The element carbon cycles in its different forms through various earth systems like atmosphere, biosphere, water bodies, and geosphere. The exchange or transfer of carbon from one earth system to another is referred to as carbon cycle (also referred to as a "biogeochemical" cycle). Carbon cycle is quite complex. All living things (plants and animals) are made up of carbon. Dead and decaying matter (dead animal and plant bodies, faecal matter, etc.) also contain carbon. Carbon combines with a gas called oxygen in the atmosphere to form carbon dioxide (CO2). This can happen, for example, when organic matter is burned or oxidised. Burning of fossil fuels like coal and petroleum (because of consumption in industries, automobiles, etc.) also leads to the release of carbon dioxide in the atmosphere. Plants take carbon dioxide during photosynthesis and release it during respiration. Other living organisms, like humans, inhale oxygen and exhale carbon dioxide during respiration. The amount of carbon dioxide in the atmosphere is approximately 0.034%. Even slightest changes in this number can cause massive changes on earth. And as we know, this is actually happening mainly due to large-scale industrialization and excessive use of fossil fuels. Carbon dioxide is a green house gas and can trap heat. Increased levels of CO2 are, thus, causing global warming of earth. Carbon is also sometimes present in the atmosphere as carbon monoxide (CO). For instance, burning of plastics can release CO in the atmosphere. Carbon monoxide is a very harmful gas and can even cause death if inhaled in excess amount. Carbon is also present in water bodies as dissolved carbon dioxide, which is used by aquatic plants to carry out photosynthesis.