Water

Water is vital to living organisms 

Water makes up about 80% of a cell's contents. It has loads of improtant functions, inside and outside of cells 

Water is a metabolite in loads of metabolic reactions, including condensation and hydrolysis reactions. 

Water is a solvent, which means that 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 because it has a high latent heat of vapourisation 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 orther organisms. 

Water molecules have a simple structure 

 A molecule of water (H2O) is one atom of oxygen (O) joined to two atoms of hydrogen (H2) 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 slightly positive charge. 

The unshared negative electrons on the oxygen atom give it a slightly negative charge. 

This makes water a polar molecule - it has a partial negative charge (delta -) on one side and a partial positive (delta +) charge on the other. 

The slightly negatively-charged oxygen atoms attract the slightly positive-charged hydrogen atoms of other water molecules. 

This attraction is called hydrogen bonding and it gives water some of it's useful properties. 

Useful properties of water 

Water is an important Metabolite 

• Many metabolic reactions involve a condensation or hydrolysis reaction 
• A hydrolysis reaction requires a molecule of water to break a bond. A condensation reaction releases a molecule of water as a new bond is formed. 
• For example, amino acids are joined together to make polypeptides by condensation reactions. Energy from ATP is released through a hydrolysis reactions. 

Water has a high latent heat of vapourisation 

It takes a lot of energy (heat) to break the hydrogen bonds between the water molecules.

This is useful for living organisms because it means that they can use water loss through evaporation to cool down (e.g. humans sweat to cool down) without losing too much water. 

Water can buffer (resist) changes in temperature 

The hydrogen bonds between water molecules can absorb a lot of energy. So water has a high specific heat capacty (it takes a lot of energy). 

This is useful for living organisms because it means that water doesn't experience rapid temperature changes. This makes water a good habitat because the temperature under water is likely to be more stable than on land. The water inside organisms also remains at a fairly stable temperature- helping them to maintian a constant internal body temperature. 

Water is good solvent

A lot of important substances in metabolic reactions are ionic (like salt). This means they're made from one positively charged atom or molecule and one negatively charged atom or molecule. 

Water is polar, 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 a positive ion. 

This means that the ions will get totally surrounded by water molecules- they'll dissolve 

There's strong cohesion between the water molecules 

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 are polar. 

Strong cohesion helps water to flow, making it great for transporting substances e.g. how water travels in columns up the xylem in plants. 

Strong cohesion also means that water has a high surface tension when it comes into contact with air. This is the reason why sweat forms droplets, which evaporate from the skin to cool an organism down. It is the reason alos that some insects are able to walk on the surface of a pond