Phosphoenol pyruvate (PEP) is the primary $CO

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Every photosynthetic plant moves according to the Calvin cycle, but in some plants, there is a primary stage to the Calvin Cycle called the C4 pathway. Plants in tropical desert regions commonly move accordingly to the C4 pathway. Here, a 4-carbon compound called oxaloacetic acid (OAA) is the first product of carbon fixation. Such plants are special and have specific adaptations as well.
The C4 pathway commences with a molecule called phosphoenolpyruvate (PEP) which is a three-carbon molecule. This is the primary CO₂ acceptor and the carboxylation, that takes place with the help of an enzyme called PEP carboxylase. They yield a 4-C molecule known as oxaloacetic acid (OAA).
In due course, it is converted into another 4-carbon compound called malic acid. Later, they are shifted from mesophyll cells to bundle sheath cells. Here, OAA is crumbled to yield carbon dioxide and a 3-C molecule.
The CO₂ thus formed, is utilized in the Calvin cycle, whereas the 3-C molecule is shifted back to mesophyll cells for regeneration of PEP.
Sugarcane, corn, and shrubs are some examples of plants that follow the C4 pathway. Calvin pathway is a very common pathway in both C3 plants and C4 plants, but it takes place only in the mesophyll cells of the C3 Plants and not in the C4 Plants.