Carbon fixation is a process found in photosynthesis in autotrophic plants. It is when the three carbon dioxide molecules taken in each time there is a turn in the Calvin cycle in the dark reactions of photosynthesis. These three carbon dioxide molecules undergo the chemical reactions in the Calvin cycle.

The end result is that there is a net output of a single G3P; and in two turns of the Calvin cycle when two G3P are output, then they are combined to form organic compounds, specifically monosaccharides.