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Related Concept Videos

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The multi-protein complex photosystem II (PS II) harvests photons and transfers their energy through its bound pigments to its reaction center, and ultimately to photosystem I (PSI) through the electron transport chain. The pigments responsible for caputirng the light energy in photosystems include chlorophyll a, chlorophyll b, and carotenoids.
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The light reactions of photosynthesis assume a linear flow of electrons from water to NADP+. During this process, light energy drives the splitting of water molecules to produce oxygen. However, oxidation of water molecules is a thermodynamically unfavorable reaction and requires a strong oxidizing agent. This is accomplished by the first product of light reactions: oxidized P680 (or P680+), the most powerful oxidizing agent known in biology. The oxidized P680 that acquires an electron from the...
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Although structurally similar to photosystem II (PSII), photosystem I (PSI) is has a different electron supplier and electron acceptor.
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Photosystems01:32

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Photosystems are multiprotein complexes that form the functional units of photosynthesis in plants, algae, and cyanobacteria. They are found embedded in the membrane of tiny sac-like structures called thylakoids placed inside the chloroplast.
Functioning of Photosystems
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Reaction centers are pigment-protein complexes that initiate energy conversion from photons to chemical entities. Therefore, photochemical reaction center is a more appropriate term that describes these complexes. The Nobel laureates Robert Emerson and William Arnold provided the first experimental evidence of photochemical reaction centers by demonstrating the participation of nearly 2,500 chlorophyll molecules for the release of just one molecule of oxygen. Despite thousands of photosynthetic...
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Proteins targeted to the inner chloroplast membrane, or plastid proteins, are transported by two general pathways: the stop-transfer and the re-insertion or post-import pathways. Most plastid proteins carry N-terminal transit sequences and internal import sequences targeting it to the specific chloroplast subcompartment. Proteins targeted by the stop-transfer pathway have internal hydrophobic sequences that inhibit their translocation into the stroma. As a result, these precursors are arrested...
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Purification of Active Photosystem I-Light Harvesting Complex I from Plant Tissues
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Membrane charge affecting electron donation to PS II in chloroplasts.

Z Drechsler1, J Neumann

  • 1Department of Botany, The George S. Wise Faculty of Life Sciences, Tel Aviv University, 69978, Tel Aviv, Israel.

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|January 18, 2014
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Ethylenediaminetetraacetic acid (EDTA) treatment of chloroplasts inhibits electron flow by increasing negative charge density on thylakoid membranes. This effect is reversed by cations and chloride ions, suggesting EDTA impacts water donation sites.

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Area of Science:

  • Photosynthesis research
  • Plant biochemistry
  • Thylakoid membrane biophysics

Background:

  • Electron transport chain in photosynthesis is crucial for energy production.
  • Thylakoid membranes contain essential protein complexes for light-dependent reactions.
  • EDTA is known to chelate divalent cations like Mg(2+), potentially affecting membrane properties.

Purpose of the Study:

  • To investigate the effect of EDTA pretreatment on chloroplast electron flow.
  • To determine the role of cation valency and chloride ions in restoring electron flow.
  • To analyze changes in charge density on the thylakoid membrane after EDTA treatment.

Main Methods:

  • Chloroplasts were pretreated with EDTA (0.75 mM).
  • Electron flow rates were measured at pH > 8.5.
  • Inhibition reversal was studied using electron donors and various salt additions.
  • Charge density in the Q region was calculated for control and treated chloroplasts.

Main Results:

  • EDTA pretreatment significantly inhibited electron flow above pH 8.5.
  • Restoration of electron flow by salts showed cation valency dependence: C(3+)>C(2+)>C(+).
  • Maximal restoration required low chloride concentrations, essential for oxygen evolution.
  • EDTA treatment increased negative charge density from -1.1 to -2.0 μC/cm(2) near Q.

Conclusions:

  • EDTA treatment increases negative charge density on thylakoid membranes, likely by dissipating pH gradients and chelating Mg(2+).
  • This increased negative charge density is closely related to the water donation site in photosystem II.
  • Cations and chloride ions play critical roles in screening these negative charges and restoring electron transport.