Well in the first step of the cycle it is called carbon fixation. But what is carbon fixation? C4 carbon fixation is one of three biochemical mechanisms, along with C3 and CAM Photosynthesis, functioning in land plants to "fix" carbon dioxide (binding the gaseous molecules to dissolved compounds inside the plant) for sugar production through photosynthesis. Along with CAM photosynthesis, C4 fixation is considered an advancement over the simpler and more ancient C3 carbon fixation mechanism operating in most plants. Both mechanisms overcome the tendency of RuBiSco (the first enzyme in the Calvin cycle) to phosphire, or waste energy by using oxygen to break down carbon compounds to CO2. However C4 fixation requires more energy input than C3 in the form of ATP. C4 plants separate RuBiScO from atmospheric oxygen, fixing carbon in the mesophyll cells and using  oxaloacate and malate to ferry the fixed carbon to RuBisCO and the rest of the Calvin cycle enzymes isolated in the bundle-sheathe cells. The intermediate compounds both contain four carbon atoms, hence the name C4.
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