Living organisms remove materials from the environment for growth and other processes, and these materials are returned to the environment either in waste or when living things die & decay
Materials decay because they are decomposed by microorganisms, which are more active and break down materials faster in warm, moist and aerobic conditions
The decay process releases substances that plants need to grow
In a stable community, the processes that remove materials are balanced by processes that return materials, and so the materials are part of a constant cycle
The carbon cycle
Carbon dioxide is removed from the environment by green plants and algae during photosynthesis
The carbon from the carbon dioxide is used to make carbohydrates, fats and proteins, which make up the bodies of plants & algae
When green plants & algae are eaten by animals and these animals are eaten by other animals, some of the carbon becomes part of the fats & proteins that make up their bodies
When green plants, algae & animals respire, some of this carbon becomes carbon dioxide and is released into the atmosphere
When plants, algae & animals die, some animals & microorganisms feed on their bodies, releasing carbon as carbon dioxide by respiration
By the time the microorganisms & detritus feeders have broken down the waste products & dead bodies of organisms in ecosystems and cycled the materials as plant nutrients, all the energy originally absorbed by green plants & algae has been transferred
Combustion of wood & fossil fuels releases carbon dioxide into the atmosphere