Water

Water is the medium transport in all living things, as it is able to carry polar substances (that don't usually dissolve in organic substances) and many non-polar substances.

Structure Makes It Dipolar

• It contains 1 atom of oxygen joined to 2 atoms of hydrogen.
• The shared negative hydrogen electrons are pulled towards the oxygen atom.
• This leaves the other side of each of the hydrogen atoms with a slight positive charge.
• The unshared negative electron on the oxygen atom gives a slight negative charge.
• The negatively charged oxygen atoms of water attract the positively charged hydrogen atoms of other water molecules.
• This attraction is called hydrogen bonding, which holds the molecule together.
• The non-linear, v-shape allows the ends to attract opposite charges.

Cohesion

• Cohesion is the attraction between molecules of the same type.
• Water molecules are very cohesive (they stick together) due to their dipolar nature.
• This helps water to flow, making it great for transporting substances.

Solvent

• Waters dipolar nature makes it a solvent for other polar molecules.
• A lot of important substances in biological reactions are ionic.
• This means they're made from 1 positively charged atom/molecule and 1 negatively charged atom/molecule.
• As water is dipolar, the positive end of a water molecule will be attracted to the negative ion, and the negative end of a water molecule will be attracted to the positive ion.
• This means the substances will dissolve in water, as their ions will separate and become surrounded by water molecules.
• As many biological reactions within the cells happen in water, its ability to act as a solvent is very important.

Thermal Properties

• The hydrogen bonds allow water to be liquid at room temperature, unlike other small molecules like carbon dioxide which is a gas at room temperature.
• It's a liquid as the hydrogen bonds mean a lot of energy is required to warm water up and turn into gas.
• This gives water a high specific heat capacity and high latent heat of vaporisation.
• This is useful in living things as it means temperature fluctuations are small.
• This can also allow the body to cool down as sweating and transpiration take energy from the body. 

Surface Tension

• Surface tension is when liquids behave as if their surface is covered by a thin elastic skin.
• Water has a very high surface tension.
• This is due to the hydrogen bonds which hold the molecules together and pull them down.
• This attraction is not present between the different molecules where water and air meet.
• Causing the water layer to hold together to form a thin skin of surface tension.

Amphoteric

• Water molecules are amphoteric, which means they can act as both an acid and a base.
• The acid comes from its formation of H ions, a proton donor.
• The base comes from its formation of OH ions, a proton acceptor.
• As water molecules can both donate and accept protons it can act as a buffer in the biochemical reactions that take place in cells.
• This means help to prevent reactions in progress from changing the pH inside the cell.

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