<P> Ecological systems (ecosystems) have many biogeochemical cycles operating as a part of the system, for example the water cycle, the carbon cycle, the nitrogen cycle, etc . All chemical elements occurring in organisms are part of biogeochemical cycles . In addition to being a part of living organisms, these chemical elements also cycle through abiotic factors of ecosystems such as water (hydrosphere), land (lithosphere), and / or the air (atmosphere). </P> <P> The living factors of the planet can be referred to collectively as the biosphere . All the nutrients--such as carbon, nitrogen, oxygen, phosphorus, and sulfur--used in ecosystems by living organisms are a part of a closed system; therefore, these chemicals are recycled instead of being lost and replenished constantly such as in an open system . </P> <P> The flow of energy in an ecosystem is an open system; the sun constantly gives the planet energy in the form of light while it is eventually used and lost in the form of heat throughout the trophic levels of a food web . Carbon is used to make carbohydrates, fats, and proteins, the major sources of food energy . These compounds are oxidized to release carbon dioxide, which can be captured by plants to make organic compounds . The chemical reaction is powered by the light energy of the sun . </P> <P> It is possible for an ecosystem to obtain energy without sunlight . Carbon must be combined with hydrogen and oxygen in order to be utilized as an energy source, and this process depends on sunlight . Ecosystems in the deep sea, where no sunlight can penetrate, use sulfur . Hydrogen sulfide near hydrothermal vents can be utilized by organisms such as the giant tube worm . In the sulfur cycle, sulfur can be forever recycled as a source of energy . Energy can be released through the oxidation and reduction of sulfur compounds (e.g., oxidizing elemental sulfur to sulfite and then to sulfate). </P>

Determine why cycles in the biosphere are called biogeochemical cycles