The final product of metabolic pathways based on carbon is carbon dioxide which is released into the atmosphere. To close the carbon cycle it is neccessary to feed the carbon dioxide back to the food chain. It comprises the starting point of any food chain.
Some bacteria contain an enzyme consisting of two identical subunits.
In higher organisms capable of photosynthesis a larger enzyme with eight small and large subunits each is found. The larger of the subunits is catalytically active. The structures of enzymes from Synechococcus , tobacco and spinach were elucidated. To the right there is a side view, below is shown a view along the four-fold symmetry axis. The X-ray diffraction data of this complex gave clues to the reaction mechanism and it's stereo chemistry.