Biosphere refers to the narrow zone of the earth in which all life forms exist.
Do you know why life becomes possible in this zone?
It is because this is the zone in which all the three essentials things which are required for sustenance of life are found in a right mixture.
They are land (lithosphere), air (atmosphere) and water (hydrosphere).
In other words, this narrow zone is a place where lithosphere, atmosphere and hydrosphere meet.
This zone extends vertically into the atmosphere to about 10km, downward into the ocean to depths of about 10.4 km and into about 27,000 ft of the earth’s surface where maximum living organism have been found.
There are some life forms which are found in extreme conditions.
Two examples of this type are algae and thermophillic.
Algae which is supposed to be one of the earliest forms of life can exist even in the most hostile environment such as frozen Antarctica.
On the other extreme side, thermophillic (heat loving) bacteria usually inhabit deep sea volcanic vents having a temperature of more than 3000C.
In fact, these bacteria cannot survive in a temperature below boiling point.
Biosphere has been evolving over the time.
Horizontally the biosphere covers the entire globe, though the life may not be possible in some of the hottest and the coldest parts.
However, most living things are confined to a narrow band which permits the capture of solar energy through the process of photosynthesis, which is essential for any organic life.