Carbohydrates dark reactions

Dark reactions the short-term chemical energy is used to convert carbon dioxide into carbohydrate. We will not be concerned with these dark reactions. 

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,... 

The reduction of C02 -> carbohydrate, is a dark reaction which occurs in a number of enzymatic steps. 

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. 

The end result of the photochemical part of photosynthesis is the formation of 02, NADPH, and ATP. Much of the oxygen is released to the atmosphere, but the NADPH and ATP are utilized in a series of dark reactions that achieve the reduction of carbon dioxide to the level of a carbohydrate (fructose). A balanced equation is... 

Photosynthesis uses solar energy to synthesize carbohydrate from carbon dioxide and water. In the light reactions, the light energy drives the synthesis of NADPH and ATP. In the dark reactions (carbon-fixation reactions), the NADPH and ATP are used to synthesize carbohydrate from C02 and H20. 

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... 

The dark reactions (also called the carbon-fixation reactions) use the ATP and NADPH produced by the light reactions to convert carbon dioxide into carbohydrate. The final products are sucrose and starch. 

NAD PH. The dark reactions use the ATP and NADPH produced by light reactions to reduce carbon atoms from C02 to a more reduced state as carbohydrates (Figure 11.5). 

Significantly, the membrane orientation of CF]-CFq is reversed compared with that of the mitochondrial ATP synthase (Figure 19.25). Thus, protons flow out of the thylakoid lumen through ATP synthase into the stroma. Because CF] is on the stromal surface of the thylakoid membrane, the newly synthesized ATP is released directly into the stromal space. Recall that NADPH formed through the action of photosystem I and ferredoxin-NADP+ reductase also is released into the stromal space. Thus, ATP and NADPH, the products of the light reactions ofphotosynthesis, are appropriately positioned for the subsequent dark reactions, in which CO is converted into carbohydrate. 

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. 

In land plants and green algae (chlorophytes), photosynthesis has two distinct stages, the light reactions, which convert light energy to ATP and NADPH and the dark reactions, which convert CO2 to carbohydrate using ATP and NADPH. Both occur in the chloroplasts (Fig. 13.4). 

Further, CO2 hydrogenation reaction is regarded as one of artificial photosynthesis technologies. It is well known that photosynthesis by plants consists of two main reactions, that is, the fight reaction and the dark reaction. In the fight reaction, water is decomposed to H2 and O2 under the solar visible fight irradiation. Produced O2 is emitted into the atmosphere. On the other hand, produced H2 is carried by NADP+ as NADPH. In the dark reaction, adsorbed CO2 is hydrogenated by H2 fi om NADPH and CO2 is fixed finally as carbohydrates under the non-... 

NADPH Reduced form of nicotinamide adenine dinucleotide phosphate. An energy-rich compound produced by the light-reaction of photosynthesis. It is used to synthesize carbohydrates in the dark-reaction. 

In photosynthesis there are two types of processes. The first are light reactions, divided into photosystems I and II, which are directly dependent on solar energy the others are dark reactions, which can take place without light. In the dark reactions, the products of the light reactions, ATP and NaDPH-ase, are used for the reduction of CO2 to form carbohydrates. The dark reactions have been mapped in great detail. 

The incorporation of COz into carbohydrate by eukaryotic photosynthesizing organisms, a process that occurs within chloroplast stroma, is often referred to as the Calvin cycle. Because the reactions of the Calvin cycle can occur without light if sufficient ATP and NADPH are supplied, they have often been called the dark reactions. The name dark reactions is somewhat misleading, however. The Calvin cycle reactions typically occur only when the plant is illuminated, because ATP and NADPH are produced by the light reactions. Therefore light-independent reactions is a more appropriate term. Because of the types of reactions that occur in the Calvin cycle, it is also referred to as the reductive pentose phosphate cycle (RPP cycle) and the photo synthetic carbon reduction cycle (PCR cycle). 

In subaerial C3 plants substrate and reactant (s and r, respectively, in Fig. 5.56) for photosynthesis are both gaseous (atmospheric) C02, which flows through the Calvin cycle (the dark reactions of photosynthesis see Box 1.10) to yield simple carbohydrates (p), which are in turn the source of various metabolic intermediates. The source of the intracellular (kinetic) isotopic fractionation during C fixation is the enzyme rubisco (D-ribulose 1,5-diphosphate carboxylase/oxygenase). There is also an isotopic fractionation resulting from the passage of C02 into the cell. Passive diffusion of C02, at a rate , favours 12C, but the fractionation is small... 

In C4 plants (Hatch 1977), the light (02-producing) and dark (carbohydrate-producing) reactions of photosynthesis are separated spatially. Moreover, a sort of molecular turbocharger is used to raise the pressure of CO2 at rubisco s reaction site. Carbon is initially fixed in mesophyll cells, relatively close to the surface of the leaf The reaction utilized is shown below. 

The Calvin cycle-the so-called dark reactions of photosynthesis-does not occur solely in the dark. In fact, the dark reactions are stimulated by light, but do not directly use the energy of light to function. Instead, they use the NADPH and ATP generated by the light reactions to fix atmospheric carbon dioxide into carbohydrates. 

In the dark reactions of photosynthesis, NADPH and ATP produced by the light reactions are used in the reductive synthesis of carbohydrate from C02 and water. 

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