Water & the Hydrogen Bond (Cambridge (CIE) A Level Biology): Revision Note

Water molecules: hydrogen bonds

• 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 so they are covalently bonded

• 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^-)

    • A weak positively charged region on the hydrogen atoms(δ⁺)

  • This results in the asymmetrical shape of water molecules

  • 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

• 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