Ecosystems contain two kinds of commodities: matter (nutrients) and energy. Nutrients cycle through the ecosystem, available for repeated use by organisms. These cycles of use and reuse are called biogeochemical cycles. Energy instead is used to convert low-energy carbon dioxide into high-energy carbohydrate, then passes through one or more organisms and is then lost to the ecosystem. Eventually, all of the energy that enters the ecosystem is lost as heat. To study and explain the movement of energy through an ecosystem, ecologists concentrate on issues of just who is eating whom.