Dark reaction cycle

Fig. 6.3 The Calvin cycle or the dark reactions of photosynthesis see Cooper and also Stryer in Further Reading.

Because photosystem 11 and the cytochrome b/f complex release protons from reduced plastoquinone into the lumen (via a Q. cycle), photosynthetic electron transport establishes an electrochemical gradient across the thylakoid membrane (see p. 126), which is used for ATP synthesis by an ATP synthase. ATP and NADPH+H", which are both needed for the dark reactions, are formed in the stroma. 

On p. 407, the initial step of the dark reactions in plant photosynthesis (in the Calvin cycle) is shown at the top left. 

The dark reaction of photosynthesis (Figure 2.3) is so-called as it does not require light to proceed. It does, however, require the products of the light reaction to operate and it will not, therefore, take place in the absence of light. It was discovered by Calvin and is often known as the Calvin cycle. 

The dark reaction involves the fixation of carbon dioxide to form carbohydrates. The ATP and the NADPH produced in the light reaction drive this carbon fixation. It might be thought that the interruption of the Calvin cycle would also produce effective herbicides but this is not the case. There are two reasons why. First, the reaction is not an energetic reaction whose interruption would lead to the destruction of cellular components and second, the enzymes involved in the process are present in very high amounts. If an enzyme is to be targeted as a key step in the metabolism of a plant, it is important that it is present in small amounts and that it is not turned over very quickly. If an enzyme is abundant,... 

Figure 2.3 The dark reaction (the Calvin cycle) of photosynthesis...

The light reactions illustrated in Figure E9.1 are accompanied by a sequence of dark reactions leading to the formation of carbon intermediates and sugars. This sequence of reactions, called the Calvin cycle, incorporates C02 into carbohydrate structures. 

There ensues a series of dark reactions or conformational changes that have the effect of greatly activating the imine linkage of the all-frans-rhodopsin towards hydrolysis. On hydrolysis, all-frawj-retina] is released and is unable to recombine with opsin until it is reconverted to the 11-cis isomer. The trans-to-cis rearrangement is a thermal rather than a photochemical reaction and is catalyzed by the enzyme retinal isomerase. The cycle of reactions is summarized in Figure 28-13. 

The resultant NADPH and ATP provide the reducing power and free energy to drive the reduction of carbon dioxide (the dark reactions) via the pentose phosphate pathway (or Calvin cycle l7>) and lead ultimately to the synthesis of glucose according to the overall stoichiometry below. 

The dark reactions (carbon-fixation reactions) use the ATP and NADPH produced by the light reactions to fix carbon dioxide as carbohydrate sucrose and starch. The reactions form a cycle (the Calvin cycle) in which the enzyme ribulose bisphosphate carboxylase (rubisco), located in the stroma, condenses a C02 molecule with ribulose 1,5-bisphosphate to produce two molecules of 3-phosphoglycerate. Other reactions then regenerate the ribulose... 

Frasch (45, 46) has shown that the OEC can catalyze an azide-insensitive catalase reaction in the dark. The activity can be directly associated with the OEC because (1) competitive inhibitors of water oxidation are also competitive inhibitors of the catalase activity and (2) the K for water oxidation and catalase activity are essentially identical. The enzyme apparently cycles in this case between S0 and S2. Mano and co-workers (47) showed that the Si/S i states are also competent to carry out catalase reactions however, this reaction is highly pH-depen-dent. For example, at pH 8.8, the Si state can oxidize H202 to 02, but S i is incapable of completing the reaction cycle however, if the pH is lowered to pH 6 steady-state measurements of oxygen evolution can be gathered. Just as is the case with water oxidation, these catalase reactions... 

PGA (phosphoglycerate) A three-carbon molecule formed when carbon dioxide is added to ribulose biphosphate (RuBP) during the dark reaction of photosynthesis (Calvin, or Calvin-Benson Cycle). PGA is converted to PGAL, using ATP and NADPH. 

PGAL (phosphoglyceraldehyde) A substance formed from PGA during the dark reaction of photosynthesis. Some PGAL leaves the cycle and can be converted to glucose, while other PGAL molecules are used to reform ribulose biphosphate (RuBP) to continue the dark reaction. 

The discussion of the light reactions of photosynthesis in Chapter 19 leads naturally into a discussion of the dark reactions—that is, the components of the Calvin cycle—in Chapter 20. This pathway is naturally linked to the pentose phosphate pathway, also covered in Chapter 20, because in both pathways common enzymes interconvert three-, four-, five-, six-, and seven-carbon sugars. 

Photosynthesis proceeds in two parts the light reachons and the dark reactions. The light reactions, which were discussed in Chapter 19. transform light energy into ATP and biosynthetic reducing power, NADPH. The dark reactions, which constitute the Calvin cycle, named after Melvin Calvin, the biochemist who elucidated the pathway, reduce carbon atoms from their fully oxidized state as carbon dioxide to the more reduced state as a hexose. The components of the Calvin cycle and called the dark reactions because, in contrast with the light reactions, these reactions do not directly depend on the presence of light. 

The dark reaction, known as the Calvin cycle, uses the reducing power of NADPH as well as the free energy stored in the ATP to assimilate carbon dioxide in the form of carbohydrates. The way by which Nature achieves carbon fixation is via the reaction of CO2 with ribulosebiphosphate (RuBP) to give two molecules of 3-phosphoglycerate, a process which is catalyzed by the enzyme RuBP-carboxylase. The phosphogylcerate is converted further to fructose 6-phosphate, the final product of the Calvin cycle. The overall reaction, despite its complex mechanism, corresponds to the simple Eq. (16) above. 

Fig. 5.43. Schematic bifurcation diagram for CH3CHO + O2 reaction showing separate branches corresponding to dark reaction and steady glow, with upper branch losing stability at a Hopf bifurcation as Ta is reduced to give limit cycle (cool-flame) oscillations. The simple limit cycle also loses stability as is reduced further and a complex oscillation corresponding to the multi-stage ignition will emerge but cannot be adequately represented...

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... 

We need to split two waters per CO2 fixed, so we double this total to 12 protons, generating 3 or 4 ATP. Only 3 ATP are required per CO2 in the dark reactions, so we probably make a slight profit of just under 1 ATP per CO2 over that which is required. However, sometimes, some of the electrons from ferredoxin Eire fed back to cyt-bb/f, short-circuiting the Z-scheme. This generates ATP via the Q-cycle but no NADPH. This is called cyclic phosphorylation and enables the cell to make even more ATP, even when NADPH is not required (Fig. 13.11). Depending on the exact stoichiometry of the various complexes (still not nailed down completely), cyclic phosphorylation may be essential to satisfying the Calvin cycle s requirements. 

Fig. 13.13 The dark reactions involve three distinct parts carbon fixation (Rubisco), gluconeogenesis, and RuBP regeneration (Calvin cycle).

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... 

---