Why can humans get nitrogen for their diets by breathing air?
Nitrogen in the air is N2. N2 is inert (non reactive) and this bond takes a rather large amount of energy to break. Humans lack the right machinery (enzymes) to break it. Even having enough energy and the enzymes not enough. The reaction needs to be done in a low O2 environment. The low oxygen environment is a requirement of nitrogenase, the enzyme that does this reaction. Consequently, only some organisms can do this trick and only when the conditions (high energy and low O2) are right. Unfortunately, animals like to breathe too much (we are highly oxygenated) to ever take advantage of local N2 fixation. Most organisms that fix nitrogen are fairly simple bacteria. This allows them the flexibility to survive the low O2 environments required. Even higher plants like legumes that fix nitrogen do it through association with N2 fixing bacteria.
