Root nodules of leguminous plants harbour symbiotic bacteria, Rhizobium, which fixes atmospheric nitrogen and provides the plant with most or all of its nitrogen requirements. In return, they have access to a rich supply of carbohydrate. The fixation of atmospheric nitrogen $\left(\mathrm{N}_2\right)$ to ammonia $\left(\mathrm{NH}_3\right)$ occurs within the nodules, using the enzyme nitrogenase but this enzyme is very sensitive to oxygen and gets inhibited in oxygen rich environment. This problem is solved by leghaemoglobin, a pinkish-red, oxygen-binding, iron containing protein present in root nodules. It functions as oxygen buffer, scavanges oxygen and provides the enzyme nitrogenase, a low oxygen environment in which nitrogen fixation can occur.