Stroma, chloroplast

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... 

Moderately damaged cells were characterized by the presence of several, often exceedingly large, crystalloids in the chloroplast stroma (Fig. U, 5). The crystalline bodies occurred... 

The chloroplast stroma contains all the enzymes necessary to convert the triose phosphates produced by C02 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-bisphos-phate 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 (see Section 20.3). 

Regulation of Starch and Sucrose Synthesis Sucrose synthesis occurs in the cytosol and starch synthesis in the chloroplast stroma, yet the two processes are intricately balanced. What factors shift the reactions in favor of (a) starch synthesis and (b) sucrose synthesis ... 

In the photosynthetic cells of plants, fatty acid synthesis occurs not in the cytosol but in the chloroplast stroma (Fig. 21-8). This makes sense, given that NADPH is produced in chloroplasts by the light reactions of photosynthesis ... 

Photosystem I contains three iron-sulfur clusters firmly associated with the reaction center. These are designated Fe-Sx, Fe-SA, and Fe-SB in figure 15.17. The cysteines of Fe-Sx are provided by the two main polypeptides of the reaction center, which also bind P700 and its initial electron acceptors Fe-SA and Fe-SB are on a separate polypeptide. The quinone that is reduced in photosystem I probably transfers an electron to Fe-Sx, which in turn reduces Fe-SA and Fe-SB. From here, electrons move to ferredoxin, a soluble iron-sulfur protein found in the chloroplast stroma, then to a flavoprotein (ferredoxin-NADP oxidoreductase), and finally to NADP+. 

Transport of two electrons from photosystem II through the cytochrome bhf complex to photosystem I results in the movement of four protons from the chloroplast stroma to the thylakoid lumen. The proton translocation probably occurs in a Q cycle resembling that illustrated in figure 14.11. Two more protons are released in the lumen for each molecule of H20 that is oxidized to 02, and one additional proton is removed from the stroma for each molecule of... 

Chloroplast type of CuZn-SOD isozyme is localized in the chloroplast stroma of most plants, whereas Fe-SOD occurs in chloroplasts of Euglena, the moss Marchantia polymorpha and several species of seed plant. So far Mn-SOD has not been detected in chloroplasts in a soluble form but occurs in a thylakoid membrane-bound form.23,24) The cytosolic CuZn-SOD, which is distinguishable from the chloroplastic CuZn-SOD in terms of amino acid sequence, occurs in cell compartments other than chloroplasts and in nonphotosynthetic tissues.14)... 

The chlorophyll-protein complexes are oriented in the lamellar membranes in such a way that the electron transfer steps at the reaction centers lead to an outward movement of electrons. For instance, the electron donated by Photosystem II moves from the lumen side to the stromal side of a thylakoid (see Figs. 1-10 and 5-19). The electron that is donated back to the trap chi (Pgg0) comes from H20, leading to the evolution of 02 by Photosystem II (Eq. 5.8). The 02 and the H+ from this reaction are released inside the thylakoid (Fig. 5-19). Because 02 is a small neutral molecule, it readily diffuses out across the lamellar membranes into the chloroplast stroma. However, the proton (H+) carries a charge and hence has a low partition coefficient (Chapter 1, Section 1.4A) for the membrane, so it does not readily move out of the thylakoid lumen. 

We next consider the main function of a leaf, photosynthesis, in terms of the conductances and the resistances encountered by CO2 as it diffuses from the turbulent air, across the boundary layers next to the leaf surface, through the stomata, across the intercellular air spaces, into the mesophyll cells, and eventually into the chloroplasts. The situation is obviously more complex than the movement of water vapor during transpiration. Indeed, CO2 not only must diffuse across the same components encountered by water vapor moving in the opposite direction5 but also must cross the cell wall of a mesophyll cell, the plasma membrane, part of the cytosol, the membranes surrounding a chloroplast, and some of the chloroplast stroma. Resistances are easier to deal with than are conductances for the series of components involved in the pathway for CO2 movement, so we will specifically indicate the resistance of each component. 

Figure 8-11. Schematic cross section near the periphery of a mesophyll cell (Fig. 1-1), indicating the sequential anatomical components across which CO2 diffuses from the intercellular air spaces to the carboxylation enzymes in the chloroplast stroma.

Figure 8-12. Joining of (a) the diffusion steps for C02 crossing the boundary layer, the rest of the gas phase, and then the liquid-phaseresistances of themesophyll cells and their chloroplasts to reach the chloroplast stroma with (b) the biochemistry in the chloroplasts, as described by a Michealis-Menten formalism (Eqs. 3.28 and 8.27).

The reductive pentose phosphate cycle is the only fundamental carboxylating mechanism in plants. In C3 plants the entire process of photosynthesis (the light reactions and the RPP cycle) occurs within chloroplasts. The enzymes catalysing steps in the RPP cycle are water-soluble and are located in the soluble portion (chloroplast stroma or extract). 

Figure 2.5 compares the orientation of the ATP synthase F0/Fj complex in mitochondria with that in chloroplasts. The lumen enclosed by the thylakoid membrane is slightly acidic it corresponds to the mitochondrial intermembrane space where electron transport first pumps protons (H+). In chloroplasts, ATP is made as protons diffuse from the thylakoid lumen through the membrane to the chloroplast stroma (Fig. 2.4). In mitochondria, ATP is made as protons diffuse from the mitochondrial intermembrane space through the inner mitochondrial membrane to the mitochondrial lumen or matrix.

Each chloroplast is bounded by an envelope of a highly permeable outer membrane and a nearly impermeable inner membrane, the two membranes being separated by a narrow, inter-membrane compartment [see Fig. 13 (C) and (C )]. The outer membrane allows small molecules to pass through, while the inner membrane presents a barrier which allows only certain metabolites or ATP to pass through with the help of special transport proteins embedded in the membrane. Enclosed by the inner membrane is the stroma, a concentrated solution containing the enzymes necessary for CO fixation, i.e., its conversion into carbohydrates. The stroma also contains the chloroplast s own DNA, RNA and ribosomes involved in the synthesis of proteins. This chloroplast stroma is analogous to the matrix in mitochondria. 

The terminal electron acceptor of PS I is the 11-kDa, water-soluble ferredoxin (Fd) present in the chloroplast stroma. The ultimate electron source for Fd reduction is water, transferred by way of cytochrome bj and the PS-I electron carriers (see Fig. 1). The transferred electron finally goes to reduce NADP to NADPH, which is needed for CO2 fixation. Reduction of NADP is mediated by the enzyme ferredoxin- NADP -reductase, or FNR, boundd to the outer surface of the non-appressed region of the thylakoid membrane. 

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). 

After the synthesis of the plastocyanin precursor in cytoplasm, the first import signal mediates the transport of the protein into the chloroplast stroma. After this signal is removed by a protease, a second import signal mediates the transfer of the protein into the thylakoid lumen. Plastocyanin then binds a copper atom, folds into its final three-dimensional structure, and associates with the thylakoid membrane. 

Ascorbic acid (vitamin C) is utilized as a cofactor to stabilize the chloroplast stroma, in quenching free radicals and reacting with hydroxy radicals and in the biosynthesis of tartaric acid and oxalic acid (6), important organic acids in grapes, and many vegetables. The effect of CA on ascorbic acid content differs with commodity,... 

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