Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Biosynthesis of Amino Acids and Nucleotides
The ability to fix atmospheric nitrogen is limited to a relatively few organisms.

Only a relatively small number of microorganisms and plants have the ability to fix atmospheric nitrogen. These organisms include certain free-living Bacteria, such as cyanobacteria (blue-green Algae), which inhabit not only fresh and salt waters but also soils, as well as A number of soil-dwelling bacteria, notably Azotobacter. The primary major product of Nitrogen Fixation in these organisms is ammonia (NH₃), which can be utilized by other life forms either directly or after its conversion into soluble compounds such as nitrites, nitrates, and Amino Acids.

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Fig. 22-26. The Nitrogen Cycle. The total amount of nitrogen fixed annually by the biosphere exceeds 10¹¹ kg.

A different mechanism of Nitrogen fixation is characteristic of the legume family, which includes peas, beans, clover, and alfalfa. This mechanism—known as symbiotic nitrogen fixation—is based on the interaction between a host plant and symbiotic bacteria inhabiting its ROOT nodules. The Enzymes involved in nitrogen fixation belong to the root-nodule bacteria, while the plant, in turn, supplies certain essential components for this process that the bacteria lack (Fig. 22-27). Along with legumes, certain other plant species also possess the ability to fix atmospheric nitrogen; however, the overwhelming majority of non-leguminous plants and all animal species lack this capability.

The vital importance of nitrogen for life is evident from the fact that nearly all cultivated crops require more soluble nitrogen compounds for optimal growth than are naturally present in the soil. We can supply them with nitrogen in the form of "natural" fertilizers, such as manure or guano (Section 19.20), or apply chemical fertilizers to the soil, such as ammonium nitrate (NH₄NО₃). Another way to provide crops with the necessary nitrogen is through crop rotation. Without The addition of nitrogen fertilizers, corn can grow in the same field for no more than one or two seasons; however, if every two or three years the field is planted with peas, beans, alfalfa, or clover, these plants, thanks to their capacity for symbiotic nitrogen fixation, will enrich the soil with sufficient nitrogen to subsequently sustain the growth of corn.

Fig. 22-27. Root nodules of a leguminous plant (Lotus corniculatus) where nitrogen fixation takes place. The nitrogen-fixing symbiotic bacteria form large aggregates closely associated with the root Cells. These cells supply the bacteria with specific factors required for nitrogen fixation, notably leghemoglobin, which has a very high affinity for oxygen. Oxygen is a potent inhibitor of Nitrogenase.