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1.1.1Unit 1: Lifestyle, Transport, Genes and Health

The Importance of Water — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 4 parts Lesson 8 of 29 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Part 1 of 4Structure of the Water Molecule

1.1.1 The Importance of Water

Everything biologically useful about water follows from its polar molecules and the hydrogen bonds between them.

Covalent bonds
  • Each water molecule has one oxygen atom sharing electrons with two hydrogen atoms through covalent bonds.
Polarity
  • Oxygen attracts the shared electrons more strongly,
    • so it carries a slight negative charge (δ−) and each hydrogen a slight positive charge (δ+).
Dipole
  • Because the charge is unevenly distributed across the molecule, water is described as a polar molecule or dipole.
Hydrogen bonds
  • The δ+ hydrogen of one molecule is attracted to the δ− oxygen of another, forming weak but numerous hydrogen bonds.

Part 2 of 4Properties and Their Importance

Properties and Their Importance
Solvent
  • Polar water molecules surround ions and polar molecules such as glucose,
    • so reactions can occur in solution and substances can be transported in blood and sap.
Cohesion and surface tension
  • Hydrogen bonds hold molecules together, enabling columns of water to rise in xylem and giving a surface on which small organisms can live.
High specific heat capacity
  • A lot of energy is needed to raise the temperature of water,
    • so aquatic habitats and cell contents resist rapid temperature changes.
High latent heat of vaporisation
  • Much energy is taken from the surface as water evaporates,
    • so sweating and transpiration give an effective cooling effect.

Part 3 of 4Further Roles of Water

Metabolite
  • Water takes part in reactions such as photosynthesis and hydrolysis, where it is used as a reactant.
Transport medium
  • Blood plasma and phloem sap are mainly water, which carries dissolved nutrients, hormones and wastes around organisms.
Density of ice
  • Ice is less dense than liquid water because of the lattice of hydrogen bonds, so it floats and insulates the water below it.
Support
  • Water inside plant cells creates turgor pressure that keeps non-woody stems upright, and gives a hydrostatic skeleton in some animals.

Part 4 of 4Exam Checklist & Tips

1.1.1 – Key Points for the Exam

  • Polar molecule A molecule with an uneven distribution of charge, giving partially positive and negative regions.
  • Hydrogen bond A weak attraction between a δ+ hydrogen atom and a δ− atom on another molecule.
  • Cohesion The attraction between molecules of the same substance, such as water to water.
  • Solvent A liquid in which other substances dissolve.
  • Specific heat capacity The energy needed to raise the temperature of 1 kg of a substance by 1 °C.

Common Mistakes

  • Saying that hydrogen bonds are inside the water molecule; they are between neighbouring molecules.
  • Describing water as a charged molecule; it is overall neutral but polar.
  • Writing that water has a low specific heat capacity; it is high, which buffers temperature.
  • Confusing cohesion (water with water) with adhesion (water with another surface).

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