water

• Functions of water


  • Reactant in important chemical reactions, including hydrolysis reactions
  • Solvent for biological reactions
  • Transports substances
  • Helps with temperature control
  • Habitat for organisms
• Solvent


  Substances can dissolve in it
• Most biological reactions take place in solution (e.g. in the cytoplasm of eukaryotic and prokaryotic cells)
• Transports substances


  Water's liquid and solvent properties allow it to easily transport materials like glucose and oxygen around plants and animals
• Water


  • Has a high specific heat capacity
  • Has a high latent heat of evaporation
• Habitat


  The fact that water helps with temperature control, is a solvent and becomes less dense when it freezes means many organisms can survive and reproduce in it
• 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 partial negative charge on one side and a partial positive charge on the other.
• The slightly negatively-charged oxygen atoms attract the slightly positively-charged hydrogen atoms of other water molecules. This attraction is called hydrogen bonding and it gives water some of its useful properties.
• Hydrogen bonds give water a high specific heat capacity - this is the energy needed to raise the temperature of 1 gram of a substance by 1 °C.

  The hydrogen bonds between water molecules can absorb a lot of energy.

  So water has a high specific heat capacity - it takes a lot of energy to heat it up. This means water doesn't experience rapid temperature changes, which is one of the properties that makes it a good habitat - the temperature under water is likely to be more stable than it is on land.
• It takes a lot of energy (heat) to break the hydrogen bonds between water molecules. So water has a high latent heat of evaporation - a lot of energy is used up when water evaporates (changes from a liquid to a gas). This is useful for living organisms because it means water's great for cooling things.

  This is why some mammals, like us, sweat when they're too hot. When sweat evaporates, it cools the surface of the skin.
• Cohesion is the attraction between molecules of the same type (e.g. two water molecules). Water molecules are very cohesive (they tend to stick together) because they're polar. This helps water to flow, making it great for transporting substances. It also helps water to be transported up plant stems in the transpiration stream.
• At low temperatures water freezes - it turns from a liquid to a solid.

  Water molecules are held further apart in ice than they are in liguid water because each water molecule forms four hydrogen bonds to other water molecules, making a lattice shape. This makes ice less dense than liquid water - which is why ice floats. This is useful for living organisms because, in cold temperatures, ice forms an insulating layer on top of water - the water below doesn't freeze. So organisms that live in water, like fish, don't freeze and can still move around.
• A lot of important substances in biological reactions are ionic (like salt, for example). This means they're made from one positively-charged atom or molecule and one negatively-charged atom or molecule (e.g. salt is made from a positive sodium ion and a negative chloride ion). Because water is polar, the slightly positive end of a water molecule will be attracted to the negative ion, and the slightly negative end of a water molecule will be attracted to the positive ion. This means the ions will get totally surrounded by water molecules - in other words, they'll dissolve.
• Water's polarity makes it useful as a solvent in living organisms.

  E.g. in humans, important ions can dissolve in the water in blood and then be transported around the body.