what is the first step in the nitrogen cycle when gaseous nitrogen is converted into ammonia
Nitrogen cycle Conversion of nitrogen into nitrates and nitrites through atmospheric, industrial and biological processes is called as nitrogen fixation. Atmospheric nitrogen must be processed, or "fixed", in a usable form to be taken up by plants. Between 5x1012 and 10x1012 g per year are fixed by lightning strikes, but most fixation is done by free-living or symbiotic bacteria known as diazotrophs. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is converted by the bacteria into other organic compounds. Most biological nitrogen fixation occurs by the activity of Mo-nitrogenase, found in a wide variety of bacteria and some Archaea. Mo-nitrogenase is a complex two-component enzyme that has multiple metal-containing prosthetic groups.[3] An example of free-living bacteria is Azotobacter. Symbiotic nitrogen-fixing bacteria such as Rhizobium usually live in the root nodules of legumes (such as peas, alfalfa, and locust trees). Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates. Because of this relationship, legumes will often increase the nitrogen content of nitrogen-poor soils. A few non-legumes can also form such symbioses. Today, about 30% of the total fixed nitrogen is produced industrially using the Haber-Bosch process,[4] which uses high temperatures and pressures to convert nitrogen gas and a hydrogen source (natural gas or petroleum) into ammonia.[5]
Cell cycle The first phase within interphase, from the end of the previous M phase until the beginning of DNA synthesis, is called G1 (G indicating gap). It is also called the growth phase. During this phase, the biosynthetic activities of the cell, which are considerably slowed down during M phase, resume at a high rate. The duration of G1 is highly variable, even among different cells of the same species.[3] In this phase, the cell increases its supply of proteins, increases the number of organelles (such as mitochondria, ribosomes), and grows in size. In G1 phase, a cell has three options. (1) To continue cell cycle and enter S phase (2) Stop cell cycle and enter G0 phase for undergoing differentiation. (3) Get arrested in G1 phase hence it may enter G0 phase or re-enter cell cycle. The deciding factor is availability of nitrogens and storage of energy rich compounds at the deciding point called check point. This check point is called the restriction point or START and is regulated by G1/S cyclins, which cause transition from G1 to S phase. Passage through the G1 check point commits the cell to division.