Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Metabolism: An Overview
There is a nitrogen cycle in the biosphere

In addition to sources of carbon, oxygen, and energy, All living organisms require a source of nitrogen. Nitrogen is essential for the Biosynthesis OF AMINO Acids, as well as purine and pyrimidine bases—the nitrogen-containing building blocks from which Proteins and Nucleic Acids are subsequently assembled. Here we encounter a familiar distinction: living organisms vary widely in the chemical form in which they can assimilate nitrogen. Almost all higher animals must obtain at least a portion of their required nitrogen in the form of Amino Acids. For example, the human and white rat diet must include 10 of the 20 common amino acids in preformed form, because their bodies cannot synthesize these amino acids from simpler precursors. Plants can generally utilize ammonia or soluble nitrates as their sole nitrogen source. Relatively few organisms possess The ability to assimilate (fix) gaseous nitrogen (N₂), which accounts for about 80% of our atmosphere. However, because the Earth's crust contains very little inorganic nitrogen in the form of soluble salts, all living organisms ultimately depend on this atmospheric nitrogen and on the organisms capable of fixing it. Nitrogen is fixed, for example, by cyanobacteria (formerly known as blue-green Algae). Cyanobacteria lead an independent existence because they are completely autotrophic. They not only assimilate atmospheric nitrogen, but are also capable of Photosynthesis, meaning they can meet their entire carbon requirement from atmospheric CO₂. Almost all other species of nitrogen-fixing Bacteria reside in the soil. Some of them live as symbionts in the ROOT nodules of certain plant species, primarily members of the legume family, and carry out symbiotic Nitrogen Fixation there.

Nitrifying bacteria oxidize ammonia to nitrites and nitrates, while denitrifying bacteria convert nitrates back into ammonia. Thus, alongside the giant cycles of Carbon and Oxygen (Fig. 13-1), our biosphere also features a nitrogen cycle, in which colossal amounts of nitrogen undergo cyclic transformations (Fig. 13-2). The cycles of carbon, oxygen, and nitrogen, driven by the activities of many species of living organisms, undoubtedly depend on maintaining a delicate balance between producers and consumers in the biosphere (Fig. 13-3). These large-scale cycles of matter in the biosphere are accompanied by an equally massive cycle of energy. Photosynthetic organisms capture solar energy and produce energy-rich CARBOHYDRATES and other Organic compounds, while heterotrophic organisms utilize these Organic compounds as Energy Sources. In the METABOLISM of every Organism participating in these metabolic cycles, which expends energy on various types of work, some fraction of the metabolizable (useful) form of energy is lost, while The amount of non-metabolizable (unavailable, useless) energy increases. At almost every stage of these biological cycles, heat and Other forms of energy are dissipated into the environment—that is, they transition into a disordered and unusable form for living organisms. Consequently, the flow of energy in the biosphere is a unidirectional rather than a cyclic process, because useful energy cannot be regenerated from unavailable, dissipated energy. Carbon, oxygen, and nitrogen undergo continuous Circulation, participating in cycle after cycle, whereas useful energy continually degrades into an unavailable form.

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Fig. 13-2. The Nitrogen Cycle in the biosphere

Fig. 13-3. Solar Energy Flow and the cycles of carbon, oxygen, and nitrogen illustrated in a single ecosystem. In this isolated ecosystem, photosynthesis by grassy vegetation fixes atmospheric CO₂, generates organic compounds, and releases oxygen. Soil microorganisms fix atmospheric nitrogen, converting it into ammonia and nitrates, which are then utilized by plants as nitrogen sources for the synthesis of proteins and nucleic acids. Zebras obtain oxygen from the air and acquire the carbon and amino acids they need from plants by oxidizing starch, protein, and Other components of plant matter. Lions prey on zebras, and their excrement returns to the soil, where microorganisms process it, completing the cycle.

Solar energy serves as the driving force behind this entire cycle. However, at each link in the food chain, less than 10% of the acquired useful energy is converted into biomass; the remaining energy is dissipated into the environment and becomes unavailable. Of all the solar energy reaching this ecosystem, less than 0.1% is stored in the bodies of lions. This is why a very vast territory is required to feed a herd of zebras, and a large herd of zebras to feed a pair of lions.

Let us now shift our focus from these macroscopic aspects of metabolism to the metabolic events occurring within living Cells at the microscopic level. We must keep in mind, however, that each Cell type is characterized by its own unique requirements for specific sources of carbon, oxygen, and nitrogen, as well as corresponding energy sources. Cellular metabolism is a system of enzymatic transformations of both matter and energy, beginning with initial substrates and culminating in The biosynthesis of living matter.