Figure 1.

C₄ plants, cyanobacteria, and algae concentrate CO₂ around the enzyme Rubisco using different strategies. (a) In the mesophyll cells of C₄ plants, HCO₃⁻ is added to the three-carbon molecule phosphoenolpyruvate (PEP) by the enzyme PEP carboxylase, generating the four-carbon molecule malate or aspartate. The malate or aspartate is then decarboxylated to form the three-carbon molecule pyruvate and release CO₂ in bundle sheath cells, where Rubisco is expressed. The biophysical CCMs of (b) cyanobacteria and (c) algae actively transport HCO₃⁻ into the cell, then convert it to CO₂ at a site of clustered Rubisco, which is either the (b) carboxysome or (c) pyrenoid