Energy is the capacity to do work. It is the flow of energy that makes the work of life (metabolism) possible. From the smallest unit of life—the cell—to the largest organism, all living things obtain, modify, expend, and release energy and in doing so adhere to very explicit energy laws of the universe.
Of the tremendous amount of energy received from the sun, less than 1 percent is captured by green plants and converted through photosynthesis to usable energy. Some solar energy drives the geological processes that make the earth habitable for organisms then is lost back into space. Fully a third never makes it through the Earth's atmosphere to reach the surface. The small percentage that is captured in photosynthesis runs the entire Earth ecosystem.