The biological conversion of atmospheric nitrogen into ammonia in the group of plants known as legumes (peas, beans lentils and clover etc.) is achieved by the action of soil bacteria which infect the plant tissues through the root hairs. The bacteria multiply and grow using nutrients and energy from the plant, producing small growths or nodules on the roots. In the nodules the bacteria can then convert atmospheric nitrogen into ammonia.

(i) The root bacteria use a complex enzyme to enable the conversion of atmospheric nitrogen into ammonia. In Figure 4, the graph shows the rate at which they produce ammonia against increasing temperature. Explain how and why the rate of ammonia production differs in zones 1, 2 and 3 shown on the graph, and what point X represents.