A photosystem consists of a light-harvesting complex and a reaction center. Pigments in the light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center. The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor. The excited electron must then be replaced. In a photosystem II, the electron comes from the splitting of water, which releases oxygen as a waste product.
In b photosystem I, the electron comes from the chloroplast electron transport chain discussed below. The actual step that converts light energy into chemical energy takes place in a multiprotein complex called a photosystem , two types of which are found embedded in the thylakoid membrane, photosystem II PSII and photosystem I PSI Figure 2. The two complexes differ on the basis of what they oxidize that is, the source of the low-energy electron supply and what they reduce the place to which they deliver their energized electrons.
Both photosystems have the same basic structure; a number of antenna proteins to which the chlorophyll molecules are bound surround the reaction center where the photochemistry takes place. Each photosystem is serviced by the light-harvesting complex, which passes energy from sunlight to the reaction center; it consists of multiple antenna proteins that contain a mixture of — chlorophyll a and b molecules as well as other pigments like carotenoids.
In short, the light energy has now been captured by biological molecules but is not stored in any useful form yet. The energy is transferred from chlorophyll to chlorophyll until eventually after about a millionth of a second , it is delivered to the reaction center.
Up to this point, only energy has been transferred between molecules, not electrons. Figure 2. The electron transport chain moves protons across the thylakoid membrane into the lumen. Each seedling was fertilized with g of NPK and 40 g of magnesium sulfate. The data were collected in July between p.
The selected leaves were subjected to a 20 minutes period of adaptation to darkness, sufficient for complete oxidation of the reaction centres. Absorbance ABS referred to the absorption of photons by the chlorophyll molecules in the antenna complex. The second, SFI No , refers to the energy that is dissipated or lost from photosynthetic electron transport Strasser et al.
It is defined as:. E xperimental design and statistical analysis: The experiment was arranged in a completely randomized design with four species and ten replicates per species. The computer program used to conduct the statistical analyses was SAEG 5. Examining the capture and use of energy by the species used in this study, it was found that with increased specific fluxes per reaction centre , B.
With regard to the phenomenological fluxes, it was observed that E. This lack of photochemical efficiency resulted in higher dissipation of energy in the form of heat and fluorescence, especially in I. The high values of energy dissipation per reaction centre and consequently per cross section, can be explained by the fact that B. This could have occurred through damage caused to the active reaction centres, resulting in low photochemical efficiency in PS II f Po table 1.
The low density of active reaction centres may have also overloaded the working reaction centres in B. However, in compensation, this could have greatly increased the dissipation of light energy in the form of heat and fluorescence resulting in lower efficiency in PS II figure 1A and table 1.
As for the f Po , it was found the values ranged from 0. The low values for f Po found in B. Analyzing the efficiency of electron transfer beyond Q A , it was found that E. This suggests that the probability of excited energy entering in the electron transport chain was greater in E. The low values of y 0 in S. These proteins, called PsaA and PsaB, form the heterodimeric protein structure of the reaction center.
The organization of cofactors in the PSI reaction center is shown in Figure 7. Figure 7. The "special pair" of chlorophyll a molecules called P is shown in red. The other chlorophyll a species that are part of the pathway of electron transfer are shown in green, and the phylloquinone molecules are shown in dark blue. The primary electron acceptor is the four iron four sulfur cluster called F X , shown here as a red iron and yellow sulfur cube. Once again, a symmetric disposition of the cofactors is revealed by the crystal structure, but this time there's a significant difference in the electron acceptor.
In PSI, the cofactor "legs" come together at a single electron acceptor called F X , an iron-sulfur protein consisting of four Fe atoms and four inorganic sulfides.
The present view is that excitation of P results in charge separation via electron transfer down either "leg" of cofactors chlorophylls A and A 0 , phylloquinone. Figure 7 represents this by way of the two arrows, one from each "leg", directed at the primary electron acceptor F X. Photooxidized P is reduced by the water soluble copper protein called plastocyanin, or in some cases by a water soluble cytochrome.
Details of these reactions can be found in Berg, et al. Summary Photosynthetic reaction centers can be shown to have remarkably similar structures, comprised of two branches of cofactors that are made up of dimeric chlorophylls, in the case of light absorption, and either pheophytins or monomeric chlorophylls that are the components of the branches that transfer electrons to the ultimate electron acceptors, quinones or an iron-sulfur cluster.
In bacteria and in photosystem II, only one branch is functional, and that branch delivers electrons to a tightly bound quinone, which then reduces a second, exchangeable quinone. The function of the "inactive" branch in the reaction centers is not obvious, and remains an interesting enigma concerning the structure and function of photosynthetic reaction centers. The crystal structure of plant photosystem I. Nature , Berg, J.
Chapter Freeman and Company, New, NY Dekker, J. Supramolecular organization of thylakoid membrane proteins in green plants. Acta , Jordan, P. Three-dimensional structure of cyanobacterial photosystem I at 2. Deisenhofer, J. Crystallographic refinement at 2. Ferreira, K. Architecture of the photosynthetic oxygen-evolving center. Science , Liu, Z. Crystal structure of spinach major light-harvesting complex at 2.
Once the light energy has been absorbed directly by the pigment molecules, or passed to them by resonance transfer from antenna pigments, they release two electrons into an electron transport chain. Light is made up of small bundles of energy called photons. If a photon with the right amount of energy hits an electron it will raise the electron to a higher energy level. Electrons are most stable at their lowest energy level or ground state, the orbit in which the electron has the least amount of energy.
Electrons in higher energy levels can return to ground state in a manner analogous to a ball falling down a staircase.
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