We look at the nitrogen and carbon cycle as they are connected to organic chemistry.
Here is a link to this: Chemistry of amino acids and protein structure
If you go to this and start to panic, relax. This is just the explanation of how the carbon and nitrogen cycle interact with living organisms.
The main points to get out of this is that for life on earth to work, materials such as carbon, nitrogen, and water need to be used over and over again, So the glass of water you are drinking at one time was a rain drop or could have been dinosaur spit.