A level Biology exam revision resources written by A level Examiners


The roles water plays in living organisms which depend on hydrogen bonds, are listed below.   Exam Advice

  1. Because water is polar it can dissolve many substances forming aqueous solutions

    This allows the molecules of dissolved substances to move freely, enabling chemical reactions to occur in solution.

    In addition, dissolved substances can easily be transported in solution e.g. mineral ions in xylem and in blood plasma.

  2. Water has a high specific heat capacity, this means it takes a lot of energy to increase the temperature of water.

    This minimises temperature fluctuations inside cells (which is good for enzyme action), and also ensures that the sea temperature is very stable.

  3. Water has a high latent heat of evaporation, this means it takes a lot of energy to change state from a liquid to a gas.

    This allows organisms to cool themselves by producing sweat, which can evaporate.

  4. Cohesion : Water molecules form hydrogen bonds with adjacent water molecules, which means there are cohesive forces that prevent them separating. In plants this keeps columns of water together in xylem. On the surface of ponds it allows small organisms to literally walk on the surface of the water e.g. pond skaters.

Water plays other roles in living organisms, and these additional roles also contribute to its Biological importance.

  • Water is transparent which allows light penetration so aquatic plants can photosynthesise.
  • It can be used as a chemical reactant e.g. in hydrolysis and photosynthesis.
  • It can act as a lubricant e.g. synovial fluid and mucus.
  • It provides internal support e.g. turgid plant cells hold up the stems of herbaceous plants.
  • Water is less dense when frozen, thus ice floats and insulates the water below, allowing aquatic life to survive.

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