Earth is the ultimate life support system. A multiplicity of biogeochemical cycles operate to maintain 'the balance of nature' . Oxygen is produced and carbon dioxide is fixed as biomass by autotrophic photosynthesizers energized by solar radiation.
Heterotrophic organisms metabolize oxygen and plant biomass to produce carbon dioxide.
Temperature is regulated by the complex interplay of solar radiation, radioactive decay within the Earth's interior, and a host of oceanic and atmospheric factors that are not well understood. Pure water is provided by natural weather cycles. Natural products are recycled by the ecosystem as one organism's waste is another organism's food. When man ventures beneath the sea or into space he must take his environment with him. For short duration missions it is practical to take an ample supply of food, oxygen, and the gases from which water and electricity are generated.