Phosphate antiport systems

FIGURE 20-15 The Pi-triose phosphate antiport system of the inner chloroplast membrane. This transporter facilitates the exchange of cytosolic P for stromal dihydroxyacetone phosphate. The products of photosynthetic carbon assimilation are thus moved into the cytosol... 

FIGURE 20-15 The P -triose phosphate antiport system of the inner... 

Fatty acids are generated cytoplasmically while acetyl-CoA is made in the mitochondrion by pyruvate dehydrogenase.This implies that a shuttle system must exist to get the acetyl-CoA or its equivalent out of the mitochondrion. The shuttle system operates in the following way Acetyl-CoA is first converted to citrate by citrate synthase in the TCA-cycle reaction. Then citrate is transferred out of the mitochondrion by either of two carriers, driven by the electroos-motic gradient either a citrate/phosphate antiport or a citrate/malate antiport as shown in Figure 2-2. 

Most ATP-requiring reactions occur in the cytosol and produce ADP and orthophosphate. Since most ATP is formed by mitochondrial oxidative phosphorylation (in appropriate cells) from ADP and orthophosphate, these molecules must traverse the inner membrane. ATP and ADP are translocated by the specific adenine-nucleotide-transport system. This antiport system is widely distributed in the membrane and exchanges one mitochondrial ATP for one cytoplasmic ADP. The carrier selectively binds and transports ADP inwards and ATP outwards. The phosphate enters the mitochondrion via a different antiport system, the phosphate carrier, which exchanges it for a hydroxyl ion. 

A FIGURE 8-28 The phosphate and ATP/ADP transport system in the inner mitochondrial membrane. The coordinated action of two antiporters (purple and green) results in the uptake of one ADP - and one HP04 - in exchange for one ATP -, powered by the outward translocation of one proton during electron transport. The outer membrane is not shown here because it is permeable to molecules smaller than 5000 Da. 

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