What Do Plants Do With Nitrogen?

[pnyf]

So I know that the bacteria in a cycled tank convert ammonia to nitrite to nitrate. It is also my understanding that that live plants will also use the different types of nitrogen. What does the plant do with the nitrogen? I'm trying to think back to my high school science classes, but am coming up with a blank.
is it nitrogen + CO2 get processed into O2 and water?

 

[SO19Firearms]

Wiki

 

[pnyf]

That's interesting. It sounds like the plants do a whole bunch of different things with the nitrogen then