Noesis
Level 2 — RecallBiomolecules — Proteins & Nucleic Acids

Biomolecules — Proteins & Nucleic Acids

⏱ 30 minutes40 marksprintable — key stays hidden on paper

Chapter 1.4 — Biomolecules: Proteins & Nucleic Acids

Level: 2 — Recall (definitions, standard problems, short derivations) Time Limit: 30 minutes Total Marks: 40

Instructions

Answer all questions. Marks are shown in brackets. Use clear labelled diagrams where asked.

Q1. State the four chemical elements always present in every protein, and name one additional element found in some proteins. (3 marks)

Q2. Draw the general structure of an amino acid. Clearly label the amino group, carboxyl group, hydrogen atom, R group, and the central (alpha) carbon. (5 marks)

Q3. (a) Explain how a peptide bond is formed between two amino acids, naming the type of reaction. (3 marks) (b) A polypeptide contains 150 amino acids joined in a single chain. How many peptide bonds are present, and how many water molecules were released during its synthesis? (2 marks)

Q4. Define primary structure of a protein, and state why it is important in determining the protein's shape. (3 marks)

Q5. Compare the alpha helix and beta pleated sheet by completing a table with the following features: shape/arrangement, and the bond that stabilises each. (4 marks)

Q6. Distinguish between tertiary and quaternary structure of proteins, giving one example of a protein for each. (4 marks)

Q7. (a) Define denaturation. (2 marks) (b) List three factors that can cause a protein to denature. (3 marks)

Q8. (a) Name the three components of a single nucleotide. (3 marks) (b) Classify each of the following bases as a purine or a pyrimidine: adenine, cytosine, guanine, thymine, uracil. (3 marks)

Q9. Complete the table comparing DNA and RNA for: sugar, number of strands, and one base that differs between them. (3 marks)

Q10. (a) In a DNA molecule, complementary base pairing occurs. Complete the base pairing rules: A pairs with ___ ; G pairs with ___. (2 marks) (b) A DNA sample contains 30% adenine. Using Chargaff's rule, calculate the percentage of guanine. (2 marks)

Q11. State the food test, reagent colour change, and the biomolecule detected for Biuret test and iodine test. (4 marks) (Answer only 4 of the 40 — bonus if attempted; core marks = 40 across Q1–Q10 above.)

End of Paper

Answer keyMark scheme & solutions

Q1. (3 marks)

  • Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N) — 2 marks (all four required; deduct for missing)
  • Additional element: Sulfur (S) [accept phosphorus in some proteins] — 1 mark Why: proteins are built from amino acids whose backbone contains C, H, O, N; sulfur appears in the R groups of cysteine/methionine.

Q2. (5 marks)

  • Central α-carbon shown — 1
  • Amino group (–NH₂) — 1
  • Carboxyl group (–COOH) — 1
  • Hydrogen atom (–H) — 1
  • R group (variable side chain) — 1
        H
        |
  H₂N — Cα — COOH
        |
        R

Why: all 20 amino acids share this backbone; only R differs, giving each its properties.

Q3. (a) (3 marks)

  • The carboxyl (–COOH) group of one amino acid reacts with the amino (–NH₂) group of the next — 1
  • A molecule of water is removed → condensation (dehydration synthesis) reaction — 1
  • A covalent peptide bond (C–N) forms between them — 1

(b) (2 marks)

  • Peptide bonds = 150 − 1 = 149 — 1
  • Water molecules released = 149 (one per bond formed) — 1 Why: n amino acids in one chain form (n−1) bonds, each by losing one water.

Q4. (3 marks)

  • Primary structure = the specific sequence/order of amino acids in a polypeptide chain — 2
  • Importance: this sequence determines how the chain folds, hence the protein's final 3-D shape and function — 1

Q5. (4 marks) — 1 mark per correct cell

Feature Alpha helix Beta pleated sheet
Shape/arrangement Coiled/spiral Folded sheet / parallel strands zig-zag
Stabilising bond Hydrogen bonds Hydrogen bonds
Why: both are secondary structures held by hydrogen bonds between backbone C=O and N–H groups; they differ in geometry.

Q6. (4 marks)

  • Tertiary: overall 3-D folding of a single polypeptide chain (1) — example: myoglobin / lysozyme / enzyme (1)
  • Quaternary: two or more polypeptide chains (subunits) joined together (1) — example: haemoglobin / collagen / insulin (1)

Q7. (a) (2 marks) — Denaturation = loss of the protein's specific 3-D shape/structure (1) causing loss of function, without breaking peptide bonds/primary structure (1). (b) (3 marks) — any three, 1 each: high temperature; extremes of pH (strong acid/alkali); heavy metals/salts; organic solvents (e.g. alcohol); UV radiation; mechanical agitation.

Q8. (a) (3 marks) — Pentose sugar (1); phosphate group (1); nitrogenous base (1). (b) (3 marks) — Purines: adenine, guanine. Pyrimidines: cytosine, thymine, uracil. (½ each, rounded; award 3 for all correct, 2 for ≥3 correct, 1 for ≥1 correct.)

Q9. (3 marks) — 1 mark each row

Feature DNA RNA
Sugar Deoxyribose Ribose
Strands Double Single
Differing base Thymine Uracil

Q10. (a) (2 marks) — A pairs with T (1); G pairs with C (1). (b) (2 marks) — By Chargaff: %A = %T = 30%, so A+T = 60%; remaining G+C = 40%; %G = %C = 40/2 = 20% — 2 marks (1 for method, 1 for answer).

Q11. (4 marks, bonus)

  • Biuret test: reagent = Biuret (Cu²⁺ in alkali); blue → purple/violet; detects protein — 2
  • Iodine test: iodine solution; orange-brown → blue-black; detects starch — 2
[
  {"claim":"150-amino-acid chain has 149 peptide bonds and releases 149 water molecules","code":"n=150; bonds=n-1; water=n-1; result=(bonds==149 and water==149)"},
  {"claim":"With 30% adenine, guanine is 20% by Chargaff's rule","code":"A=30; T=A; G=(100-(A+T))/2; result=(G==20)"},
  {"claim":"Complementary G equals C percentage","code":"A=30; T=A; GC=100-(A+T); G=GC/2; C=GC/2; result=(G==C)"},
  {"claim":"Adenine and guanine are the two purines among the five listed bases","code":"purines={'adenine','guanine'}; bases={'adenine','cytosine','guanine','thymine','uracil'}; result=(len(purines & bases)==2 and len(purines)==2)"}
]