Oxygen Cycle

It involves the inter- change of oxygen between the elemental form of gaseous O2, contained in a huge reservoir in the atmosphere, and chemically bound O in CO2, H2O, minerals, and organic matter. It is strongly tied with other elemental cycles, particularly the carbon cycle. Elemental oxygen becomes chemically bound by various energy-yielding processes, particularly combustion and metabolic processes in organisms. It is released in photosynthesis. This element readily combines with and oxidizes other species such as carbon in the process of aerobic respiration, or carbon and hydrogen in the combustion of fossil fuels such as methane:

CH4 + 2O2 → CO2 + 2H2O

Elemental oxygen also oxidizes inorganic substances such as iron(II) in minerals:

4FeO4 + O2 → 2Fe3O3

A particularly important aspect of the oxygen cycle is stratospheric ozone O3. A relatively small concentration of ozone in the stratosphere, more than 10 km high in the atmosphere, filters out ultraviolet radiation in the wavelength range of 220-330 nm, thus protecting life on the Earth from the highly damaging effects of this radiation. The oxygen cycle is completed by the return of elemental O2 to the atmosphere. The only significant way in which this is done is through photosynthesis mediated by plants.

REACTIONS OF ATMOSPHERIC OXYGEN:

Some of the primary features of the exchange of oxygen among the atmosphere, geosphere, hydro-sphere, biosphere, and anthrosphere are summarized. The oxygen cycle is critically important in atmospheric chemistry, geochemical transformations, and life processes. Oxygen in the troposphere plays a strong role in processes that occur on the Earth's surface. Atmospheric oxygen takes part in energy-producing reactions, such as the burning of fossil fuels:

CH4(in natural gas) +2O2→ CO2+2H2O

Oxygen is returned to the atmosphere through plant photosynthesis:

CO2 + H2O + hv→ {CH2O} + O2

All molecular oxygen now in the atmosphere is thought to have originated through the action of photosynthetic organisms, which shows the importance of photosynthesis in the oxygen balance of the atmosphere. It can be shown that most of the carbon fixed by these photosynthetic processes is dispersed in mineral formations as humic material, only a very small fraction is deposited in fossil fuel beds. Therefore, although combustion of fossil fuels consumes large amounts of O2, there is no danger of running out of atmospheric oxygen.

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Nitrogen Cycle

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