Matter flows within the biosphere through recycling by organisms and biochemical cycles. Organisms merely transform matter and do not deplete it, so it is possible for matter to cycle in various biological systems. Examples of matter flow within the biosphere include the water cycle and nutrient cycles.
The water cycle contributes to matter flow in the biosphere through its passing between organisms and the environment. When water enters the roots of a plant, it can either be used by the plant or get passed to another organism through consumption. Transpiration occurs when water evaporates from the leaves of a plant and re-enters the atmosphere, where it eventually condenses into rain and falls back to the earth, continuing the cycle.
The nutrient cycle includes other elements and compounds necessary for life and subsequently exchanged by various organisms. Examples are the nitrogen cycle, the carbon cycle and the phosphorus cycle. The nitrogen cycle occurs when specific bacteria transform atmospheric nitrogen into a form usable by other organisms, such as when nitrogen gas is converted into ammonia through nitrogen fixation. These bacteria can also transform nitrogen into nitrates or nitrites, which can then form the basis of proteins. Consumers that eat the producers of protein then transfer this nutrient throughout other layers of the biosphere. Decomposers transform dead organic material back into ammonia, where it is returned to the soil as the cycle continues.