Thylakoid matrix

The doubly reduced Qb can take up two protons from the surrounding medium, forming plastohydro-quinone, QbH2. Eventually QbH21s released from its binding site to the thylakoid matrix where it can interact with the cytochrome Z>6/complex and thus transfer its two electrons to photosystem I eventually. The site vacated by QbH2 is then replenished with an unreduced PQ molecule from the plastoquinone pool, this replacement plastoquinone now ready to function as Qb and accept electron from newly formed Qa . The plastoquinone pool contains 5-10 PQ molecules per reaction center, with each PQ able to store two electrons. This series of reactions is summarized in Pig. 6. 

The thylakoid matrix is an extensive internal membrane system inside the chloroplasts, which are small organelles in the plant cells. The inner lumen of the membrane system maintains a pH of 5, while the outer compartment, called stroma, has a pH of 8. The energy picked up from the photons is used to establish and maintain this difference. 

Like mitochondria, chloroplasts (when illuminated) pump protons across their membranes (Fig. 23-18). However, while mitochondria pump protons to the outside, the protons accumulate on the inside of the thylakoids. The ATP synthase heads of coupling factor CEj are found on the outside of the thylakoids, facing the stromal matrix, while those of F, lie on the insides of mitochondrial membranes. However, the same mechanism of ATP formation is used in both chloroplasts and mitochondria (Chapter 18). 

FIGURE 1. Thin section of part of an isolated chloroplast showing the internal thylakoid membrane system which consists of appressed grana lamellae (g) and non-appressed stroma lamellae (s) embedded in the stroma protein matrix and surrounded by a double membrane envelope (e). 

ATP synthetase was the first thylakoid complex to be positively localized by Miller and Staehelin [51] who demonstrated unequivocally by antibody labelling studies that CF, was present only in non-appressed membranes. The bulky CF, component that protrudes some 9-14 nm into the stromal matrix would prevent ATP synthetase being located in the appressed membrane regions that approach one another to = 3 nm under illumination [6]. 

Figure 19.25. Comparison of Photosynthesis and Oxidative Phosphorylation. The light-induced electron transfer in photosynthesis drives protons into the thylakoid lumen. The excess protons flow out of the lumen through ATP synthase to generate ATP in the stroma. In oxidative phosphorylation, electron flow down the electron-transport chain pumps protons out of the mitochondrial matrix. Excess protons from the intermembrane space flow into the matrix through ATP synthase to generate ATP in the matrix.

The reaction center thus evolves the oxygen necessary for assimilating CO in the form of NADPH and pumps protons from the matrix to the inside of the thylakoid membrane. The proton gradient drives subsequent energy storage by adenosine triphosphate (ATP) synthesis and the ATP is later used in carbon-fixation reactions to make carbohydrates. 

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.

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... 

Most reviewers5 8 now argue that photosynthetic oxygen evolution results from a sequential four-step electron-transfer process in which oxidizing equivalents from chi fl+- are accumulated in a "charge-storing" complex to accomplish the concerted four-electron oxidation of two H2O molecules to one O2 molecule. The photooxidant (chi + ) and reductant (pheo O are one-electron transfer agents, and the matrix is the lipoprotein thylakoid membrane. Hence, evaluation and consideration of the one-electron redox potentials for PS 11 components within a lipoprotein matrix are necessary in order to assess the thermodynamic feasibility of proposed mechanistic sequences. 

Note that the membrane orientation of CFi-CFy is reversed compared with that of the mitochondrial ATP synthase (Figure 19.25). However, the functional orientation of the two synthases is identical protons flow from the lumen through the enzyme to the stroma or matrix where ATP is synthesized. Because CF is on the stromal surface of the thylakoid nic/ii-brane, the newly synthesized ATP is released directly into the stromal space. Likewise, NADPH formed by photosystem I is released into the stromal space. Thus, ATP and NADPH, the products of the light reactions oj... 

The internal membrane vesicles (thylakoids) are fused into stacks (grana), which reside in a matrix (the stroma). All the chlorophyll in the cell is contained in the thylakoid membranes, where the light-induced production of ATP takes place during photosynthesis. [Courtesy of Biophoto Associates/M. C. Ledbetter/Brookhaven National Laboratory]... 

Electron microscopy of sectioned chloroplasts shows ATP synthase complexes as knobUke projections on the outside (stromal or N) surface of thylakoid membranes these complexes correspond to the ATP synthase complexes seen to project on the inside (matrix or N) surface of the inner mitochondrial membrane. Thus the relationship between the orientation of the ATP synthase and the direction of proton pumping is the same in chloroplasts and mitochondria. In both cases, the Fi portion of ATP synthase is located on the more alkaline (N) side of the membrane through which protons flow down their concentration gradient the direction of proton flow relative to Fi is the same in both cases P to N (Fig. 19-58). 

The internal structure of chloroplasts (Figure 17.4c) resembles that of the mitochondrion (see here). Note the presence of an outer, relatively permeable membrane and an inner membrane that is selectively permeable. The stroma of the chloroplast is analogous to the mitochondrial matrix. Immersed in the stroma are many flat, saclike membrane structures called thylakois which are stacked like coins. The stacks are called grana. Grana are irregularly interconnected by thylakoid extensions called stroma lamellae. The thylakoid membrane encloses the lumen (or interior) of the thylakoid. 

There are analogies in structure and role between the mitochondrial matrix (Figure 15.2a) and the chloroplast stroma and also between the inner membrane of the mitochondrion and the thylakoid membrane of the chloroplast. Absorption of light and all of the light reactions occur within or on the thylakoid membranes. ATP and NADPH produced by these reactions are released into the surrounding stroma, where the synthetic dark reactions occur. 

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