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You are watching: Where do electrons get their energy in photosystem i

Cooper GM. The Cell: A molecule Approach. Second edition. Sunderland (MA): Sinauer Associates; 2000.


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During photosynthesis, energy from sunshine is harvested and used to drive the synthesis of glucose indigenous CO2 and also H2O. By converting the energy of sunshine to a usable kind of potential chemical energy, photosynthesis is the ultimate resource of metabolic energy for all organic systems. Photosynthesis takes place in two unique stages. In the light reactions, power from sunlight drives the synthesis of ATP and NADPH, combination to the formation of O2 indigenous H2O. In the dark reactions, therefore named due to the fact that they perform not call for sunlight, the ATP and NADPH created by the irradiate reactions journey glucose synthesis. In eukaryotic cells, both the light and also dark reaction of photosynthesis happen within chloroplasts—the irradiate reactions in the thylakoid membrane and also the dark reactions within the stroma. This section discusses the light reactions that photosynthesis, i m sorry are concerned oxidative phosphorylation in mitochondria. The dark reaction were questioned in detail in thing 2.


Electron circulation through Photosystems I and also II

Sunlight is soaked up by photosynthesis pigments, the most abundant of i beg your pardon in plants are the chlorophylls. Absorption of irradiate excites an electron come a higher energy state, thus converting the power of sunlight to potential chemistry energy. The photosynthetic pigments room organized into photocenters in the thylakoid membrane, each of which includes hundreds of pigment molecule (Figure 10.20). The many pigment molecule in each photocenter act together antennae to absorb light and also transfer the energy of your excited electrons to a chlorophyll molecule that serves as a reaction center. The reaction center chlorophyll then transfers that high-energy electron come an agree molecule in an electron carry chain. High-energy electrons space then transferred v a collection of membrane carriers, combination to the synthetic of ATP and also NADPH.


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Figure 10.20

Organization that a photocenter. Each photocenter is composed of thousands of antenna colors molecules, i beg your pardon absorb photons and transfer power to a reaction center chlorophyll. The reaction facility chlorophyll climate transfers that is excited electron to an acceptor (more...)


The ideal characterized photosynthesis reaction facility is that of the bacter Rhodopseudomonas viridis, the structure of which was established by Johann Deisenhofer, Hartmut Michel, Robert Huber, and their partner in 1985 (Figure 10.21). The reaction facility consists of 3 transmembrane polypeptides, bound come a c-type cytochrome top top the exterior next of the membrane. Energy from sunshine is captured by a pair that chlorophyll molecules well-known as the unique pair. Electrons are then transferred from the distinct pair to an additional pair that chlorophylls and also from over there to various other prosthetic teams (pheophytins and quinones). From over there the electrons room transferred come a cytochrome bc complicated in i m sorry electron transport is combination to the generation the a proton gradient. The electrons room then moved to the reaction facility cytochrome and also finally went back to the chlorophyll one-of-a-kind pair. The reaction center thus converts the power of sunshine to high-energy electrons, the potential power of which is converted to a proton gradient through the cytochrome bc complex.


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Figure 10.21

Structure of a photosynthesis reaction center. The reaction center of R. Viridis is composed of three transmembrane protein (purple, blue, and also beige) and also a c-type cytochrome (green). Chlorophylls and other prosthetic groups are colored yellow. (Courtesy (more...)


The proteins affiliated in the light reactions of photosynthesis in tree are organized into 5 complexes in the optical membrane membrane (Figure 10.22). Two of this complexes are photosystems (photosystems I and II), in which irradiate is soaked up and transferred to reaction center chlorophylls. High-energy electrons are then transferred with a collection of carrier in both photosystems and in a 3rd protein complex, the cytochromebfcomplex. Together in mitochondria, this electron transfers room coupled come the transfer of protons right into the thylakoid lumen, thereby developing a proton gradient across the optical membrane membrane. The power stored in this proton gradient is climate harvested by a fourth protein facility in the optical membrane membrane, ATP synthase, i m sorry (like the mitochondrial enzyme) couples proton circulation back across the membrane to the synthetic of ATP.


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Figure 10.22

Electron transport and ATP synthesis throughout photosynthesis. Five protein complexes in the thylakoid membrane role in electron transport and also the synthesis of ATP and NADPH. Photons are took in by complexes of colors molecules associated with photosystems (more...)


