This is a crude analogy but it may help some understand what is occurring when plants fix carbon. With water condensation, water is changing from a gaseous phase to a solid phase. Then if it freezes, it's final form is solid. When carbon is fixed, carbon dioxide is being transformed from a gaseous form to a sold form through the energy derived from photosynthesis. The fixation of carbon is both a physical and chemical change. The gas CO2 is converted into a sugar, a solid