Chapter 8 Carbon dioxide assimilation

Abstract

Publisher Summary This chapter discusses the carbon dioxide assimilation in plants. All oxygenic (oxygen-evolving) organisms from the simplest prokaryotic cyanobacteria to the most complicated land plants have a common pathway for the reduction of CO 2 to sugar phosphates. This pathway is known as the reductive pentose phosphate (RPP), Calvin-Benson or C 3 cycle. Although the RPP cycle is the fundamental carboxylating mechanism, a number of plants have evolved adaptations in which CO 2 is first fixed by a supplementary pathway and then released in the cells in which the RPP cycle operates. One of these supplementary pathways, the C 4 pathway, involves special leaf anatomy and a division of biochemical labor between cell types. Plants endowed with this pathway, through greater efficiency, are able to flourish under conditions of high light intensity and elevated temperatures. A second supplementary pathway was first found in species of the Crassulaceae and is called Crassulacean acid metabolism (CAM). These plants are often found in dry areas and fix CO 2 at night into C 4 acids. During the day, the leaves can close their stomata to conserve water, while CO 2 released from the C 4 acids is converted to sugar phosphates by the RPP cycle using absorbed light energy. CO 2 fixation is also found in many bacteria, both photosynthetic and non-photosynthetic. The purple sulfur and purple nonsulfur bacteria employ the RPP cycle as do plants. The photosynthetic green bacteria, however, use a group of ferredoxin-linked carboxylases in a pathway known as the reductive carboxylic acid cycle.