Describe and explain the Nitrogen Cycle.

For a question like this it is very useful to draw a diagram of the cycle linking to where it occurs to aid visual learning and retention of each stage. For example, I like to teach this by showing the cycle moving throughout a landscape as it progresses from nitrogen in the atmosphere to in the ground. Colours can be used to annotate what organism is required for the stage to take place and enzymes that aid reactions that take place. Markschemes often give marks for simply naming stages and linking them to the bacteria involved therefore it is vital you learn this in order to pick up straightforward marks. Being a substantial topic it can come up on the 25 mark essay questions so I have included plenty of detail in order to access all those marks. I would answer a question like so: Nitrogen is used by all organisms to make proteins and nucleic acids therefore it is essential to life. The nitrogen cycle starts with nitrogen fixation whereby nitrogen gas is "fixed" or changed into more useful compounds by nitrogen fixing bacteria. Nitrogen gas is reduced to ammonia which dissolves to form ammoniun ions. This reaction is catalysed by the enzyme nitrogenase and requires a significant amount of energy is the form of 15 molecules of ATP. The nitrogen fixing bacteria live in the soil but are particularly abundant in the roots of leguminous plants e.g. peas. The next stage is nitrification whereby nitrifying bacteria can oxidise (losing electrons) ammonia to nitrate in two parts. Firstly, forming nitrite ions then nitrate ions. This reaction releases energy which the bacteria utilise. During ammonification, saprobiotics break down protein to detritus to form ammonia in two stage. Firstly they break down proteins to amino acids using extracellular protease enxzymes. Next they remove amino groups from amino acids using deaminase enzymes to produce ammonia. Lastly, dentrification is the anaerobic process that converts nitrates to nitrogen gas which are then lost to the air. The cycle then begins again.