Various animals and plants breathe oxygen, and the end product of respiration – CO2 – is released into the atmosphere.
All living things breathe, as a result of this process, carbon in organic substances, in the form of carbon dioxide, enters the atmosphere again. Carbon dioxide is also formed during the mineralization of organic matter by microorganisms. In living matter, the processes of carbon assimilation and its release during breathing are practically balanced. Only about 1% of the carbon is deposited in the form of peat, that is, it is removed from the cycle. In the hydrosphere, carbon is contained in dissolved form (carbon dioxide, carbonic acid, carbonic acid ions). Here its reserves are much greater than in the atmosphere. Hydrosphere carbon is also used by living organisms in the process of photosynthesis and for the construction of calcareous skeletons (sponges, intestinal, mollusks, etc.). Carbon is constantly exchanged between the World Ocean and the hydrosphere, and in the ocean a significant amount of carbon is removed from the cycle and deposited in the form of poorly soluble carbonates.
Carbon also enters the atmosphere as a result of human activities: when burning organic mineral fuel – coal, gas, oil and its processed products, etc. Carbon dioxide is formed during the combustion of fuel and enters the atmosphere during decay of organic matter, fermentation, respiration, from sedimentary rocks due to chemical processes that occur at high temperatures, when burning combustible materials. All this is carbon dioxide of biogenic origin.