Carbon Cycle
Carbon constitutes 49% of dry weight of organisms. Out of total quantity of global carbon, 71% is formed dissolved in oceans. Thus ocean is a reservoir of carbon. Fossil fuels also represent a reservoir of carbon. Carbon cycle is very simple. The key events here are the complementary reactions of respiration and photosynthesis. Respiration takes carbohydrates and oxygen and releases carbon dioxide, water and energy. Photosynthesis takes carbon dioxide and water and produces carbohydrates and oxygen. Photosynthesis takes energy from the sun and stores it in the carbon-carbon bonds of carbohydrates; while respiration release that energy.
The main reservoirs for carbon dioxide are in the oceans and in rock. Carbo dioxide dissolves readily in water. Once there, it may precipitate (fall out of solution) as solid rock known as calcium carbonate (limestone). Corals and algae encourage this reaction and build up limestone reefs in the process.
On land and in the water, plants take up carbon dioxide and convert it also carbohydrates through photosynthesis. This carbon in the plants now has three possible fates. It can be liberated to the atmosphere by the plant through respiration, it can be consumed by an animal, or it can remain as it when the plant dies.
Animals obtain all their carbon through their food, thus, all carbon in biological system comes from plants (autotrophs). In the animal,(consumers) the carbon also has the same three possible fates. Carbon from plants or animals that is released to the atmosphere through respiration will either be taken up by a plant in photosynthesis or dissolved in the oceans. When an animal or a plant dies, there are two possible fates of carbon, it can either be released to the atmosphere by decomposers, or it can be buried intact and ultimately form coal, oil or natural gas (fossil fuels). The fossil fuels can be mined and burned in the future; releasing carbon dioxide to the atmosphere or the carbon in limestone and other sediments can only be released to the atmosphere. When they are subducted and brought to volcanoes, or when they are pushed to the surface and slowly weathered away. Humans have a great impact on the carbon cycle because when we burn fossil furls we release excess carbon dioxide into the atmosphere.