Photosynthesis electron flow

While this electron flow takes place, the cytochrome on the periplasmic side donates an electron to the special pair and thereby neutralizes it. Then the entire process occurs again another photon strikes the special pair, and another electron travels the same route from the special pair on the periplasmic side of the membrane to the quinone, Qb, on the cytosolic side, which now carries two extra electrons. This quinone is then released from the reaction center to participate in later stages of photosynthesis. The special pair is again neutralized by an electron from the cytochrome. 

The pathway of electron flow is called the Z scheme of photosynthesis because the diagram from p680 to p700 resembles the letter Z (Figure 7.10) [37,39]. 

N2-fixing organisms, the enzyme is found in both the vegetative cells and the heterocysts. In A. variabilis and A. nidulans the enzyme is thought to be membrane bound. Examination of the interrelationship between photosynthesis and respiration has led to speculation that the role of the enzyme may be to control electron flow in respiration and photosynthesis (Appel and Schulz 1998, 2000) (Fig. 2.2D). 

Photosynthesis is the reduction of C02 by electrons from water with the help of visible irradiation producing carbohydrate and oxygen. The outline of the electrone flow is expressed by Fig. 1. The electron from water is pumped up twice by photosystems II and I (PS II and I), where chlorophyll (Chi) molecules play the main role for the excitation, energy concentration, and charge separation. 

Tetranuclear iron-sulfur clusters are key relay stations in the electron flow in photosynthesis. Photosystem I comprises three subunits, PsaA, PsaB and PsaC. The latter contains two [Fe4S4] centres FA and FB. The core subunits PsaA and B, respectively, house a [Fe4S4] centre denoted FX in addition to other, organic cofactors. The role of this latter cluster was probed in preparations partially devoid of PsaC. It was concluded from the results that FX has a major role in controlling the electron transport through PS I.236 Since the final acceptor of the electrons in PS I is a ferredoxin with a [Fe2S2] cluster it was of interest to study a... 

When electrons flow from photosystem I to photosystem II, protons are transported across the chloroplast membranes as indicated in Figure E9.1. This aspect of photosynthesis will be discussed in a later section. 

Fig. 19.1 Energy cycle on the earth represented by electron flow driven by solar energy where electrons are provided by water. This energy cycle is supported by photosynthesis represented by Eq.(19.1).

Although catalytic water oxidation (dark reaction) is the first and important reaction of the electron flow in the photosynthesis represented by Fig. 19.1 whereby water is used as the source of electrons provided to the whole system, its catalyst and reaction mechanism are not yet established.10-13) In the photosynthesis Mn-protein complex works as a catalyst for the difficult four-electron oxidation of two molecules of water to liberate one 02 molecule (Eq. (19.2)). It is inferred that at least four Mn ions are involved in the active center, but its structure is not yet completely elucidated. 

Photosystems I and II operate in concert. Their interaction is described in the Z scheme (shown in outline in Figure 18). In photosystem II, the primary oxidant is able to remove electrons from water. These electrons are transported to photosystem I via plastoquinone and plastocyanin to replace PSI electrons that have been used in the reduction of iron-sulfur proteins and transferred via NADP to 0O2. Electron flow between PSII and PSI is accompanied by the synthesis of Atp 367 These oxidizing and reducing aspects of photosynthesis can be separated and other substrates incorporated. 

However, some evidence of a significant electrical conductance in biomaterials was already available in the 1960s. For example, significant conductance was found (Digby, 1965) in crustaceans. Indirect support also came from mechanisms involving electron flow which seemed necessary to explain phenomena in photosynthesis, in enzyme reactivity, and in the energy-producing activities in mitochondria. 

Michael Wasielewski of Northwestern University asked Thomas Moore about the type of light fluxes being used to investigate the solar flux. He also asked, Since we all know that photosynthesis has control mechanisms that actually modify electron flow, based on light flux, what kind of prospectus or perspective do we have for control mechanisms in such systems Moore explained that one of the factors that seems to limit natural photosynthesis is the diffusion of carbon dioxide into the system for fixing, so it is important in photosynthesis to throttle back the powerful oxidant when carbon dioxide is limiting. There is a control mechanism called nonphotochemical quenching that is related to the... 

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. 

We begin our discussion of electron flow in photosynthesis with the water oxidation step ... 

For cyclic electron flow, an electron from the reduced form of ferredoxin moves back to the electron transfer chain between Photosystems I and II via the Cyt bCyclic electron flow does not involve Photosystem II, so it can be caused by far-red light absorbed only by Photosystem I — a fact that is often exploited in experimental studies. In particular, when far-red light absorbed by Photosystem I is used, cyclic electron flow can occur but noncyclic does not, so no NADPH is formed and no O2 is evolved (cyclic electron flow can lead to the formation of ATP, as is indicated in Chapter 6, Section 6.3D). When light absorbed by Photosystem II is added to cells exposed to far-red illumination, both CO2 fixation and O2 evolution can proceed, and photosynthetic enhancement is achieved. Treatment of chloroplasts or plant cells with the 02-evolution inhibitor DCMU [3-(3,4-dichlorophenyl)-l, 1-dimethyl urea], which displaces QB from its binding site for electron transfer, also leads to only cyclic electron flow DCMU therefore has many applications in the laboratory and is also an effective herbicide because it markedly inhibits photosynthesis. Cyclic electron flow may be more common in stromal lamellae because they have predominantly Photosystem I activity. 

Many organic compounds involved in photosynthesis accept or donate electrons (see Table 5-3). The negatively charged electrons spontaneously flow toward more positive electrical potentials (A > 0), which are termed redox potentials for the components involved with electron flow in chlo-roplast lamellae (Fig. 1-10) or the inner membranes of mitochondria (Fig. 1-9). Redox potentials are a measure of the relative chemical potential of electrons accepted or donated by a particular type of molecule. The oxidized form plus the reduced form of each electron transfer component can be regarded as an electrode, or half-cell. Such a half-cell can interact with other electron-accepting and electron-donating molecules in the membrane, in which case the electrons spontaneously move toward the component with the higher redox potential. 

Another class of energy storage compounds consists of redox couples such as NADP+-NADPH (Table 6-1). The reduced form, NADPH, is produced by noncyclic electron flow in chloroplasts (Chapter 5, Section 5.5C). Photosynthesis in bacteria makes use of a different redox couple, NAD+-NADH. The reduced member of this latter couple also causes an... 

The activities of chloroplasts and mitochondria are related in various ways (Fig. 6-7). For instance, the O2 evolved by photosynthesis can be consumed during respiration, and the CO2 produced by respiration can be fixed by photosynthesis. Moreover, ATP formation is coupled to electron flow in... 

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