Thylakoid membranes development

Mawson BT, Morrissey PJ, Gomez A and Melis A. (1994). Thylakoid membrane development and differentiation assembly of the chlorophyll a-b light-harvesting comp lex and evidence for the origin of Mr= 19, Mr=17.5 and 13.4 kDa proteins. Plant Cell Physiol. 35, 341-351. 

Observations in chloroplasts played a key role in the development of the chemiosmotic theory of oxidative phosphorylation, which we discussed in chapter 14. Andre Jagendorf and his colleagues discovered that if chloroplasts are illuminated in the absence of ADP, they developed the capacity to form ATP when ADP was added later, after the light was turned off. The amount of ATP synthesized was much greater than the number of electron-transport assemblies in the thylakoid membranes, so the energy to drive the phosphorylation could not have been stored in an energized... 

Advances in the study of photosynthetic manganese and the water oxidation complex have been accelerated by the development of techniques for the isolation of photosystem II particles by Triton-X and/or digitonin treatment of thylakoid membranes (188,189). Freeze-fracture electron microscopy indicates the particles are highly purified membrane fragments almost entirely devoid of photosystem I components (190). The lumenal side of the photosystem II membrane is exposed, allowing direct access to the water oxidation enzyme complex. These PSII preparations contain four atoms of manganese per PSII reaction center and possess large amounts of 02 activity (191, 192). 

Figure 6-5 indicates that the C>2-evolution step and the electron flow mediated by the plastoquinones and the Cyt b(f complex lead to an accumulation of H+ in the lumen of a thylakoid in the light. This causes the internal H+ concentration, c, or activity, to increase. These steps depend on the light-driven electron flow, which leads to electron movement outward across the thylakoid in each of the two photosystems (see Fig. 5-19). Such movements of electrons out and protons in can increase the electrical potential inside the thylakoid (E ) relative to that outside ( °), allowing an electrical potential difference to develop across a thylakoid membrane. By the definition of chemical potential (fij = jx + RT In cij 4- ZjFE Eq. 2.4 with the pressure and gravitational terms omitted see Chapter 3, Section 3.1), the difference in chemical potential of H+ across a membrane is...

The isolated CFq-CFi has been incorporated into phospholipid liposomes and shown to carry in this form most of the energy-transducing functions which it catalyses within the thylakoid membranes. Thus, the reconstituted ATP synthase carries out ATP-dependent proton translocation resulting in both a 4pH and a developing across the reconstituted liposomes [72,73] an uncoupler-sensitive ATP-Pj exchange reaction [39] and ATP formation driven by artificially imposed 4pH and Ail/ [39,74,75], or by electric field pulses [56]. The ATP synthase proteolipo-somes provide the simplest system available today for the study of electrochemical-gradient-driven phosphorylation. 

Fig. 15. Conceptual development of a membane vesicle subjected to voltage pulses to create a potential difference across the membrane. (A) A1 pm-dlameter sphere of water is Imagined placed between two platinum electrodes 1 mm apart (B) The water sphere is replaced by a sphere of lipid (C) The Interior of the lipid sphere is replaced by a sphere of water, resulting in a lipid shell surrounding an aqueous medium to form the equivalent of a membrane vesicle. See text for details. (D) A schematic representation of a chloroplast thylakoid membrane containing ATP synthase to be subjected to voltage pulses and then the amount of ATP formed determined. Plots of actually measured ATP formation by voltage pulses (E) or light pulses (F) as a function of the number of pulses. (A), (B), (C), (E) and (F) from Witt (1987) Examples for the cooperation of photons, excitons, electrons, electric fields and protons in the photosynthesis membrane. Nouveau Journal deChimie 11 97 (D) adapted from Bauermeister, Schlodderand Graber(1988) Electric field-driven ATP synthesis catalyzed by the membrane-bound ATP-synthase from chloroplasts. Ber Bunsenges Phys Chem 92 1037.

Absorption of Light Energy and the Fate of the Electronically Excited States Light-Harvesting Pigment Molecules Development of Some Modern Concepts of Photosynthesis The Thylakoid Membrane of Chloroplasts... 

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