The Role of Water in Living Organisms (Cambridge (CIE) AS Biology): Revision Note

Water molecules: 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 ionic compounds (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)

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 create 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 from the surface of the organism in order to lose a great amount of heat energy

• This provides a cooling effect for living organisms, for example the transpiration from leaves or evaporation of water in sweat from the skin