Matter cycles and energy flow

Atoms move between organisms and the nonliving environment. Plants take up nutrients, animals obtain them through food, and decomposers return materials to forms that can be reused. Carbon also moves through photosynthesis, respiration, and other processes. Energy behaves differently from matter: it passes through food webs and much becomes heat that is no longer available to the organisms for work. Ecosystems need continuing energy inputs, even while nutrient atoms can cycle repeatedly.