Water & the Hydrogen Bond (CIE 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