• 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