Dark reactions, of photosynthesis

The fixation of carbon dioxide to form hexose, the dark reactions of photosynthesis, requires considerable energy. The overall stoichiometry of this process (Eq. 22.3) involves 12 NADPH and 18 ATP. To generate 12 equivalents of NADPH necessitates the consumption of 48 Einsteins of light, minimally 170 kj each. However, if the preceding ratio of l ATP per NADPH were correct, insufficient ATP for COg fixation would be produced. Six additional Einsteins would provide the necessary two additional ATP. Prom 54 Einsteins, or 9180 kJ, one mole of hexose would be synthesized. The standard free energy change, AG°, for hexose formation from carbon dioxide and water (the exact reverse of cellular respiration) is +2870 kj/mol. 

RCII may subsequently have been transformed into RCI by formation of the Fx cluster and eventually the capturing of a soluble 2[4Fe-4S] protein as an RC-associated subunit. These additions would have allowed electrons to leave the space of the membrane and serve for reductive processes in the dark reactions of photosynthesis. Our present knowledge concerning distribution of HiPIPs among species indicate that this electron carrier would have been invented only lately within the branch of the proteobacteria. Tbe evolutionary driving... 

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

In contrast to the biological CO2 fixation during the dark reaction of photosynthesis where very low concentrations of CO2 from air can be fixed at room temperature, most reactions with transition-metal complexes or with organic substrates either require high partial pressure of CO2 or high temperatures. An exception was published quite recently, the rapid fixation of CO2 and O2 from air at a palladium(O) complex 10 (Scheme 11) [77]. 

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. 

In the dark, the production of ATP and NADPH by photophosphorylation, and the incorporation of C02 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 synthesis of ATP, described in Chapter 19. They do not, in... 

There are features in this conversion that closely resemble the dark reactions of photosynthesis, which build a C6 chain (fructose) from Cs chains (Section 20-9). For example, the reactants are either phosphate esters or mixed anhydrides, and the phosphorylating agent is ATP ... 

Dark reactions. Reactions that can occur in the dark, in a process that is usually associated with light, such as the dark reactions of photosynthesis. 

Figure 11.5 Chloroplast and information about light and dark reactions of photosynthesis...

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. 

Fundamentals of Biochemistry A Textbook Dark Reaction of Photosynthesis... 

Part of the dark reactions of photosynthesis is interconversion of sugars with an enzyme called transketolase using thiamine pyrophosphate, TPP, as a catalyst (Section 8.12.8). Provide a reasonable mechanism for this enzymatic reaction. In addition to water, there are weak general acids and general bases present in the active site at pH 7. 

Starch and sucrose, key substrates for the development of dental caries, are exclusively synthesized by plants. They are made in plant leaves by a process called photosynthesis, which utilizes sunlight as the energy source. This chapter outlines the light and dark reactions of photosynthesis and compares the light reaction with mitochondrial electron transport (Sect. 1). The key dark reaction, the production of phosphoglycerate by the enzyme ribulose bisphosphate carboxylase (rubisco), is described along with the production of fructose, sucrose, and starch (Sect. 2). The chapter concludes with a detailed discussion of the roles of starch and sucrose in plant metabolism (Sect. 3). 

The thylakoids and stroma are the sites of the so-called light and dark reactions of photosynthesis, respectively. This compartmentalization of photosynthetic functions was recognized by Park and Pon when they broke open the chloroplasts, separated the contents into thylakoid and stroma fractions and examined their properties. The specific activities of the thylakoids include photochemical reactions, electron transport, oxygen evolution, ATP synthesis and NADP reduction, while the stroma contains enzymes for CO2 fixation driven by ATP and NADPH and other biochemical reactions in the dark. Our understanding and appreciation of the detailed structure and organization of the thylakoid membranes has increased tremendously in recent years. Further discussion of thylakoid structure will be continued in section VII on page 26. 

Let us return to the so-called dark reaction of photosynthesis. In it, both the NADPH2 and the ATP formed in the light reactions are consumed in the fixation of C02. The fixation reactions were charted by Melvin Calvin and his co-workers in Berkeley, California (for which Calvin received the Nobel Prize for 1961), with the use of radioactive carbon dioxide. During these reactions, C02 is made to combine with a pentose (5-carbon) sugar, ribulose diphosphate, to give an unstable 6-carbon intermediate which breaks down to two molecules of the 3-carbon phosphoglyceric acid. 

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... 

The increase in 02 and concomitant decrease in C02 caused by the evolution of oxygenic autotrophs on the Earth, has resulted in an undersaturation of the main (andancient) carboxylating enzyme, ribulose-l,5-bisphosphate carboxylase, responsible for the first step in carbon fixation—the dark reaction of photosynthesis. To palliate this difficulty, a number of species of marine phytoplankton have evolved carbon concentrating mechanisms that all involve some forms of... 

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. 

Part of the energy of from the sunlight is captured in a process called photophosphorylation in which ADP is phosphorylated to form ATP. The remaining reactions of photosynthesis are not dependent upon light and are part of what are called the dark reactions of photosynthesis. They do NOT, however, occur only in the dark. They are, in fact, stimulated by light. 

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. 

The dark reactions of photosynthesis are actually stimulated by the light reactions. The central enzyme in the dark reactions, ribulose-l,5-bisphosphate carboxylase, is stimulated by high pH and C02 and... 

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