Thylakoid Membranes in Chloroplasts

Most of the arguments described in the sections on bacterial signal peptides and membrane proteins seem to be valid for the eukaryotic systems, as well as the translocation phenomena across the ER membrane (Sakaguchi, 1997). They seem to be also true for the translocation system across the mitochondrial inner membrane protein into the intermembrane space and the system across the thylakoid membrane in chloroplasts. Although the TAT-dependent pathway has not been found in the ER, it exists on the thylakoid membrane (and possibly on the inner membrane of mitochondria). 

Simpson, D.J. and Robinson S.P. 1984. Freeze-fracture ultrastructure of thylakoid membranes in chloroplasts from manganese-deficient plants. Plant Physiol. 74, 735-741. 

Like Complex III of mitochondria, cytochrome b6f conveys electrons from a reduced quinone—a mobile, lipid-soluble carrier of two electrons (Q in mitochondria, PQb in chloroplasts)—to a water-soluble protein that carries one electron (cytochrome c in mitochondria, plastocyanin in chloroplasts). As in mitochondria, the function of this complex involves a Q cycle (Fig. 19-12) in which electrons pass, one at a time, from PQBH2 to cytochrome bs. This cycle results in the pumping of protons across the membrane in chloroplasts, the direction of proton movement is from the stromal compartment to the thylakoid lumen, up to four protons moving for each pair of electrons. The result is production of a proton gradient across the thylakoid membrane as electrons pass from PSII to PSI. Because the volume of the flattened thylakoid lumen is small, the influx of a small number of protons has a relatively large effect on lumenal pH. The measured difference in pH between the stroma (pH 8) and the thylakoid lumen (pH 5) represents a 1,000-fold difference in proton concentration—a powerful driving force for ATP synthesis. 

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 transport of proteins into chloroplasts also occurs by more than one mechanism. An SRP-dependent pathway may be needed only for insertion of proteins into membranes.594 Other proteins, among which are the 23-kDa and 16-kDa photosystem II proteins (Chapter 23), enter by a pathway related to the Tat pathway of bacteria. In thylakoids this pathway is directly dependent upon the large pH difference (A pH) across die thylakoid membrane. In contrast to the bacterial Sec pathway, the A pH pathway seems to be able to transport completely folded proteins. 

Some proteins enter membranes immediately after synthesis. The translocon channel is not required. However, in E. coli an additional protein YidC is needed.603 Homologs of this protein are found in mitochondria (Oxal protein) and in thylakoid membranes of chloroplasts (Alb3 protein).608 These proteins may function in cotranslational insertion. If a protein carries a... 

Figure 17.2 illustrates our model for splitting water by solar energy. I" is important that all the redox reactions involved in thf system be reversible. The quinone compound in the organic solvent combines the two photocatalytic reactions, and its function can be compared to the electron relaying molecules in thylakoid membranes of chloroplasts. Electron transfer reactions via quinone compouncs in artificia systems have been studied as a model of photosynthesis22-23 and in an electrochemical system for acid concentration.24 ... 

In the Z scheme, photosystem II, the cytochrome b6f complex and photosystem I operate in series to move electrons from H20 to NADP+ and to create an electrochemical potential gradient for protons across the thylakoid membrane. In addition to this linear pathway, chloroplasts in some plant species may use a cyclic electron-transfer scheme that includes photosystem I and the cytochrome b6f... 

Granum (pi. grana) A closely packed stack of thylakoid membranes in a chloroplast. 

The Tat and YidC systems are also found in certain organelles in eukaryotic cells. The former is present in the thylakoid membrane in plant cells, and the latter is found both in the thylakoid membrane (where it is called the Albino3 system) and in the inner mitochondrial membrane (where it is called the Oxalp system). Both chloroplasts and mitochondria appear to have unique systems for importing proteins across their outer and inner membranes, and these systems also handle membrane proteins. 

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. 

---