Generally defined, the portion of the Earth where life is found is called the biosphere, and consists of three parts. This is commonly accepted, although geologists or other specialized earth scientists sometimes define the biosphere more narrowly to include only the life itself -- the bacteria, algae, plants and animals, including humans, that inhabit the Earth. Under these more narrow definitions, the biosphere forms a fourth part of the Earth system and interacts with the other three.
The biosphere is the portion of the Earth where life occurs -- the portions of the land, water and air that hold life. These parts are known, respectively, as the lithosphere, hydrosphere and atmosphere. However, some portions of each may not support life -- for example, the upper regions of the atmosphere.
The lithosphere is the terrestrial part of the biosphere -- the land mass. With the exception of the lower mantle and core, the lithosphere supports a variety of life from bacteria to large mammals. The weathering of the lithosphere crust forms soil, which provides minerals to support life.
The hydrosphere is the aquatic part of the biosphere -- the water. Unlike the lithosphere and the atmosphere, every portion of the hydrosphere supports life. Specially-adapted bacteria grow in hot springs, tube worms form the basis of sulfur-based communities around deep-sea, hydrothermal vents, and in more hospitable regions, life abounds. Water-dwelling individuals of virtually every taxonomic group of plants and animals have been identified as important parts of the biosphere. Water is essential to life, and the hydrosphere plays an important part in atmosphere formation.
The lower regions of the atmosphere contain gases that are essential for plant and animal respiration, and birds and other life can be found up to around 2,000 meters above the Earth's surface. The atmosphere also plays critical roles in shaping the biosphere by deflecting harmful radiation from the sun and determining weather patterns.
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