Figure 3.12 Alternative photosynthetic pathways. (a) C3 plants conduct photosynthesis in mesophyll cells during the day. (b) C4 plants separate the steps of photosynthesis in space. The initial CO2 assimilation step occurs in the mesophyll cells and the remaining steps occur in the bundle sheath cells. (c) CAM plants separate the steps of photosynthesis in time. Assimilation of CO2 happens at night and the remaining steps occur during the day.