-C4 is the alternative pathway of Calvin cycle (C3 cycle) taking place during the dark phase (light-independent stage) of photosynthesis. In the C4 cycle the first stable compound is 4 carbon compound, namely oxaloacetic acid. Hence it is called C4 cycle. It is a process of carbon dioxide fixation.
-C3 plants have the disadvantage that in hot dry conditions their photosynthetic efficiency suffers because of a process called photorespiration. When the CO2 concentration in the chloroplasts drops below about 50 ppm, the catalyst rubisco that helps to fix carbon begins to fix oxygen instead.
-The problem of photorespiration is overcome in C4 plants by a two-stage strategy that keeps CO2 high and oxygen low in the chloroplast where the Calvin cycle operates.
