Human earth shapers/ETHS101/Earthsystems/Biosphere

=Biosphere=

Overview

 * 1) Life on earth can only live in a very thin layer.
 * 2) This is because the second law of thermodynamics means that all life needs a more-or-less continuous input of high grade energy (that's what food really is).
 * 3) The energy almost all comes from the sun via photosynthesis, which converts carbon dioxide and water into food and oxygen.
 * 4) Photosynthesis only occurs where there is enough light (i.e. above ground, or in top layer of the ocean, rivers or lakes).
 * 5) All living things also need enough water to survive.
 * 6) Most life lives in shallow water or just above the ground
 * 7) Some food falls down into the deep ocean, so there is life there
 * 8) Some food is carried down into the soil so there is lots of life there as well
 * 9) Life doesn't exist anywhere else on earth

Where Life is
Life on earth exists in a tiny proportion of the earth - the biosphere. Calling the biosphere a “sphere” is bit misleading, it is really a very thin layer smeared on top of the geosphere. Life only exists in this small volume because living organisms depend on having suitable amounts of a set of different things, and this happy combination only occurs in very restricted places. The first and most important thing is that life needs energy in a usable form (food) because of the second law of thermodynamics, as explained in this mini-lecture.

Where the biosphere is and why
So, all life needs energy in a usable form ("food") because of the second law of thermodynamics. Life gets this energy either directly via photosynthesis or from eating things that obtained their food from photosynthesis. The only exception that we know of is a set of interesting organisms that exploit energy stored in chemicals created geologically. If you want more information on these, do a web search for “chemoautotroph”.

This need to get energy from photosynthesis is the first reason that the biosphere is so small. Photosynthesis needs light, carbon dioxide and water, so life is centred on the places where these are available together. This is explained in this second mini-lecture