Most plants such as soybeans are called C3 species because they go through photosynthesis by using C3 carbon fixation, where CO2 is first incorporated into a 3-carbon compound called PGA, by using the enzyme RUBISCO for carbon fixation and Calvin cycle. Other plants, like pigweeds are called C4 plants because Carbon is first incorporated into a 4-carbon compound activating the PEP enzyme carboxylase for carbon fixation and RUBISCO for the Calvin cycle. The difference between the C3 and C4 species anatomy is in a C3 plant, photosynthesis occurs in a single chloroplast of the mesophyll cell (on the surface of the leaf) whereas in a C4 plant, C4 pathway separates carbon fixation from the Calvin cycle in an anatomic structure called Kranz anatomy, consisting of the mesophyll cell and the bundle sheath cells. The difference between the species is that C3 is inefficient in carbon fixation in comparison to C4 plant because both Carbon fixation and the Calvin cycle take place in the mesophyll cells, which is packed with RUBISCO and RUBISCO has a strong affinity for O2 than CO2. This is a problem because increase O2 causes increase photorespiration.