Chloroplast Calvin cycle

FIGURE 22.27 Light-induced pH changes in chloroplast compartments. Illumination of chloroplasts leads to proton pumping and pH changes in the chloroplast, such that the pH within the thylakoid space falls and the pH of the stroma rises. These pH changes modulate the activity of key Calvin cycle enzymes. 

The pathway is depicted in Fig. 35. The Calvin cycle, taking place in the chloroplast stroma of plants, is a primary source of carbon for all organisms and of central importance for a variety of biotechnological applications. The set of reactions, summarized in Table VIII, is adopted from the earlier models of... 

Photosynthesis in vascular plants takes place in chloroplasts. In the C02-assimilating reactions (the Calvin cycle), ATP and NADPH are used to reduce C02 to triose phosphates. These reactions occur in three stages the fixation reaction itself, catalyzed by rubisco reduction of the resulting 3-phosphoglycerate to glyceraldehyde 3-phosphate and regeneration of ribulose 1,5-bisphosphate from triose phosphates. 

Triose phosphates produced by the Calvin cycle in bright sunlight, as we have noted, may be stored temporarily in the chloroplast as starch, or converted to sucrose and exported to nonphotosynthetic parts of the plant, or both. The balance between the two processes is tightly regulated, and both must be coordinated with the rate of carbon fixation. Five-sixths of the triose... 

Calvin cycle 752 plastids 752 chloroplast 752 amyloplast 752 carbon-fixation reaction 753 ribulose 1,5-bisphosphate 753 3-phosphoglycerate 753 pentose phosphate pathway 753 reductive pentose phosphate pathway 753 C3 plants 754 ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) 754 rubisco activase 757... 

The photosynthetic process in green plants occurs in subcellular organelles called chloroplasts. These organelles resemble mitochondria they have two outer membranes and a folded inner membrane called the thy-lakoid. The apparatus for photosynthesis, including the chlorophyll reaction centers and electron carriers, is in the thylakoid membrane. The chemical reactions of the Calvin cycle take place in the stroma, the region around the thylakoid membrane. 

Much of the glyceraldehyde 3-phosphate produced by the Calvin cycle in chloroplasts is exported to the cytosol and used to produce the disaccharide, sucrose. First the glyceraldehyde 3-phosphate is converted to fructose 6-phos-phate and glucose 1-phosphate. The chemical reactions involved are essentially a reversal of glycolysis (see Topic J3). The glucose 1-phosphate is then converted to UDP-glucose and this reacts with fructose 6-phosphate to synthesize sucrose 6-phosphate ... 

In contrast to the previous results, Weigel (1985 a, b) reported that in mesophyll protoplasts of Valerianella locusta and in intact chloroplasts of Spinacea oleraceae cadmium affects photosynthesis by inhibition of several reaction steps of the Calvin cycle and not by interaction with the electron transport or photophosphorylation (cf. section on photosynthetic C02 fixation). 

Thioredoxin in Regulation of Calvin Cycle Enzymes Motohashi and colleagues used thioredoxin as a hook to fish out from plant extracts the proteins that are activated by thioredoxin. To do this, they prepared a mutant thioredoxin in which one of the reactive Cys residues was replaced with a Ser. Explain why this modification was necessary for their experiments. [Motohashi, K., Kondoh, A., Stumpp, M.T., Hisabori, T. (2001) Comprehensive survey of proteins targeted by chloroplast thioredoxin. Proc. Natl. Acad. Sci. USA 98, 11,224-11,229.]... 

The C4 cycle can be viewed as an ATP-dependent C02 pump that delivers C02 from the mesophyll cells to the bundle-sheath cells, thereby suppressing photorespiration (Hatch and Osmond, 1976). The development of the C4 syndrome has resulted in considerable modifications of inter- and intracellular transport processes. Perhaps the most striking development with regard to the formation of assimilates is that sucrose and starch formation are not only compartmented within cells, but in C4 plants also may be largely compartmented between mesophyll and bundle-sheath cells. This has been achieved together with a profound alteration of the Benson-Calvin cycle function, in that 3PGA reduction is shared between the bundle-sheath and mesophyll chloroplasts in all the C4 subtypes. Moreover, since C4 plants are polyphyletic in origin, several different metabolic and structural answers have arisen in response to the same problem of how to concentrate C02. C4 plants have three distinct mechanisms based on decarboxylation by NADP+-malic enzyme, by NAD+-malic enzyme, or by phosphoenolpy-ruvate (PEP) carboxykinase in the bundle-sheath (Hatch and Osmond, 1976). 

