1. an organism which can photosynthesize (such as a plant or some bacteria) will absorb carbon dioxide (CO2) from the air and incorporate it into its body or turn it into organic matter.
  2. other organisms which cannot photosynthesize will eat the organic matter, or the photosynthesizing organism, and release CO2 gas as a waste product back into the air.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.