The role of water

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The role of water

• describe how hydrogen bonding occurs between
  water molecules, and relate this, and other
  properties of water, to the roles of water in
  living organisms

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Water

• 60 of living things
• 2/3 of the earths surface

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Hydrogen bonds between water molecules

• Hydrogen bonds form when a slightly negatively
  charged part of a molecule comes close to a
  slightly positively charged hydrogen molecule in
  the same or another molecule.
• This is most easily seen in water
• Hydrogen bonds are not strong bonds. They are
  often described as interactions.
• However, in some polymers, many thousands of
  hydrogen bonds can form and this helps to
  stabilise the structure of the molecule
• Heating can easily break hydrogen bonds (e.g.
  when water molecules move apart becoming a gas)

N.B. The symbol d (delta) above refers to the
charge in that part of the molecule. In water,
negatively charged electrons tend to hang
around the oxygen molecule, making the hydrogen
molecules slightly positive. The positive
hydrogen atoms in one molecule are attracted to
the negative oxygen atoms in another molecule
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Covalent bonds between atoms

• Covalent bonds are bonds that involve atoms
  sharing electrons
• One oxygen atom has 8 electrons in total, 2 in
  the first shell, and 6 in the outer shell.
• The outer shell has, in fact, 8 spaces, so there
  are 2 spare
• Hydrogen only has one electron, so 2 hydrogen
  atoms lend their electron to oxygen to fill up
  its outer shell making it more stable. This is
  called a covalent bond

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Task

• Using Cambridge Biology Book 1, p105-108, write
  an essay to explain the role of water in living
  organisms (15). It must cover the following
  points
• The structure of water
• The relationship between water, heat and
  temperature
• Water as a solvent
• Density and viscosity
• Cohesion and surface tension

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Mark Scheme

• Covalent bonding between atoms within water
• Hydrogen bonding between water molecules (weaker
  than covalent)
• Reference to unfair electron sharing towards
  oxygen
• High specific heat capacity (a lot of heating to
  increase temperature as heat is used to break
  bonds)
• Large changes in external temperature does not
  affect internal body temperature
• Organisms in the sea have a stable external
  environment (as no great fluctuations in
  temperature)
• A lot of latent heat needed (energy needed to
  turn water into a gas)
• Cools plant leaves in hot climates
• Solvent- tiny charges on its molecules attract
  other charged molecules (dissolving is easier)
  Cl- and Na
• Blood and xylem as examples of dissolving
• Water molecules pulled close to each other
  (dense)
• Organisms have similar density so are able to
  swim
• Fairly viscous as water molecules must be pushed
  apart (streamlining is an adaptation to deal with
  this
• Cohesion (water sticking together) allows
  moveable column (plants)
• Surface tension- water molecules on surface
  pulled together and downwards so small animals
  can walk on the surface