Calvin cycle (aka Calvin-Benson Cycle or Carbon Fixation) Series of biochemical, enzyme-mediated reactions during which atmospheric carbon dioxide is reduced and incorporated into organic molecules, eventually some of this forms sugars. In eukaryotes, this occurs in the stroma of the chloroplast. 

The Q pathway for the transport of CO2 starts in a mesophyll cell with the condensation of CO2 and phosphoenolpyruvate to form oxaloacetate, in a reaction catalyzed by phosphoenolpyruvate carboxylase. In some species, oxaloacetate is converted into malate by an NADP+-linked malate dehydrogenase. Malate goes into the bundle-sheath cell and is oxidatively decarboxylated within the chloroplasts by an NADP+-linked malate dehydrogenase. The released CO2 enters the Calvin cycle in the usual way by condensing with ribulose 1,5-bisphosphate. Pyruvate formed in this decarboxylation reaction returns to the mesophyll cell. Finally, phosphoenolpyruvate is formed from pyruvate by pyruvate-Pi dikinase. 

Chloroplast ferredoxin is a small water soluble protein M W 000) containing an Fe-S center [245]. Its midpoint potential ( — 0.42 V [246]) is suitable for acting as an electron acceptor from the PSI Fe-S secondary acceptors (Centers A and B) and as a donor for a variety of functions on the thylakoid membrane surface and in the stroma. Due to its hydrophylicity and its abundance in the stromal space, ferredoxin is generally considered as a diffusable reductant not only for photosynthetic non-cyclic and cyclic electron flow, but also for such processes as nitrite and sulphite reduction, fatty acid desaturation, N2 assimilation and regulation of the Calvin cycle enzyme through the thioredoxin system [245]. Its possible role in cyclic electron flow around PSI has already been discussed. The mobility of ferredoxin along the membrane plane could be an essential feature of this electron transfer process the actual electron acceptor for this function and the pathway of electron to plastoquinone is, however, still undefined. 

Chloroplasts Light capturing processes and electron transport oxidative phosphorylation for photosynthesis Calvin cycle (dark reactions of photosynthesis). 

The dark reaction takes place in the stroma within the chloroplast, and converts CO2 to sugar. This reaction doesn t directly need light in order to occur, but it does need the products of the light reaction (ATP and another chemical called NADPH). The dark reaction involves a cycle called the Calvin cycle in which CO2 and energy from ATP are used to form sugar. Actually, notice that the first... 

CO2 are fixed into sugar (glucose) and mediated by the enzyme rubisco (ribulose-l-5-biphosphate carboxylase). It occurs in the stroma of chloroplasts. The Calvin cycle is also known as the dark reaction, as opposed to the first-stage light reactions. 

The utilization of carbon dioxide by ATP and NADPH occurs in the chloroplast matrix, (outside the thylakoid lumen). A series of reactions assimilates carbon dioxide (Fig. 2.6), the Calvin cycle or dark reaction, and generates fructose 6-phosphate. Fructose 6-phosphate is the immediate precursor of glucose 6-phosphate for the synthesis of starch in the... 

Fig.2.10 Phosphoglycerate utilization, (a) During the day. Photosynthesis in the chloroplast makes starch until there is no more room. The Calvin cycle continues to make triose phosphate, which exits the chloroplast in exchange for organic phosphate (Pi) entering the chloroplast and converting ADP to ATP. In the cytosol, the triose phosphate is mostly converted to sucrose but also to small amounts of other compounds such as amino acids for transport throughout the plant, (b) During the night. Phosphorylase is activated and it breaks up the starch to glucose 6-phosphate from which triose phosphate is made. The triose phosphate is exchanged for Pi. The Pi is a substrate for phosphorylase and keeps it active. Once in the cytosol, the triose phosphate is transferred mostly to mitochondria for respiration...

The dark reaction (Calvin cycle) uses the NADPH and ATP to make glyceraldehyde 3-phosphate (triose phosphate), which is metabolized initially to starch, sucrose, and cellulose. Starch and sucrose are the major plant storage products. Starch is synthesized from ADP-glucose in the chloroplast, sucrose from fructose 6-phosphate and UDP-glucose in the leaf cytosol. 

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