The role of water

1. 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

2. Water 60% of living things 2/3 of the earth’s surface

3. 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 δ (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

4. 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

5. 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

7. 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