Thylakoid development

The objectives of this study are twofold. Firstly, to resolve the factors determining the depression of 0 when mature maize leaves are chilled in the light. Secondly, to examine the perturbations in thylakoid development that may be associated with the loss of photosynthetic ability when maize leaves are grown at low temperatures. 

Acyl lipid asymmetry during oat thylakoid development... 

DEVELOPMENT OF A CHEMILUMINESCENCE DETECTION OF HERBICIDES RELATIVE TO THE MEDIATED INHIBITION OF THYLAKOIDS IN A ji-FLUIDIC SYSTEM... 

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... 

Plastoquinone is one of the most important components of the photosynthetic electron transport chain. It shuttles both electrons and protons across the photosynthetic membrane system of the thylakoid. In photosynthetic electron flow, plastoquinone is reduced at the acceptor side of photosystem II and reoxidized by the cytochrome bg/f-complex. Herbicides that interfere with photosynthesis have been shown to specifically and effectively block plastoquinone reduction. However, the mechanisms of action of these herbicides, i. e., how inhibition of plastoquinone reduction is brought about, has not been established. Recent developments haVe brought a substantial increase to our knowledge in this field and one objective of this article will be to summarize the recent progress. 

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 5-19. Schematic representation of reactions occurring at the photosystems and certain electron transfer components, emphasizing the vectorial or unidirectional flows developed in the thylakoids of a chloroplast. Outwardly directed election movements occur in the two photosystems (PS I and PS II), where the election donors are on the inner side of the membrane and the election acceptors are on the outer side. Light-harvesting complexes (LHC) act as antennae for these photosystems. The plastoquinone pool (PQ) and the Cyt b(f complex occur in the membrane, whereas plastocyanin (PC) occurs on the lumen side and ferredoxin-NADP+ oxidoreductase (FNR), which catalyzes electron flow from ferredoxin (FD) to NADP+, occurs on the stromal side of the thylakoids. Protons (H+) are produced in the lumen by the oxidation of water and also are transported into the lumen accompanying electron (e ) movement along the electron transfer chain.

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...

At variance with Hill s scheme [3], which has been discussed above in its recent developments, a three-light reaction scheme has been proposed by Arnon and coworkers [4,59]. According to this scheme, Fd and subsequently NADP would be reduced by PS II directly, and PS II would perform two different photoacts with two acceptors Fd and Q (Qa ) [4]. The role of PS I would be limited to the performance of cyclic photophosphorylation, catalysed by Fd as the electron carrier. Recent experiments showing that PS Il-enriched, inside-out thylakoid vesicles are capable of low rates of NADP reduction upon addition of Fd, FNR and plasto-cyanin [67] have been designed to investigate the view that only PS II is required to transfer electrons from water to NADP. However, the presence of PS I in the preparations, though in low proportions, was not ruled out, and the cause of the absolute requirement for PC, which is known to be oxidized by P-700 [29], was unexplained. 

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. 

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