Biosphere
Ecology can be studied at several levels: population level (individuals of
the same species in the same or similar environment), biocenosis level (or
community of species), ecosystem level, and biosphere level.
The outer layer of the planet Earth can be divided into several compartments:
the hydrosphere (or sphere of water), the lithosphere (or sphere of soils
and rocks), and the atmosphere (or sphere of air). The biosphere (or sphere
of life) is all living matter on the planet or that portion of the planet
occupied by life. It reaches well into the other three spheres, although
there are no permanent inhabitants of the atmosphere. Relative to the volume
of the Earth, the biosphere is only the very thin surface layer which
extends from 11,000 meters below sea level to 15,000 meters above.
It is thought that life first developed in the hydrosphere, at shallow
depths, in the photic zone. (Recently, though, a competing theory has
emerged, that life originated around hydrothermal vents in the deeper ocean.)
Multicellular organisms then appeared and colonized benthic zones.
Photosynthetic organisms gradually produced the chemically unstable
oxygen-rich atmosphere that characterizes our planet. Terrestrial life
developed later, after the ozone layer protecting living beings from UV (ultra-violet)
rays formed. Diversification of terrestrial species is thought to be
increased by the continents drifting apart, or alternately, colliding.
Biodiversity is expressed at the ecological level (ecosystem), population
level (intraspecific diversity), species level (specific diversity), and
genetic level. Recently technology has allowed the discovery of the deep
ocean vent communities. This remarkable ecological system is not dependent
on sunlight but bacteria, utilizing the chemistry of the hot volcanic vents,
are at the base of its food chain.
The biosphere contains great quantities of elements such as carbon, nitrogen,
hydrogen and oxygen. Other elements, such as phosphorus, calcium, and
potassium, are also essential to life, yet are present in smaller amounts.
At the ecosystem and biosphere levels, there is a continual recycling of all
these elements, which alternate between the mineral and organic states.
While there is a slight input of geothermal energy, the bulk of the
functioning of the ecosystem is based on the input of solar energy. Plants
and photosynthetic microorganisms convert light into chemical energy by the
process of photosynthesis, which creates glucose (a simple sugar) and
releases free oxygen. Glucose thus becomes the secondary energy source which
drives the ecosystem. Some of this glucose is used directly by other
organisms for energy. Other sugar molecules can be converted to other
molecules such as amino acids.
Cellular respiration is the process by which organisms (like mammals) break
the glucose back down into its constituents, water and carbon dioxide, thus
regaining the stored energy the sun originally gave to the plants. The
proportion of photosynthetic activity of plants and other photosynthesizers
to the respiration of other organisms determines the specific composition of
the Earth's atmosphere, particularly its oxygen level.