Chemical nutrients and energy tend to flow in the same direction for most of an ecosystem, but the main difference is that the nutrient cycle is recycled in the ecosystem while the energy flow is ultimately lost from the ecosystem to the universe at large. Energy flow is unidirectional; chemical elements can be recycled. So, the energy that enters an ecosystem as sunlight eventually flows out of the ecosystem in the form of heat. In contrast, the matter in an ecosystem is continuously recycled as atoms are combined and recombined in different ways. Energy flows from lower to higher trophic levels; chemicals cycle from higher to lower trophic levels. Energy flow is unidirectional; chemical elements can be recycled. Energy can both enter and leave an ecosystem; chemical elements always remain within a single ecosystem. The movement of nutrients through the biosphere is different from the transfer of energy because, whereas energy flows through the biosphere and cannot be reused, elements are recycled. Matter is cycled through the ecosystem, while energy flows through it. Energy transfer is unidirectional, meaning it moves in one direction from the sun to producers, consumers, and decomposers, while matter can be recycled through the various organisms in the ecosystem. Matter cycles within ecosystems and can be traced from organism to organism. Plants use energy from the Sun to change air and water into matter needed for growth. Animals and decomposers consume matter for their life functions, continuing the cycling of matter. Energy and nutrients are passed from organism to organism through the food chain as one organism eats another. Decomposers remove the last energy from the remains of organisms. Inorganic nutrients are cycled, energy is not. Energy recycles while nutrients flow in two directions. Nutrients are lost as heat while energy forms chemical compounds. Energy and nutrients are passed around through the food chain, when one organism eats another organism. Any energy remaining in a dead organism is consumed by decomposers. Nutrients can be cycled through an ecosystem but energy is simply lost over time.