The biosphere is simply "life on Earth"—the sum total, that is, of all living things on Earth. Yet the whole is more than the sum of the parts: not only is the biosphere an integrated system whose many components fit together in complex ways, but it also works, in turn, in concert with the other major earth systems. The latter include the geosphere, hydrosphere, and atmosphere, through which circulate the chemical elements and compounds essential to life. Among these elements is carbon, a part of all living things, which also cycles through the nonliving realms of soil, water, and air—just one of many vital biogeo-chemical cycles. As for the compounds on which life depends, none is more important than water, which, though it is the focal point of the hydrosphere, passes through the various earth systems as well. Organisms participate in the hydrologic cycle by providing moisture to the air through the process of transpiration, and they likewise benefit from the downward movement of moisture in the form of precipitation. These and many other interactions make it easy to see why scientists speak of Earth as a system—and why some go even further and call it a living thing.