Why do plants need nitrogen?

Plants use nitrogen by absorbing either nitrate or ammonium through the roots. Most of the nitrogen is used by the plant to produce protein (in the form of enzymes) and nucleic acids. Nitrogen is readily transported through the plant from older tissue to younger tissue. Therefore, a plant deficient in nitrogen will show yellowing in the older leaves due to the underdevelopment or destruction of chloroplasts.(1)The reason why plants cant use nitrogen from the air is because it is in its gas form N2. Plants cant use nitrogen in this form so it therefore needs to be turned into either nitrate or ammonium before it can be utilised by the plant. This can be done through nitrogen fixation which can be accomplished by bacteria for example.