Q1. Differentiate between a Prosthetic Group and a Co-enzyme.

Answer: Both are organic cofactors required for enzyme activity, but they differ in the nature of their association with the enzyme.

Prosthetic Group

  • Tightly and permanently bound to the apoenzyme
  • Remains attached even when the enzyme is not active
  • Essential structural component of the enzyme
  • Example: Haem in catalase and peroxidase

Co-enzyme

  • Loosely and temporarily associated with the apoenzyme
  • Binds only during catalysis and then dissociates
  • Often derived from vitamins
  • Example: NAD⁺, NADP⁺ (from niacin)

Memory tip: Prosthetic = Permanent | Co-enzyme = Temporary

Q2. Describe the Watson–Crick model of DNA.

Answer: The Watson–Crick model explains the three-dimensional structure of DNA (B-DNA).

Key features:

  • DNA consists of two polynucleotide strands coiled into a double helix
  • Backbone made of deoxyribose sugar and phosphate
  • Nitrogenous bases face inward and pair specifically
  • A = T (2 hydrogen bonds) and G ≡ C (3 hydrogen bonds)
  • Strands are antiparallel (5′→3′ and 3′→5′)
  • One turn of helix has 10 base pairs
  • Pitch of helix = 34 Å, distance between base pairs = 3.4 Å

Memory tip: A–T (2 bonds), G–C (3 bonds), 10 bp per turn

Q3. Explain competitive inhibition with an example.

Answer: Competitive inhibition occurs when an inhibitor competes with the substrate for the enzyme’s active site.

Key points:

  • Inhibitor resembles the substrate structurally
  • Binds to the active site of enzyme
  • Reduces rate of reaction
  • Effect can be overcome by increasing substrate concentration

Example:

  • Malonate inhibits succinic dehydrogenase by competing with succinate

Memory tip: Competitive = Same site, similar shape

Q4. Why do starch and cellulose react differently with iodine?

Answer:

  • Starch has a helical structure (amylose) that traps iodine molecules, producing a blue-black color
  • Cellulose has straight, unbranched β-glucose chains and lacks helical cavities
  • Hence, cellulose does not show any color change with iodine

Memory tip: Helix holds iodine → blue color

Q5. What is meant by the living state?

Answer: The living state is a non-equilibrium steady state maintained by continuous metabolic activity.

Key points:

  • Living systems continuously perform work
  • Constant input of energy is required
  • Equilibrium means no work → death

Memory tip: Life = non-equilibrium

Q6. Explain the four levels of protein structure.

Answer:

Primary structure

  • Linear sequence of amino acids
  • Held by peptide bonds

Secondary structure

  • Local folding into α-helix or β-pleated sheet
  • Stabilized by hydrogen bonds

Tertiary structure

  • Overall 3D folding of a polypeptide
  • Maintained by disulfide bonds, ionic bonds, hydrogen bonds
  • Essential for biological activity

Quaternary structure

  • Association of two or more polypeptide chains
  • Example: Hemoglobin (4 subunits)

Memory tip: 1° sequence → 2° fold → 3° shape → 4° subunits

Q7. Define activation energy. How do enzymes affect it?

Answer: Activation energy is the minimum energy required for reactants to reach the transition state.

Role of enzymes:

  • Lower the activation energy
  • Stabilize the transition state
  • Increase rate of reaction
  • Do not change overall free energy of reaction

Memory tip: Enzymes make reactions easier, not different

Q8. Differentiate between anabolic and catabolic pathways.

Answer:

Anabolic pathways

  • Synthesis of complex molecules
  • Require energy (ATP)
  • Example: photosynthesis, protein synthesis

Catabolic pathways

  • Breakdown of complex molecules
  • Release energy
  • Example: respiration, glycolysis

Memory tip: Anabolic = build | Catabolic = break

Q9. Name the bonds present in proteins, polysaccharides, and nucleic acids.

Answer:

  • Proteins: Peptide bond
  • Polysaccharides: Glycosidic bond
  • DNA/RNA: Phosphodiester bond

Memory tip: Protein–Peptide, Sugar–Glycosidic, DNA–Phosphodiester

Q10. Why are lipids found in the acid-insoluble fraction?

Answer: Although lipids have low molecular weight (<800 Da), they are insoluble in water.

Key explanation:

  • During tissue grinding, membranes break
  • Lipids form insoluble vesicles
  • Vesicles cannot pass through filter cloth
  • Hence retained with macromolecules

Memory tip: Insoluble vesicles behave like macromolecules

Q11. What are cofactors? Name their types.

Answer: Cofactors are non-protein components required for enzyme activity.

Types:

  • Prosthetic groups
  • Co-enzymes
  • Metal ions (Zn²⁺, Mg²⁺, Fe²⁺)

Memory tip: Enzyme help = cofactors

Q12. What is the effect of high temperature and extreme pH on enzymes?

Answer: High temperature or extreme pH causes denaturation of enzymes.

Key points:

  • Tertiary structure is disrupted
  • Active site loses shape
  • Enzyme becomes inactive permanently

Memory tip: Shape lost = function lost