1.7 water

• importance of water:

  • used as a reactant in cell e.g. photosynthesis
  • provides structural support in cells
  • keep organisms cool to maintain an optimum body temperature.
• what are some important properties of water?

  metabolic importance

  high heat capacity

  heat of vaporization

  cohesive properties

  useful as a solvent properties
• structure of water:

  • polarity
  • hydrogen bonding
• good solvent:

  • Water is a good solvent because ions and polar molecules can easily dissolve in it.
  • Water is a polar molecule. This means that the positive end of the water molecule attracts negative ions and the negative end will attract positive ions.
• high heat of vaporisation:

  • The specific latent heat of vaporisation is the amount of energy needed to change 1 kg of a liquid substance to a gas.
  • As liquid water heats up, hydrogen bonding makes it difficult to separate the water molecules from each other. This means that a lot of energy is needed for water to evaporate.
  • When water evaporates, energy is used up - this cools the environment where the evaporation is taking place.
  • This is why sweating helps with body temperature regulation.
• high heat capacity :

  • Specific heat capacity is the amount of heat one gram of a substance must absorb or lose to change its temperature by 1oC.
  • Water has a high heat capacity so takes a long time to heat and cool.
  • The specific heat capacity of water is much larger than sand. This is why land cools faster than the sea.
  • Water is used by warm blooded animals to more evenly disperse heat in their bodies.
• good metabolite:

  • Water is used or formed in many metabolic reactions, such as condensation and hydrolysis reactions.
  • ATP + H2O → ADP + Pi + energy
  • A bond is broken and a water molecule is used up and so this is a hydrolysis reaction.
  • ADP + Pi + energy → ATP + H2O
  • A new bond is formed and a water molecule is released and so this is a condensation reaction.
• cohesive properties:

  • The strong attraction between water molecules due to hydrogen bonds is called cohesion.
  • Cohesion produces surface tension where water meets air.
  • This is why water forms droplets when placed on a dry surface rather than being flattened out by gravity.
  • Plants use this natural phenomenon to help transport water from their roots to their leaves.