Biosphere is defined as that part of the earth and atmosphere in which many smaller ecosystems exist and operate. Three main subdivisions of the biosphere are lithosphere (solid matter); hydrosphere (liquid matter), and atmosphere or the gaseous envelope of the earth which extends up to a height of 22.5 km. The area of contact and interaction between these components is really important for life, for it is here that the entire life is confined and the basic processes of life like photosynthesis and respiration occur.

Living organisms are, mostly, confined to the parts of biosphere that receive solar radiation during the day. As stated above this includes atmosphere, the surface of land, the few meters of soil and the upper layers of water of oceans, lakes and rivers. The illuminated zone may be a few centimeters in turbid water of a river, and up to about 100 meters in clearest part of an ocean. In the ocean, the biosphere does not end where light ceases as gravity enables the energy flow to continue downward, since fecal pellets, cast skins and organisms dead and alive are always falling from the illuminated regions into the depths.

In addition to the extension of the biosphere downwards, there is a limited extension upwards also. On very high mountains, like Himalayas, the limit above which chlorophyll-bearing plants cannot live appears to be about 6,200 meters.

In simple words, biosphere can be viewed as the part of the earth consisting of oceans and the surface of the continents, together with the adjacent atmosphere (i.e. the troposphere). However, polar ice caps and the higher mountains slopes above the snow line are known as parabiosphere.

Since living organisms require inorganic metabolites from each of the subdivision of the biosphere, water from the hydrosphere, mineral from lithosphere and chemical elements from atmosphere, a brief discussion on the nature and working of each subdivision will help us to know the mechanism which influences metabolic activities of living organisms.