The Thylakoid Membrane of Chloroplasts

In green plants, chloroplasts are the site of photosynthesis, and we begin by examining their presence, location and stmcture in leaf tissue at several different organizational levels, starting with the leaf itself, as illustrated in Pig. 13. Pigs. 13 (A) and (B) are sketches of a (spinach) leaf and a magnified cross- 

Each chloroplast is bounded by an envelope of a highly permeable outer membrane and a nearly impermeable inner membrane, the two membranes being separated by a narrow, inter-membrane compartment [see Fig. 13 (C) and (C )]. The outer membrane allows small molecules to pass through, while the inner membrane presents a barrier which allows only certain metabolites or ATP to pass through with the help of special transport proteins embedded in the membrane. Enclosed by the inner membrane is the stroma, a concentrated solution containing the enzymes necessary for CO fixation, i.e., its conversion into carbohydrates. The stroma also contains the chloroplast s own DNA, RNA and ribosomes involved in the synthesis of proteins. This chloroplast stroma is analogous to the matrix in mitochondria. 

The thylakoids and stroma are the sites of the so-called light and dark reactions of photosynthesis, respectively. This compartmentalization of photosynthetic functions was recognized by Park and Pon when they broke open the chloroplasts, separated the contents into thylakoid and stroma fractions and examined their properties. The specific activities of the thylakoids include photochemical reactions, electron transport, oxygen evolution, ATP synthesis and NADP reduction, while the stroma contains enzymes for CO2 fixation driven by ATP and NADPH and other biochemical reactions in the dark. Our understanding and appreciation of the detailed structure and organization of the thylakoid membranes has increased tremendously in recent years. Further discussion of thylakoid structure will be continued in section VII on page 26. 

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The photosynthetic apparatus of modem cyanobacteria, algae, and vascular plants is more complex than the one-center bacterial systems, and it appears to have evolved through the combination of two simpler bacterial photocenters. The thylakoid membranes of chloroplasts... 

In most instances, either for mitochondrial suspensions or whole bacteria, ApH is less negative than -0.5 unit making a contribution of, at most, -30 mV to Ap. The exception is found in the thylakoid membranes of chloroplasts (Chapter 23) in which protons are pumped into the thylakoid vesicles and in which the internal pH falls dramatically upon illumination of the chloroplasts.185 The ApH reaches a value of -3.0 or more units and Ap is 180 mV, while Em remains 0. Reported values of Em for mitochondria and bacteria range from -100 to -168 mV and Ap from -140 to -230 mV.172 179 Wilson concluded that Em for actively respiring mitochondria, using malate or glutamate as substrates,... 

The proteins that participate in the light reactions of photosynthesis are located in the thylakoid membranes of chloroplasts. The light reactions result in (1) the creation of reducing power for the production of NADPH, (2) the generation of a transmembrane proton gradient for the formation of ATP, and (3) the production of O2. 

Animals and bacteria are heterotrophs they obtain carbon in various forms as food and metabolize many forms of it to provide energy and body structure. Plants are autotrophs all their carbon comes from C02 powered by photosynthesis. Photosynthesis occurs within the thylakoid membranes of chloroplasts in plant leaves, and it is mediated by chlorophyll. The light reaction splits water into 02, electrons, and protons (H+). NADPH is produced by electron transport and ATP synthesis by associated proton transport. 

Absorption of Light Energy and the Fate of the Electronically Excited States Light-Harvesting Pigment Molecules Development of Some Modern Concepts of Photosynthesis The Thylakoid Membrane of Chloroplasts... 

ATP is generated in the thylakoid membranes of chloroplasts by a complex called CFO-CFl and is very similar to the FOFl complex of mitochondria. As in mitochondria, passage of protons through the ATP synthase complex (CFO-CFl), is the driving force (and energy source) for ATP synthesis. 

How does the structure of the chloroplast affect photosynthesis In eukaryotes, the light reactions of photosynthesis take place in the thylakoid membranes of chloroplasts. A series of membrane-bound electron carriers and pigments is able to harness the light energy of the Sun. The equation for photosynthesis... 

Photosystem II (PSII) is a large, heteromeric enzyme complex with more than twenty different protein subunits and an array of cofactors, that participate in the photosynthetic electron transport in the thylakoid membranes of chloroplasts and cyanobaaeria. PSII demonstrates the oxido-reductase aaivity and couples the oxidation of H2O with the reduction of plastoquinones through a series of intermediate redox reactions. The central core of the PSII reaction center is composed of D1 and D2 proteins associated >vith redox active components. These compounds include a tetra-manganese cluster, two redox-active residues, four to six chlorophyll a molecules, two pheophytins, and plastoquinones and The secondary plastoquinone Qg is a two-electron carrier which... 

The cytochromes are probably the most important class of electron-transfer proteins. They are involved in photosynthesis, embedded in the thylakoid membranes of chloroplasts. They are also involved in the opposite process of cellular respiration in plants and animals, which occurs in the membranes of mitochondria. ... 

If reduction of the oxidized nicotinamide were to occur in an appropriately structured and carboxylate-containing protein within a membrane, however, the pickup of proton by the carboxylate could occur from one side of a membrane, thereby lowering the concentration on that side, and on oxidation the proton release could occur to a confined space on the other side, thereby increasing the proton concentration within the confined space on the other side. Our proposal is that the cyclic reduction and oxidation of redox couples within the thylakoid membrane of chloroplasts and within the inner mitochondrial membrane achieves proton pumping by means of AGap, the aqueous mediated repulsion between oil-like and charged groups that disrupts association of oil-... 

Although there are minor differences, ATPase is highly conserved across living systems. It is found in the plasma membranes of bacteria as well as in the thylakoid membranes of chloroplasts and mitochondria of animals (e.g., PDB code Ibmf). 

In thermodynamics, one deals with closed and open systems, the difference between the two being that the latter involves the exchange of matter in addition to energy (heat and work), Clearly, a cell is an open system. Similarly, organelles such as chloroplasts and mitochondria are also open systems. Other energy transducing systems of interest are found in bacteria, in visual receptors. We shall mainly focus out attention on the thylakoid membrane of chloroplasts and the cristae membrane of rnito-chondria. It is particularly noteworthy that energy transduction and material transport in these two systems are coupled the products of photosynthesis are utilized as the reactants in respiration, and vice versa. 

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