CIE A Level Biology复习笔记2.4.2 The Role of Water in Living Organisms

• Water has many essential roles in living organisms due to its properties:
  • The polarity of water molecules
  • The presence and number of hydrogen bonds between water molecules

Solvent

• As water is a polar molecule many ions (e.g. sodium chloride) and covalently bonded polar substances (e.g. glucose) will dissolve in it
  • This allows chemical reactions to occur within cells (as the dissolved solutes are more chemically reactive when they are free to move about)
  • Metabolites can be transported efficiently (except non-polar molecules which are hydrophobic)

Due to its polarity water is considered a universal solvent

High specific heat capacity

• The specific heat capacity of a substance is the amount of thermal energy required to raise the temperature of 1kg of that substance by 1°C. Water’s specific heat capacity is 4200 J/kg°C
• The high specific heat capacity is due to the many hydrogen bonds present in water. It takes a lot of thermal energy to break these bonds and a lot of energy to build them, thus the temperature of water does not fluctuate greatly
• The advantage for living organisms is that it:
  • Provides suitable habitats
  • Allows for constant temperatures within bodies and cells to be maintained (this ensures enzymes have the optimal temperatures)
    • This is because a large increase in energy is needed to increase the temperature of water

Latent heat of vaporisation

• In order to change state (from liquid to gas) a large amount of thermal energy must be absorbed by water to break the hydrogen bonds and evaporate
• This is an advantage for living organisms as only a little water is required to evaporate for the organism to lose a great amount of heat
• This provides a cooling effect for living organisms, for example the transpiration from leaves or evaporation of water in sweat on the skin

Properties of water & its role in living organisms table