Biogeochemical cycles refer to the flow of such chemical elements and compounds between organisms and the physical environment. Chemicals taken in by organisms are passed through the food chain and come back to the soil, air, and water through mechanisms such as respiration, excretion, and decomposition. Though the energy flow in an ecosystem is one way, the flow of nutrients is cyclical. Known as bio-geo-chemical cycle, the cyclical flow of nutrients required for the organisms to grow are continuously transmitted from abiotic to biotic factors and biotic to abiotic factors within an ecosystem. The flow of energy in the ecosystem is unidirectional because the energy lost as heat from the living organisms of a food chain cannot be reused by plants in photosynthesis. During the transfer of energy through successive trophic levels in an ecosystem, there is a loss of energy all along the path.