Photosynthetic Carbohydrate Synthesis:- Synthesis of Each Triose Phosphate from CO2 Requires Six NADPH and Nine ATP
The net result of three turns of the Calvin cycle is the conversion of three molecules of CO2 and one molecule of phosphate to a molecule of triose phosphate. The stoichiometry of the overall path from CO2 to triose phosphate, with regeneration of ribulose 1,5-bisphosphate, is shown in Figure 1. Three molecules of ribulose 1,5-bisphosphate (a total of 15 carbons) condense with three CO2 (3 carbons) to form six molecules of 3-phos phoglycerate (18 carbons). These six molecules of 3 phosphoglycerate are reduced to six molecules of glyceraldehyde 3-phosphate (which is in equilibrium with dihydroxyacetone phosphate), with the expenditure of six ATP (in the synthesis of 1,3-bisphosphoglycerate) and six NADPH (in the reduction of 1,3-bisphospho glycerate to glyceraldehyde 3-phosphate). The isozyme of glyceraldehyde 3-phosphate dehydrogenase present in chloroplasts can use NADP as its electron carrier and normally functions in the direction of 1,3-bisphospho glycerate reduction. The cytosolic isozyme uses NAD, as does the glycolytic enzyme of animals and other eukaryotes, and in the dark this isozyme acts in glycolysis to oxidize glyceraldehyde 3-phosphate. Both glyceraldehyde 3-phosphate dehydrogenase isozymes, like all enzymes, catalyze the reaction in both directions.
One molecule of glyceraldehyde 3-phosphate is the net product of the carbon assimilation pathway. The other five triose phosphate molecules (15 carbons) are rearranged in steps 1 to 9 to form three molecules of ribulose 1,5-bisphosphate (15 car bons). The last step in this conversion requires one ATP per ribulose 1,5-bisphosphate, or a total of three ATP. Thus, in summary, for every molecule of triose phosphate produced by photosynthetic CO2 assimilation, six NADPH and nine ATP are required. NADPH and ATP are produced in the light dependent reactions of photosynthesis in about the same ratio (2:3) as they are consumed in the Calvin cycle. Nine ATP molecules are converted to ADP and phosphate in the generation of a molecule of triose phosphate; eight of the phosphates are released as Pi and combined with eight ADP to regenerate ATP. The ninth phosphate is incorporated into the triose phosphate it self. To convert the ninth ADP to ATP, a molecule of Pi must be imported from the cytosol, as we shall see. In the dark, the production of ATP and NADPH by photophosphorylation, and the incorporation of CO2 into triose phosphate (by the so-called dark reactions), cease. The “dark reactions” of photosynthesis were so named to distinguish them from the primary light driven reactions of electron transfer to NADP and syn thesis of ATP, described in Chapter 19. They do not, in
FIGURE 1 Stoichiometry of CO2 assimilation in the Calvin cycle. For every three CO2 molecules fixed, one molecule of triose phosphate (glyceraldehyde 3-phosphate) is produced and nine ATP and six NADPH are consumed.
FIGURE 2 The Pi–triose phosphate antiport system of the inner chloroplast membrane. This transporter facilitates the exchange of cytosolic Pi for stromal dihydroxyacetone phosphate. The products of photosynthetic carbon assimilation are thus moved into the cytosol where they serve as a starting point for sucrose biosynthesis, and Pi required for photophosphorylation is moved into the stroma. This same antiporter can transport 3-phosphoglycerate and acts in the shuttle for exporting ATP and reducing equivalents .
fact, occur at significant rates in the dark and are thus more appropriately called the carbon-assimilation re actions. Later in this section we describe the regulatory mechanisms that turn on carbon assimilation in the light and turn it off in the dark. The chloroplast stroma contains all the enzymes necessary to convert the triose phosphates produced by CO2 assimilation (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate) to starch, which is temporarily stored in the chloroplast as insoluble granules. Aldolase condenses the trioses to fructose 1,6-bisphosphate; fructose 1,6-bisphosphatase produces fructose 6 phosphate; phosphohexose isomerase yields glucose 6 phosphate; and phosphoglucomutase produces glucose 1-phosphate, the starting material for starch synthesis . All the reactions of the Calvin cycle except those catalyzed by rubisco, sedoheptulose 1,7-bisphospha tase, and ribulose 5-phosphate kinase also take place in animal tissues. Lacking these three enzymes, animals cannot carry out net conversion of CO2 to glucose.
