Carbon, in the form of CO2 from air and water, is absorbed by plants when they carry out photosynthesis. It becomes carbon compounds in plant tissues.
Carbon is passed on to primary consumers when they eat the plants. It’s passed on to secondary and tertiary consumers when they eat other consumers.
On their death, both plants and animals are usually broken down by saprobiotic microorganisms known collectively as decomposers.
Saprobiotic microorganisms secrete enzymes on to the dead organisms. These enzymes break down complex molecules into smaller, soluble molecules that saprobiotic microorganisms absorb by diffusion.
The carbon in the dead organisms is then released as carbon dioxide during respiration by the decomposer.
If decay is prevented for any reason, then the organism may become fossilised into coal, oil or peat. Not all parts of organisms decompose.
The shells and bones of aquatic organisms sink to the bottom of the oceans, and over millions of years, form carbon-containing sedimentary rocks such as chalk and limestone. This carbon eventually returns to the atmosphere as these rocks are weathered.
The combustion of fossil fuels, such as coal, oil and peat, has release carbon dioxide that was previously locked up within these fuels. Deforestation has removed enormous amounts of photosynthesising biomass and so less carbon dioxide is being removed from the atmosphere.
The oceans contain a massive reserve of carbon dioxide. This store is some 50 times greater than that in the atmosphere. It helps to keep the level of atmospheric carbon dioxide more or less constant.