The Properties of Water (AQA A Level Biology)

• Water is of great biological importance. It is the medium in which all metabolic reactions take place in cells. Between 70% to 95% of the mass of a cell is water
• As 71% of the Earth’s surface is covered in water it is a major habitat for organisms
• Water is composed of atoms of hydrogen and oxygen. One atom of oxygen combines with two atoms of hydrogen by sharing electrons (covalent bonding)
• Although water as a whole is electrically neutral the sharing of the electrons is uneven between the oxygen and hydrogen atoms
  • The oxygen atom attracts the electrons more strongly than the hydrogen atoms, resulting in a weak negatively charged region on the oxygen atom (δ^-) and a weak positively charged region on the hydrogen atoms(δ⁺), this also results in the asymmetrical shape

• This separation of charge due to the electrons in the covalent bonds being unevenly shared is called a dipole. When a molecule has one end that is negatively charged and one end that is positively charged it is also a polar molecule
• Water is a polar molecule

The covalent bonds of water make it a polar molecule

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

• Hydrogen bonds are weak, when there are few, so they are constantly breaking and reforming. However when there are large numbers present they form a strong structure
• 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

The polarity of water molecules allows hydrogen bonds to form between adjacent water molecules

• 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
• Specific heat capacity is a measure of the energy required to raise the temperature of 1 kg of a substance by 1^oC
• Water has a high specific heat capacity of 4200 J / Kg ^oC 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
• The advantage for living organisms is that it:
  • Provides suitable habitats
  • Is able to maintain a constant temperature as water is able to absorb a lot of heat without big temperature fluctuations
    • This is vital in maintaining temperatures that are optimal for enzyme activity

  • Water in blood plasma is also vital in transferring heat around the body, helping to maintain a fairly constant temperature
    • As blood passes through more active (‘warmer’) regions of the body, heat energy is absorbed but the temperature remains fairly constant
    • Water in tissue fluid also plays an important regulatory role in maintaining a constant body temperature

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

Cohesion and adhesion

• Hydrogen bonds between water molecules allows for strong cohesion between water molecules
  • This allows columns of water to move through the xylem of plants and through 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 (this is what allows insects such as pond skaters to float)

• 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