The atmosphere contains about 21% oxygen. The atmospheric oxygen enters the living organisms, as a gas required in respiration.
During this process carbon dioxide and water are formed.
C6 H12 O6 + 6O2 —–> 6 CO2 + 6 H2O + energy
The metabolic water thus formed is added to all other water present in living organisms and as such it may undergo three possible fates. Some of it may be excreted and so added to the water content of the environment. Another part may be used as a building material for the formation of more living matter. A third possible fate of the water within organism is its consumption as a fundamental raw material along with the carbon dioxide in photosynthesis.
In this process the oxygen is liberated as shown by the following equation:
6 CO2 + 6 H2O + light energy
Such free oxygen may now again be used in respiration or it may be returned to the environment as molecular atmospheric oxygen, completing the cycle (Fig. 5.7). Thus oxygen enters organisms only through respiration and leaves through photosynthesis. In intervening steps the oxygen is incorporated in water, and in this form it can interlink with the water cycle or indirectly with carbon cycle.
The oxygen is the atmosphere is the source of ozone (O3). The Ozone layer protects organisms by preventing most of the ultraviolet and X-ray from reaching the earth’s surface. The most recent factor affecting the oxygen cycle of the biosphere and the oxygen budget of the earth in the man himself. In addition to inhaling oxygen and exhaling carbon dioxide, man decreases the oxygen level and increases the carbon dioxide level by burning fossil fuels.