4.8—
Relationship of Chloroplast to Cytoplasm

Chloroplasts are surrounded by an envelope which consists of two membranes, the outer and inner membranes. The inner membrane is a permeability barrier to the diffusion of many metabolites from the chloroplast to the cytoplasm and vice versa. For example, the inner membrane is practically impermeable to sucrose, which is a major product of photosynthesis. It is very slowly permeable to ATP and impermeable to pyridine nucleotides and hexosephosphates (Heber, 1974). How then are reduced carbon compounds transported into the cyto-

plasm and how is photosynthetic ATP made available for many other cell processes, such as protein synthesis?

Heldt and his colleagues (1975) have postulated that specific carriers or translocators are involved in the transfer of certain metabolites across the inner membrane of the chloroplast envelope (Heber, 1974). For example, the so-called phosphate trarnslocator facilitates the transfer of 3-phosphoglycerate (PGA), dihydroxyacetone phosphate (DHAP), glyceraldehyde-3-phosphate (GAP) and inorganic phosphate across the inner membrane in a competitive fashion. Figure 4.7 depicts the transfer of carbon and phosphate from the

Figure 4.7
Movement of carbon and phosphate energy from the chloroplasts to
the cytoplasm. ATP is formed in the cytoplasm as the result of
the transfer of dihydroxyacetone phosphate (DHAP) and
oxaloacetate (OAA). Sucrose is formed from DHAP.
(From Heber, 1974.)

chloroplast. It seems likely that sucrose is formed outside the chloroplast although this is not established. ATP and NADPH produced inside the chloroplast by photosynthetic electron flow are used in the conversion of PGA to DHAP. DHAP is exported from the chloroplast to the cytoplasm, where it is converted into sucrose or used for the generation of reducing power and ATP. The PGA can then re-enter the chloroplast.

Reducing power is transported across the inner membrane by a counter exchange of malate and oxaloacetate. NAD-dependent malate dehydrogenase occurs both in the cytoplasm and the chloroplast, while NADP-dependent dehydrogenase occurs in the chloroplast and is activated by light. Heldt et al., (1975) have postulated that the counter exchange of malate and oxaloacetate is facilitated by a dicarboxylate translocator.

Rapid transfer of metabolites between chloroplast and cytoplasm is crucial for the operation of the C₄ -dicarboxylic pathway of photosynthesis. In plants

with the C₄ -pathway, CO₂ is fixed initially by reaction with phosphoenolpyruvate in the mesophyll cells. The resulting oxaloacetate is reduced to malate or transaminated to aspartate. Depending on the species of C₄ plant, one or other of these acids is transported to the bundle sheath cells, where the C₄ -acid is decarboxylated to give CO₂ and a 3-carbon compound. The CO₂ is then refixed by the Calvin carboxylation cycle (Hatch & Slack, 1970).