The Properties of Water (AQA A Level Biology): Revision Note

Water in cells

• Water is of great biological importance. It is the medium in which all metabolic reactions take place in cells

• Water is composed of atoms of hydrogen and oxygen

  • One atom of oxygen combines with two atoms of hydrogen by sharing electrons (covalent bonding)

• Water is a polar molecule

• Hydrogen bonds form between water molecules

  • As a result of the polarity of water, hydrogen bonds form between the positively and negatively charged regions of adjacent water molecules

• Hydrogen bonds contribute to the many properties water molecules have that make them so important to living organisms:

  • An excellent solvent – many substances can dissolve in water

  • A relatively high specific heat capacity

  • A relatively high latent heat of vaporisation

  • Water is less dense when a solid

  • Water has high surface tension and cohesion

  • It acts as a reagent

Water as a metabolite

• Water is a metabolite in many metabolic reactions, including condensation and hydrolysis reactions

  • In condensation reactions, smaller molecules combine to form a larger molecule, with the removal of a water molecule

    Examples include:

    • formation of peptide bonds between amino acids to make proteins

    • formation of glycosidic bonds in carbohydrates

    • formation of ester bonds in lipids

  • In hydrolysis reactions, water is added to break a bond within a larger molecule, splitting it into smaller units

    Examples include:

    • breaking proteins into amino acids

    • breaking starch into glucose

    • breaking triglycerides into fatty acids and glycerol

Water as a 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)

High specific heat capacity

• Specific heat capacity is a measure of the energy required to raise the temperature of 1 kg of a substance by 1 ^°C

  • Water has a high specific heat capacity of 4200 J / Kg ^°C, meaning a relatively large amount of energy is required to raise its temperature

• 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, meaning:

  • stable habitats can be provided, especially for aquatic organisms

  • water absorbs lots of heat with minimal temperature change

    • This helps maintain stable internal and external temperatures, essential for enzyme function

  • water in blood plasma transfers heat around the body, aiding temperature regulation

    • As blood flows through warmer tissues, it absorbs heat without large temperature shifts

Latent heat of vaporisation

• 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 the evaporation of water in sweat on the skin

Cohesion and adhesion

• Hydrogen bonds between water molecules allow for strong cohesion between water molecules

  • This allows columns of water to move through the xylem of plants and the blood vessels in animals

  • This also enables surface tension where a body of water meets the air; these hydrogen bonds occur between the top layer of water molecules to create a sort of film on the body of water

• Water is also able to hydrogen bond to other molecules, such as cellulose, which is known as adhesion

  • This also enables water to move up the xylem due to transpiration