water

• importance of water: vital to living organisms and makes up about 80% of a cell's contents and has lots of important functions inside and outside cells
• • water is a metabolite in loads of important metabolic reactions, including condensation and hydrolysis reactions
• • water is a solvent - some substances dissolve in it
  • most metabolic reactions take place in solution (e.g. in the cytoplasm of eukaryotic and prokaryotic cells)
• • water helps with temperature control bc it has a high latent heat of vaporisation and a high specific heat capacity
• • water molecules are very cohesive (they stick together) which helps water transport in plants as well as transport in other organisms
• polarity of water:

  a molecule of water is one atom of oxygen joined to 2 atoms of hydrogen by shared electrons
• because the shared negative hydrogen electrons are pulled towards the oxygen atom, the other side of each hydrogen atom is left with a slight positive charge. The unshared negative electrons on the oxygen atom give it a slight negative charge. This makes water a polar molecule - it has a slight (partial) negative charge on one side and a slight positive charge on the other
• Hydrogen bonds are weak bonds between a slightly positively charged hydrogen atom in one molecule and a slightly negatively charged atom in another molecule
• Hydrogen bonds form between water molecules bc the slightly negatively charged oxygen atoms of water attract the slightly positively charged hydrogen atoms of other water molecules - this hydrogen bonding gives water some of its useful properties
• important meabolite:

  many metabolic reactions involve a condensation or hydrolysis reaction

  • hydrolysis reaction requires a molecule of water to break a bond
  • condensation reaction requires a molecule of water as a new bond is formed
  • e.g. amino acids are joined together to make polypeptides by condensation reactions
  • energy from ATP is released through a hydrolysis reaction
• Good solvent:

  a lot of important substances in biological reactions are ionic (like salt) - means they are made from one positively charged atom or molecule and one negatively charged atom or molecule

  because water is polar, the slightly positive charged end of a water molecule will be attracted to the negative ion and the slightly negatively charged end of the water molecule will be attracted to a positive ion
• this means that the ions will get totally surrounded by water molecules - in other words they will dissolve
• water's polarity makes it useful as a solvent (substance capable of dissolving another substance). This means living organisms can take up useful substances (e.g. mineral ions) dissolved in water and these dissolved substances can be transported around the organisms body
• high latent heat of vaporisation: water evaporates (vaporises) when the hydrogen bonds holding water molecules together are broken - allows the water molecules on the surface of the water to escape into the air as a gas
• it takes a lot of energy (heat) to break the hydrogen bonds between water molecules so a lot of energy is used up when water evaporated - means it has a high latent heat of vaporisation - lots of heat is used to change it from a liquid to a gas
• This is useful for organisms bc it means they can use water loss through evaporation to cool down without losing too much water. When water evaporated it carries away heat energy from a surface, which cools the surface and helps to lower the temperature (e.g. when humans sweat to cool down)
• latent heat of vaporisation is the heat energy that is required to change a substance from one state to another
• can buffer changes in temp: hydrogen bonds give water a high specific heat capacity - the energy needed to raise the temperature of 1g of a substance by 1 degrees c
• when water is heated a lot of heat energy is used to break the hydrogen bonds between water molecules - this means there is less heat energy available to actually increase the temperature of the water - so water has a high specific heat capacity - it takes a lot of heat energy to heat it up
• This is useful for living organisms bc it means that water doesn't experience rapid temp changes - makes water a god habitat bc the temp underwater is likely to be more stable than on land.
• The water inside an organism also remains a fairly stable temperature - helping them to maintain a constant internal body temperature
• cohesion is the attraction between molecules of the same type - water molecules are very cohesive (they tend to stick together) bc they are polar
• strong cohesion helps water to flow, making it great for transporting substances e.g. it is how water travels in columns up the xylem in plants
• strong cohesion also means that water has a higher surface tension when it comes into contact with air - the reason that sweat forms droplets which evaporate from the skin to cool an organism down. Also the reason that pond skaters and some other insects can 'walk' on the surface of a pond