Metabolite Carriers of Chloroplasts

Abstract

The chloroplast stroma represents a separate metabolic compartment within the plant cell. The envelope which surrounds it consists of two membranes. The outer one has been shown to be freely permeable to small molecules, such as nucleotides and intermediates of the Calvin cycle (Heldt and Sauer, 1971). In this respect the outer membrane of the chloroplast is very similar to the outer mitochondrial membrane (see Chap. II, 6). The functional barrier between the chloroplast stroma and the cytoplasm is the inner membrane. The metabolic function of the chloroplast is to fix CO₂. The main products of CO₂ fixation delivered by the chloroplast to the plant cell are triosephosphates (see Chap. II, 2). Therefore, CO₂ fixation occurring in the chloroplast stroma involves the uptake of CO₂ and inorganic phosphate and the release of triosephosphates. Whereas the inner membrane of the envelope appears to be freely permeable to CO₂ (Werdan et al., 1972), it is impermeable to ions. This points to the physiological necessity of a specific transport of inorganic phosphate from the cytoplasm into the chloroplast stroma and of triosephosphates in the other direction.