Apart from primary photosynthetic CO2 fixation by RuDP carboxylase (“C3 photosynthesis”), photosynthetic mechanisms are observed among higher plants by which primary CO2 fixation is catalyzed by PEP carboxylase with formation of oxalo-acetic acid (OAA). OAA is reduced to malate and, subsequently, malate is decarboxylated, and the resulting CO2 is refixed by RuDP carboxylase. Malate synthesis and malate decarboxylation with subsequent refixation of CO2 may be separated spatially in particularly differentiated tissues of a leaf (“C4 photosynthesis”) or they may be separated by time, that is, they occur in a given cell during different phases of a rhythm (Crassulacean acid metabolism, “CAM”) (see Kluge, this volume Part 5: A).