Vidaara.orgClass 12 · Chemistry
CodeVID-C12-14-T2-01
Assignment — Proteins & Amino Acids
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all working for Sections B, C and D. Only final answers are given at the end — for full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
The bond joining two amino acids in a protein is a:
- A.glycosidic bond
- B.peptide (amide) bond
- C.ester bond
- D.ether bond
2.
An amino acid carries its –NH2 and –COOH on the:
- A.β-carbon
- B.same (α) carbon
- C.different ends only
- D.ring carbon
3.
Which is an essential amino acid?
- A.Glycine
- B.Alanine
- C.Lysine
- D.Serine
4.
Denaturation does NOT break:
- A.hydrogen bonds
- B.disulphide bridges
- C.peptide bonds
- D.ionic interactions
5.
Enzymes are chemically mostly:
- A.lipids
- B.carbohydrates
- C.globular proteins
- D.nucleic acids
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Why are amino acids amphoteric?
7.
Define the isoelectric point.
8.
Name the forces stabilising the tertiary structure of a protein.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
Describe how the α-helix and β-pleated sheet differ.
10.
Explain the lock-and-key model of enzyme action.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Describe the four levels of protein structure and explain what happens during denaturation.
Answer Key
Section A — Multiple Choice Questions
- (B) peptide (amide) bond
- (B) same (α) carbon
- (C) Lysine
- (C) peptide bonds
- (C) globular proteins
Section B — Short Answer (2 marks)
- They contain both an acidic –COOH (or –COO−) and a basic –NH2 (or –NH3+) group, so they react with both acids and bases.
- The pH at which an amino acid exists mainly as the zwitterion with zero net charge and does not migrate in an electric field.
- Hydrogen bonds, disulphide (–S–S–) bridges, ionic (salt) bridges and van der Waals/hydrophobic interactions.
Section C — Short Answer (3 marks)
- α-helix: a single chain coiled as a right-handed spiral held by intramolecular H-bonds (keratin). β-sheet: stretched chains lying side by side, held by intermolecular H-bonds, forming a pleated sheet (silk fibroin).
- The substrate fits the enzyme's active site like a key in a lock; only a complementary substrate binds, the bound complex lowers the activation energy and converts to product, which is released, regenerating the enzyme.
Section D — Long Answer (5 marks)
- Primary: the sequence of amino acids linked by peptide bonds. Secondary: local folding by backbone H-bonds into α-helix or β-sheet. Tertiary: overall 3-D folding of the chain (fibrous or globular) held by H-bonds, –S–S– bridges, ionic and van der Waals forces. Quaternary: association of two or more sub-units (e.g. haemoglobin's four chains). On denaturation, heat/acid/heavy metals break these weak forces so the 2° and 3° structures unfold and activity is lost, but the primary peptide bonds remain intact.
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