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LearnYBI11WBI11 · Unit 1: Molecules, Diet, Transport and HealthMembranes, proteins, DNA and gene expressionProteins and water
Membranes, proteins, DNA and gene expression · Subtopic 6 of 8

Proteins and Water

Amino acids (H₂N–CHR–COOH) join by condensation into peptide bonds. Primary structure: amino acid sequence. Secondary: α-helix or β-pleated sheet held by hydrogen bonds between C=O and N–H. Tertiary: further folding held by hydrogen, ionic and disulfide bonds and hydrophobic interactions between R groups. Quaternary: more than one polypeptide (and maybe a prosthetic group). Globular proteins (e.g. haemoglobin: 4 chains, haem with Fe²⁺, hydrophilic outside) are soluble and metabolic; fibrous proteins (e.g. collagen: three chains in a triple helix, glycine every third residue, cross-links) are insoluble and structural. Biuret test: purple. Water is polar; hydrogen bonding gives it high specific heat capacity, high latent heat of vaporisation, cohesion and solvent properties.

Cambridge 9700 Paper 1Cambridge 9700 Paper 2Edexcel YBI11 WBI11AQA 7402 Paper 1≈ 50 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe the four levels of protein structure and their bonds.
  • Compare haemoglobin and collagen.
  • Carry out the biuret test.
  • Explain the properties of water.
Pause and think

Sickle cell anaemia is caused by one amino acid change in haemoglobin. How can one change matter so much?

Section 2 of 14

Core idea

Start hereFolding decides everything

A protein is just a chain until it folds. Get the fold wrong and it can't do its job.

The big idea

The amino acid sequence decides how a protein folds; bonds between R groups hold the 3D shape. Water's hydrogen bonds give it the properties life needs.

Levels of protein structure

Each level depends on the one before.

Worked exampleSequence diversity

How many different tripeptides are possible from 20 amino acids?

Goal
Number.
Strategy
20 choices at each position.
  1. 20×20×2020\times20\times20
    Three positions.
  2. =8000=8000
    Multiply.
Answer8000.
✓ CheckGrows as 20ⁿ ✓.
Key takeawayWater Hydrogen bonding gives water a high specific heat capacity, high latent heat of vaporisation, cohesion and solvent properties.

Summary

  • Primary → quaternary.
  • Haemoglobin globular; collagen fibrous.
  • Biuret purple.
Key words
Peptide bond
Bond between amino acids formed by condensation.
Prosthetic group
Non-protein part attached to a protein, e.g. haem.
HistoryPauling's helix. Linus Pauling proposed the α-helix in 1951, reportedly while folding paper models in bed with flu.
Fun factYour hair is protein. Perms and straighteners work by breaking and reforming disulfide bonds in keratin.
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