Water

• Water molecule


  1 oxygen atom + 2 hydrogen atoms (H₂O)
• Water

  • Dipolar molecule
  • Oxygen atoms slightly negatively charged
  • Hydrogen atoms slightly positively charged
• Hydrogen bonding


  Hydrogen atom on one water molecule attracted to oxygen on another water molecule. Without hydrogen bonding, water would be a gas
• Importance of water


  • Reactant in cells (e.g. photosynthesis, hydrolysis)
  • Provides structural support in cells
  • Keeps organisms cool to maintain optimum body temperature
• Properties of water


  • Metabolic importance
  • High heat capacity
  • High heat of vaporisation
  • Cohesive properties
  • Useful as a solvent
  • Dipolar (but no overall charge)
• Water as a good metabolite

  • Used/formed in many metabolic reactions (e.g. condensation and hydrolysis reactions)
  • Raw material formed in photosynthesis
  • Chemical reactions take place in an aqueous medium
• High heat capacity of water

  • Acts as a buffer against sudden temperature variations
  • Takes long time to heat & cool
  • Specific heat capacity of water is larger than sand so land cools faster than the sea
  • Keeps aquatic and cellular environments stable
• High heat of vaporisation of water

  • Lots of energy needed for water to evaporate
  • When water evaporates, energy used up -> cools the environment
  • Helps with body temperature regulation (sweating)
• Cohesive properties of water

  • Strong attraction between water molecules due to hydrogen bonds
  • Produces surface tension where water meets air
  • Allows water to be pulled up (e.g. in plants via xylem)
  • Allows pond skaters to walk on water surface
• Water as a liquid medium

  • Provides habitat for aquatic organisms
  • Medium for chemical reactions
  • Used for transport
• Water as a good solvent and polar

  • Dissolves ionic and polar molecules
  • Dissolves gases (e.g. oxygen & carbon dioxide)
  • Dissolves waste (e.g. ammonia & urea)
  • Dissolves inorganic ions & small hydrophilic molecules (e.g. amino acids, monosaccharides, ATP, enzymes)
• Other features of water

  • Not easily compressed
  • Transparent (allows aquatic plants to photosynthesise, light rays can penetrate the eye)