The cycling of carbon occurs through the atmosphere, ocean, fossil fuels, and the living and dead organisms. The amount of CO2 in the atmosphere is regulated by oceanic reserves and fossil fuels. Of the total quantity of global carbon, about 71% of carbon is found dissolved in oceans, while the atmosphere only contains about 1% of total global carbon. Carbon constitutes about 49% of the dry weight of living organisms and is next only to water. The majority of carbon in the atmosphere absorbed by plants is fixed during photosynthesis. An estimated 4 × 1013 kg of carbon is fixed in the biosphere annually through photosynthesis, of which a certain amount comes back to the atmosphere in the form of CO2 through respiratory activities.