polypeptide chain due to interactions between R-groups (side chains): hydrophobic interactions (main force for globular proteins), disulfide bonds (covalent, −S−S−), H-bonds between side chains, ionic bonds.
4. Quaternary structure: Assembly of 2 or more polypeptide subunits (monomers). Example: haemoglobin (4 subunits: 2α + 2β). Not all proteins have quaternary structure.
Q2. What are enzymes? Explain the factors affecting enzyme activity.
Answer
Enzymes: biological catalysts; mostly proteins (some RNA = ribozymes). Speed up biochemical reactions without being consumed. Specific to their substrate (specificity due to active site).
Factors affecting enzyme activity:
1. Temperature: Activity increases with temperature (more kinetic energy, more collisions) up to optimal temperature (~37°C for human enzymes). Above optimal: denaturation → activity drops to zero. Q₁₀ = 2 (activity doubles per 10°C rise, up to optimum).
2. pH: Each enzyme has an optimal pH. Pepsin (stomach): pH ~2. Salivary amylase (mouth): pH ~7. Trypsin (small intestine): pH ~8. Extreme pH disrupts ionic bonds and H-bonds in active site → denaturation.
3. Substrate concentration [S]: Activity increases with [S] (more substrate occupying active sites) until all active sites are saturated → Vmax reached.
4. Enzyme concentration: If substrate is in excess, increasing enzyme concentration increases reaction rate (more active sites available).
5. Inhibitors: Competitive (competes with substrate at active site; reversible; Km↑, Vmax same). Non-competitive (binds at allosteric site, changes active site shape; Km same, Vmax↓).
Exam Checklist
- Carbohydrates: Cₙ(H₂O)ₙ; monosaccharide → glycosidic bond → polysaccharide.
- Glucose: reducing sugar (free aldehyde). Sucrose: non-reducing.
- Protein primary structure: peptide bonds; 2°: H-bonds (α-helix, β-sheet); 3°: hydrophobic + disulfide; 4°: multiple subunits.
- Enzyme active site: complementary to substrate (lock & key / induced fit).
- Km: [S] at ½Vmax; low Km = high affinity.
- Vitamin A: night blindness; D: rickets; C: scurvy; B₁: beri-beri.
- DNA: deoxyribose; RNA: ribose. A=T (2 H-bonds); G≡C (3 H-bonds).
