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2.4.12 · Biological Molecules

Water and the Hydrogen Bond — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 7 parts Lesson 23 of 25 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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2.4.1 Water and the Hydrogen Bond — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the polarity of water and the hydrogen bonds between water molecules.Medium of life — all metabolic reactions in cells take place in water.Cell content — water makes up 70–95%…

  • Composition
  • Uneven sharing
  • Dipole and polarity
  • Collective strength
  • Where to draw them
  • High specific heat capacity

Part 1 of 5Importance of water

Medium of life
  • Medium of life all metabolic reactions in cells take place in water.
Cell content
  • Cell content water makes up 70–95% of the mass of a cell.
Habitat
  • Habitat about 71% of Earth's surface is covered by water.

Part 2 of 5Structure of a water molecule

Structure of a water molecule
Composition
  • Composition one oxygen atom shares electrons with two hydrogen atoms, forming covalent bonds.
Uneven sharing
  • Uneven sharing oxygen attracts the electrons more strongly than hydrogen does.
  • This makes the oxygen weakly negative (δ−) and each hydrogen weakly positive (δ+).
Shape
  • Shape the molecule is asymmetrical (bent).
Dipole and polarity
  • Dipole and polarity the separation of charge is a dipole, so water is a polar molecule.

Part 3 of 5Hydrogen bonds between water molecules

Formation
  • Formation a hydrogen bond forms between the δ+ hydrogen of one molecule and the δ− oxygen of another.
Behaviour
  • Behaviour individual bonds are weak and constantly break and reform.
Collective strength
  • Collective strength large numbers of hydrogen bonds form a strong structure.
Where to draw them
  • Where to draw them between oxygen of one molecule and hydrogen of another, not within a molecule.

Part 4 of 5Properties arising from hydrogen bonding

1Solvent
  • Solvent many substances dissolve in water.
2High specific heat capacity
  • High specific heat capacity temperature changes slowly.
3High latent heat of vaporisation
  • High latent heat of vaporisation evaporation removes much heat.
4Lower density as a solid
  • Lower density as a solid ice is less dense than liquid water.
5High surface tension and cohesion
  • High surface tension and cohesion molecules cling together.
6Reagent
  • Reagent water takes part in reactions such as hydrolysis.

Part 5 of 5More from the specification

More from the specification
Hydrogen Bonding in Water
  • Water molecules are polar with hydrogen atoms covalently bonded to oxygen,
  • forming intermolecular hydrogen bonds that give water unique cohesive and thermal properties.
High Specific Heat Capacity
  • Due to the large amount of heat energy required to break hydrogen bonds, water buffers aquatic habitats and living organisms against dramatic temperature fluctuations.
High Latent Heat of Vaporisation
  • Evaporating a small volume of water absorbs a great amount of thermal energy, providing an efficient cooling mechanism.
    • (e.g. sweating and transpiration)
Biological Solvent Properties
  • The dipolar nature of water allows it to dissolve ionic compounds and polar metabolites, enabling enzyme-catalysed reactions within living cells.
Organism
  • A living thing, such as a plant or an animal.

Key terms

TermMeaning
Polar moleculeA molecule with an uneven distribution of charge.
DipoleSeparation of charge in a molecule, with one δ+ end and one δ− end.
Hydrogen bondA weak bond between a δ+ hydrogen and a δ− atom of another molecule.
Covalent bondA bond formed by sharing electrons between atoms.
CohesionAttraction between molecules of the same substance.
Surface tensionThe tendency of a liquid surface to resist being broken, due to cohesion.

Avoid these mistakes

1
Drawing hydrogen bonds inside a water molecule — they form between molecules.
2
Calling hydrogen bonds strong — each is weak, but many together are strong.
3
Saying water is polar because it has a charge overall — it is neutral overall but has uneven charge distribution.
4
Saying oxygen is δ+ in water — oxygen is more electronegative so it is δ−.

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