One important difference in between electron carry in chloroplasts and also that in mitochondria is that the energy acquired from sunlight during photosynthesis not just is converted to ATP but also is supplied to generate the NADPH forced for succeeding conversion the CO2 come carbohydrates. This is accomplished by the usage of two various photosystems in the irradiate reactions that photosynthesis, one to generate ATP and the other to create NADPH. Electrons space transferred sequentially in between the two photosystems, with photosystem i acting to create NADPH and also photosystem II acting to generate ATP.

The pathway that electron flow starts at photosystem II, i beg your pardon is homologous come the photosynthesis reaction center of R. Viridis currently described. However, in ~ photosystem II the energy acquired from absorb of photons is supplied to separation water molecules to molecule oxygen and protons (see number 10.22). This reaction takes ar within the optical membrane lumen, for this reason the relax of proton from H2O develops a proton gradient throughout the optical membrane membrane. The high-energy electrons acquired from this process are transferred with a series of carrier to plastoquinone, a lipid-soluble carrier similar to coenzyme Q (ubiquinone) the mitochondria. Plastoquinone carries electrons from photosystem II come the cytochrome bf complex, within which electrons room transferred come plastocyanin and additional protons are pumped into the optical membrane lumen. Electron transport v photosystem II is therefore coupled to facility of a proton gradient, which drives the chemiosmotic synthesis of ATP.

From photosystem II, electron are carried by plastocyanin (a peripheral membrane protein) come photosystem I, where the absorption of extr photons again generates high-energy electrons. Photosystem I, however, does no act together a proton pump; instead, it offers these high-energy electrons to minimize NADP+ to NADPH. The reaction facility chlorophyll the photosystem ns transfers its excited electrons through a series of carrier to ferrodoxin, a little protein top top the stromal next of the optical membrane membrane. The enzyme NADP reductase climate transfers electron from ferrodoxin to NADP+, generating NADPH. The passage of electrons v photosystems I and II therefore generates both ATP and also NADPH, i m sorry are used by the Calvin cycle enzymes in the chloroplastic stroma to convert CO2 to carbohydrate (see figure 2.39).


Cyclic Electron Flow

A second electron move pathway, called cyclic electron flow, produce ATP without the synthetic of NADPH, thereby supplying additional ATP for other metabolic processes. In cyclic electron flow, light power harvested at photosystem ns is used for ATP synthesis quite than NADPH synthesis (Figure 10.23). Rather of being moved to NADP+, high-energy electron from photosystem I are transferred come the cytochrome bf complex. Electron transfer v the cytochrome bf facility is climate coupled, together in photosystem II, to the establishment of a proton gradient throughout the optical membrane membrane. Plastocyanin climate returns these electrons come photosystem ns in a lower energy state, completing a bicycle of electron deliver in i m sorry light energy harvested in ~ photosystem i is provided to pump protons at the cytochrome bf complex. Electron move from photosystem I can thus create either ATP or NADPH, relying on the metabolic demands of the cell.


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Figure 10.23

The pathway that cyclic electron flow. Light energy soaked up at photosystem i (PS I) is supplied for ATP synthesis rather than NADPH synthesis. High-energy electrons generated by photon absorption room transferred come the cytochrome bf complex rather 보다 to (more...)


ATP Synthesis

The ATP synthase of the thylakoid membrane is similar to the mitochondrial enzyme. However, the energy stored in the proton gradient across the thylakoid membrane, in comparison to the inside mitochondrial membrane, is practically entirely chemistry in nature. This is due to the fact that the thylakoid membrane, although impermeable come protons, differs from the within mitochondrial membrane in being permeable to other ions, specifically Mg2+ and also Cl-. The complimentary passage the these ions neutralizes the voltage ingredient of the proton gradient, so the energy derived from photosynthesis is conserved mostly as the distinction in proton concentration (pH) throughout the optical membrane membrane. However, since the thylakoid lumen is a closeup of the door compartment, this difference in proton concentration can be fairly large, equivalent to a differential of an ext than 3 pH units in between the stroma and the optical membrane lumen. Due to the fact that of the size of this pH differential, the total complimentary energy stored across the thylakoid membrane is similar to that stored throughout the within mitochondrial membrane.

For each pair of electron transported, 2 protons are transferred throughout the thylakoid membrane at photosystem II and two to 4 protons in ~ the cytochrome bf complex. Because four proton are required to drive the synthetic of one molecule that ATP, i of every pair that electrons v photosystems I and II by noncyclic electron flow yields between 1 and 1.5 ATP molecules. Cyclic electron flow has a reduced yield, matching to in between 0.5 and 1 ATP molecules every pair the electrons.

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