Primary Photosynthate Production:Physiology and Metabolism

This chapter explores the major photosynthetic processes in plants which lead to the production of triose phosphate from CO₂, together with some aspects of their regulation and physiology, highlighting some of the areas of metabolism which are benefiting from a molecular approach. The Calvin cycle can be described as the fixation of three molecules of CO₂ into a three-carbon sugar phosphate, triose-P, with the incorporation of one molecule of phosphate. Rubisco is a bifunctional enzyme in that it catalyzes both the carboxylation and the oxygenation of ribulose-1,5-bisphosphate (RuBP). Oxygenation of RuBP leads to the production of one molecule of glycerate-3-P and one of glycollate-2-P. The photorespiratory pathway involves three subcellular compartments, the chloroplasts, peroxisomes, and mitochondria. The rate at which carbon is withdrawn from the Benson-Calvin cycle is controlled by the rate at which starch synthesis and sucrose synthesis operate, and the link between events in the chloroplast and sucrose synthesis in the cytosol is provided by the phosphate translocator.