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What's the difference of electron transport chain …
We turn now, for summary and orientation to the overall process of photosyntheticelectron transport, to the fourth and final topic, i.e., a functional description of howthree of the four major protein complexes function to transport electrons from H2Oto NADP+.
Photosynthesis occurs inside chloroplasts. Chloroplasts contain chlorophyll, a green pigment found inside the thylakoid membranes. These chlorophyll molecules are arranged in groups called photosystems. There are two types of photosystems, Photosystem II and Photosystem I. When a chlorophyll molecule absorbs light, the energy from this light raises an electron within the chlorophyll molecule to a higher energy state. The chlorophyll molecule is then said to be photoactivated. Excited electron anywhere within the photosystem are then passed on from one chlorophyll molecule to the next until they reach a special chlorophyll molecule at the reaction centre of the photosystem. This special chlorophyll molecule then passes on the excited electron to a chain of electron carriers.
Photosynthesis and Electron Transport Chain - BIOL …
Shortly afterwards, Robin Hill showed that a variety of substances stimulatedphotosynthetic oxygen evolution (by acting as electron acceptors). Thus, any substancethat accepts H2O-derived electrons from the photosynthetic electron transportchain is termed a Hill reagent, and oxygen evolution is called the Hill reaction. Clearly,one set of reactions (which we now call photosynthetic electron transport) harvests lightand produces an intermediate reductant (which we now know is NADPH) and another set ofreactions (the RPPP) uses the reductant to reduce CO2. Of course, CO2is the common natural Hill reagent.
As mentioned in the introduction, we will discuss four major components of thephotosynthetic electron transport system. These components are shown as the differentsymbols keyed in the upper right hand corner of the diagram. Two techniques(freeze-fracture and two-phase polymer systems that enable investigators to isolatephysically very similar biological structures [such as the different chloroplast membranesor the cis vs. trans faces of Golgi] have shown that the four components are not equallydistributed through out the membranes. To facilitate our later discussions, I would ask you to commit to memory now: (1) The ATPase and PSI are localizedexclusively in the non-appressed regions. (2) PSII is localized mostly in the appressedregions. (3) The b-f complex is abundant in both the appressed and non-appressed regions.
Electron Transport in Photosynthesis - HourlyBook
A final historical question regards the stoichiometry of electron transport. Theoxidation of 2 H2Os to 1 O2 is a four-electron process. Usualbiochemical oxidation/reduction reactions involve one or two electrons. This situationimplies that there must be a mechanism to "store" electrons. The followingoverhead is the widely accepted Kok model (1970) that provides as explanation of theJoliot (1960's) data depicted above it.
Photophosphorylation is the production of ATP using the energy of sunlight. Photophosphorylation is made possible as a result of chemiosmosis. Chemiosmosis is the movement of ions across a selectively permeable membrane, down their concentration gradient. During photosynthesis, light is absorbed by chlorophyll molecules. Electrons within these molecules are then raised to a higher energy state. These electrons then travel through Photosystem II, a chain of electron carriers and Photosystem I. As the electrons travel through the chain of electron carriers, they release energy. This energy is used to pump hydrogen ions across the thylakoid membrane and into the space within the thylakoid. A concentration gradient of hydrogen ions forms within this space. These then move back across the thylakoid membrane, down their concentration gradient through ATP synthase. ATP synthase uses the energy released from the movement of hydrogen ions down their concentration gradient to synthesise ATP from ADP and inorganic phosphate.
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Photosynthetic electron transport - FSU
In addition to producing NADPH, the light dependent reactions also produce oxygen as a waste product. When the special chlorophyll molecule at the reaction centre passes on the electrons to the chain of electron carriers, it becomes positively charged. With the aid of an enzyme at the reaction centre, water molecules within the thylakoid space are split. Oxygen and H+ ions are formed as a result and the electrons from the splitting of these water molecules are given to chlorophyll. The oxygen is then excreted as a waste product. This splitting of water molecules is called photolysis as it only occurs in the presence of light.
Electron Transport Chain - YouTube
The data show that one brief flash of light (just long enough for one photochemicalevent in each of the two photosystems) results in no O2 evolution. A secondflash of light also causes no O2 evolution. On the third flash, however, thereis oxygen production. Again, on the fourth flash, there is no oxygen production. In suchan experiment, there was O2 evolution only every fourth flash. (Eventually, ofcourse, the pattern dampened because occasionally some individual photosystems wouldabsorb two photons or no light at all.) These data provide the interpretation that the OEC("on" PSII) exists in five states. The S1-state is most stable. Indarkness, S2 and S3 revert to the S1-state, and, in allconditions, S4 spontaneously "decays" to S0, anotherstable state. In darkness, however, OECs are in the S1-state, being threeflashes away from O2 evolution. In brief summary, evolution of 1 O2,as we have said so often, requires the absorption of 8 photons--each photosystem absorbsone photon for each electron "transported" from water to NADP+.
Electron transport chain - Wikipedia
If the light intensity is not a limiting factor, there will usually be a shortage of NADP+ as NADPH accumulates within the stroma (see light independent reaction). NADP+ is needed for the normal flow of electrons in the thylakoid membranes as it is the final electron acceptor. If NADP+ is not available then the normal flow of electrons is inhibited. However, there is an alternative pathway for ATP production in this case and it is called cyclic photophosphorylation. It begins with Photosystem I absorbing light and becoming photoactivated. The excited electrons from Photosystem I are then passed on to a chain of electron carriers between Photosystem I and II. These electrons travel along the chain of carriers back to Photosystem I and as they do so they cause the pumping of protons across the thylakoid membrane and therefore create a proton gradient. As explained previously, the protons move back across the thylakoid membrane through ATP synthase and as they do so, ATP is produced. Therefore, ATP can be produced even when there is a shortage of NADP+.
Electron Transport in Photosynthesis - HyperPhysics Concepts..
1Cyanobacteria and all photosynthetic eukaryotes conduct"oxygenic" photosynthesis. I.e., they extract electrons from H2O andthe byproduct is O2. Although I have referred to this as photosynthesis withoutqualification, you should at least be aware that some prokaryotes utilize other than H2Oas an electron donor. As an example, one taxon extracts electrons from H2S; theproduct, elemental S, is found in large deposits owing to these organisms, just as O2in the atmosphere accumulated from oxygenic organisms. We will not discuss non-oxygenicphotosynthesis, except in a historical context, in this course in plantphysiology. However, as we have stressed, biochemistry is very similar among allorganisms. Indeed, important insights into plant photosynthesis have come from studieswith bacteria. The first integral membrane protein to be crystallized is a part of thephotosynthetic machinery of a non-oxygenic photosynthetic bacterium. The crystallizationpermitted detailed structural analysis. As an unabashed plant chauvinist, I would take theposition that this work was most important because inferences about the structure of partof the photosynthetic machinery ("Photosystem II") of plants could be drawn fromthis bacterium's photosynthetic apparatus. This work was recognized by a 1988 Nobel Prize(to Michel and Deisenhofer, of München). (A not-so-subliminal message is that advances inunderstanding often rely on technical advancesin this case,protein crystallization. You will recall the importance of the patch-clamp technique toinvestigations of membrane transport. This work, too, was recognized by a Nobel Prize (toNeher, of Göttingen).)
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