In C4 cycle, CO2 taken from the atmosphere is accepted by phosphoenolpyruvic acid (PEP) present in the chloroplasts of mesophyll cells of these leaves, leading to the formation of a 4-C compound, oxaloacetic acid (OAA), which, in turn, is converted to another 4-C acid, the malic acid which enters into the chloroplasts of bundle sheath cells and there undergoes oxidative decarboxylation yielding pyruvic acid (a 3-C compound) and CO2. CO2 released in bundle sheath cells reacts with Ribulose-1,5-biphosphate (RuBP) already present in the chloroplasts of bundle sheath cells and thus Calvin cycle starts from here. Pyruvic acid then re-enters mesophyll cells and regenerates PEP (phosphoenol pyruvic acid). CO2 after reacting with RuBP gives rise to sugars and other carbohydrates. Mesophyll cells have PEP carboxylase and pyruvate orthophosphate dikinase enzyme while the bundle sheath cells have decarboxylase and complete enzymes of the Calvin cycle. In C4 plants, there are 2 carboxylation reactions, primarily in mesophyll chloroplast followed by in bundle sheath chloroplast and hence we can say that the two times CO2 fixation in C4 plants is separated by space.
Crassulacean acid metabolism plants (CAM plants): This dark CO2 fixation pathway proposed by Ting (1971). It occurs in succulent or fleshy plants like Cactus, Sedum, Opuntia, orchid, pineapple, and Bryophyllum helping them to continue photosynthesis under extremely dry conditions. The stomata of succulent plants remain closed during the day and open during the night to avoid water loss (Scotactive stomata). They store CO2 during the night in the form of malic acid in the presence of enzyme PEP carboxylase. The CO2 stored during the night is used in the Calvin cycle during day time. Succulents refix CO2 released during respiration and use it during photosynthesis. This diurnal change in acidity in the plant body was first discovered in crassulacean plants e.g. Bryophyllum. So it is called as crassulacean acid metabolism. In CAM plants, the two time CO2 fixation is separated by time i.e. in the night time, initial CO2 fixation results into formation of organic acid which undergoes decarboxylation in the daytime and is followed by the second time carboxylation.