<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> <P> Although the Earth constantly receives energy from the sun, its chemical composition is essentially fixed, as additional matter is only occasionally added by meteorites . Because this chemical composition is not replenished like energy, all processes that depend on these chemicals must be recycled . These cycles include both the living biosphere and the nonliving lithosphere, atmosphere, and hydrosphere . </P> <P> The chemicals are sometimes held for long periods of time in one place . This place is called a reservoir, which, for example, includes such things as coal deposits that are storing carbon for a long period of time . When chemicals are held for only short periods of time, they are being held in exchange pools . Examples of exchange pools include plants and animals . </P>

Example of the relationship between energy and the cycles of matter
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