Synthesis, Export and Partitioning of the End Products of Photosynthesis

Abstract

Chloroplasts are the metabolic factories of plant cells. They are the site of various biosynthetic pathways such as carbon, nitrogen and sulfur assimilation, fatty acid biosynthesis, amino acid biosynthesis, isoprenoid biosynthesis and secondary metabolite biosynthesis, to mention just a few. Many of these metabolic pathways require the rapid and controlled exchange of precursors, intermediates and end products between the chloroplast stroma and the surrounding cytosol. However, two lipid bilayer membranes, the inner and outer chloroplast envelope membranes, form a permeation barrier between plastid stroma and the cytosol. Transporters in the inner plastid envelope membrane catalyze the efficient and specific exchange of metabolites between plastid stroma and other cellular compartments, thereby integrating plastidal metabolism into the metabolic networks in plant cells. Metabolite transporters not only catalyze the flux of metabolites between compartments, they also represent information pathways that communicate the metabolic status between compartments. A classic example is the coordination of sucrose biosynthesis in the cytosol with starch biosynthesis in the stroma by the triosephosphate/phosphate translocator (Flügge, 1995; Flügge, 1999).