The below information was taken from http://www.liquidgasplants.com/co2.html
Carbon dioxide is a chemical compound composed of two oxygen atoms bonded covalently to a single carbon atom. Carbon dioxide is produced by all microorganisms, animals, plants and fungi during respiration and is used by plants during photosynthesis to make sugars which may either be consumed again in respiration or used as the raw material for plant growth. Thus, carbon dioxide is a major component of the carbon cycle. Carbon dioxide is generated as a by-product of the burning of fossil fuels or vegetable matter, among other chemical procedures. Inorganic carbon dioxide is the output of volcanoes and other geothermal processes like hot springs etc.
Production of CO2