Carbon Dioxide Metabolism in Photosynthesis

Abstract

All oxygenic organisms from the simplest prokaryotic cyanobacteria to the most complicated land plants reduce atmospheric CO2 to sugar phosphates by a pathway known as the reductive pentose phosphate (RPP) pathway. This pathway located in chloroplast is the primary carboxylating mechanism in plants and is comprised of thirteen reactions catalyzed by eleven enzymes distributed in three distinct phases namely, carboxylation, reduction and the regeneration. Ribulose-1, 5-bis phosphate carboxylase/oxygenase (Rubisco) catalyzes the primary carbon fixation reaction. This enzyme also catalyzes the fixation of oxygen in a process known as photorespiration, which competes directly with fixation of CO2. At air levels of CO2 and O2, for every three CO2 molecules fixed, approximately one molecule of O2 is fixed by Rubisco. This competition between CO2 and O2 and energy costs associated with photorespiration largely determine the efficiency of C3 photosynthesis in air. Although the RPP pathway is the fundamental carboxylating mechanism, a number of plants have evolved adaptations in which CO2 is first fixed by a supplementary pathway and then released in the cells in which the RPP cycle operates. One of the supplementary pathways, the C4 pathway involves special leaf anatomy, i.e. the Kranz anatomy. In this pathway, CO2 is initially fixed in the cytosol of mesophyll cells by the enzyme phosphoenol pyruvate (PEP) carboxylase to form four carbon compound (malate or aspartate) which is then translocated to the bundle sheath cells and gets decarboxylated in the chloroplasts. The CO2 produced is then refixed by Rubisco. Based on the mechanism of decarboxylation, the crops possessing this pathway have been subgrouped into NAD-ME type, NADP-ME type and PCK-type. Plants endorsed with this pathway have no or very little photorespiration, greater efficiency and are able to grow under conditions of high light intensity and elevated temperature. A second supplementary pathway found in species of the Crassulaceae is called Crassulacean acid metabolism (CAM). The plants possessing this pathway fix CO2 in the night into C4 acids. During the day, CO2 released from decarboxylation of C4 acids is converted to sugar phosphates by RPP cycle.