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AS · Practice questions · Proteins

Sequence, fold, function.

Six original Cambridge-style questions on protein structure, from a single amino acid to the haemoglobin molecule and the collagen fibre.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Level, then bond.

01
Recall
[3 marks]

Describe the general structure of an amino acid.

  • A central carbon atom bonded to an amine group (NH2) and a carboxyl group (COOH). ✓
  • Also bonded to a hydrogen atom. ✓
  • And to a variable R group, which differs between the 20 amino acids. ✓
02
Explain
[3 marks]

Describe how a peptide bond is formed and how it is broken.

  • Formed by condensation between the carboxyl group of one amino acid and the amine group of another. ✓
  • A molecule of water is released and a C-N (peptide) bond forms. ✓
  • Broken by hydrolysis: water is added (catalysed by protease enzymes). ✓
03
Analysis
[4 marks]

Explain the meaning of the terms secondary structure and tertiary structure, naming the bonds involved in each.

  • Secondary: coiling into an α-helix or folding into a β-pleated sheet. ✓
  • Held by hydrogen bonds between the C=O and N-H groups of the polypeptide backbone. ✓
  • Tertiary: further folding of the whole polypeptide into a specific 3D shape. ✓
  • Held by hydrophobic interactions, hydrogen bonds, ionic bonds and disulfide bonds between R groups. ✓
04
Calculation
[2 marks]

Each α-globin chain of human haemoglobin has 141 amino acids and each β-globin chain has 146. Calculate the total number of peptide bonds in one haemoglobin molecule.

  • Peptide bonds per chain = amino acids − 1: 140 per α chain and 145 per β chain. ✓
  • Total = 2 × 140 + 2 × 145 = 570. ✓
05
Explain
[4 marks]

Relate the structure of haemoglobin to its function in transporting oxygen.

  • Quaternary structure of four polypeptides: two α-globin and two β-globin chains. ✓
  • Each chain has a haem group containing Fe2+, which binds one O2 reversibly, so one molecule carries four O2. ✓
  • Globular, with hydrophilic R groups outside, so it is soluble in the cytoplasm of red blood cells. ✓
  • Iron is essential: without it haem cannot bind oxygen. ✓
06
Explain
[4 marks]

Explain how the structure of collagen molecules and collagen fibres relates to their function.

  • Three polypeptide chains wound into a triple helix, held by hydrogen bonds; glycine at every third position allows tight packing. ✓
  • Neighbouring molecules are joined by covalent cross-links. ✓
  • Molecules are staggered so their ends do not line up, avoiding weak points; they form fibrils and then fibres. ✓
  • This gives very high tensile strength and little stretch, suited to tendons, skin and artery walls; collagen is insoluble. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.