Thylakoid membrane proton gradient across

The enzyme ferridoxin (Fd) NADP + oxidoreductase accepts the electron from Fd, one at a time, as it proceeds from its oxidized form through a semiquinone intermediate to its fully reduced form. The enzyme then transfers a hydride ion to NADP converting to its reduced state NADPH. Uptake of a proton in the reduction of NADP+ further contributes to the proton gradient across the thylakoid membrane driving ATP synthesis. 

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. 

Answer Illumination of chloroplasts in the absence of ADP and P sets up a proton gradient across the thylakoid membrane. When ADP and P, are added, ATP synthesis is driven by the gradient. In the absence of continuous illumination, the gradient soon becomes exhausted and ATP synthesis stops. 

The light catalyzed oxidation of water by the PSII RC complex yields three products, O2, electrons, and protons (Fig. 1). Oxygen gas diffuses out of the cells while the protons accumulate in the lumenal space, since the latter cannot cross the lipophilic thylakoid membranes. During electron transfer from PSII to PSI, two electrons, coupled with two protons, are transported from the stromal side (outside) of the thylakoid membrane to the lumen (inside). This also contributes to the accumula tion of a proton gradient across the membrane. The proton gradient creates a proton... 

Mitchell s theory holds that an electrochemical proton gradient across the membrane (which is only slightly permeable to many ionized species and particularly to H ") is formed by the vectorial transport of into the thylakoid lumen coupled to electron transport, as a consequence of the alternate disposition across the membrane of electron carriers which can bind protons and others which cannot be protonated. 

The oxidation of plastoquinol results in the release of two protons into the thylakoid lumen. In the second half of the Q cycle (Section 18.3.4). cytochrome hf reduces a second molecule of plastoquinone from the Q pool to plastoquinol, taking up two protons from one side of the membrane, and then reoxidizes plastoquinol to release these protons on the other side. The enzyme is oriented so that protons are released into the thylakoid lumen and taken up from the stroma, contributing further to the proton gradient across the thylakoid membrane (Figure 19.18). 

A Proton Gradient Across the Thylakoid Membrane Drives ATP Synthesis... 

The proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP. The ATP synthase of chloroplasts (also called CFq-CFj) closely resembles the ATPsynthesizing assemblies of bacteria... 

When the plastohydroquinone becomes fully oxidized at the lumenal surface ofthe membrane, it loses two electrons and also releases two protons to the (inside) lumen phase. Thus accompanying the reduction of one plastoquinone molecule by PS 11 and its subsequent reoxidation, there is a net transfer of two protons from the (outside) stromal phase into the (inside) lumen phase. Thus with the splitting of two water molecules by PS 11 to form one oxygen molecule, four protons are translocated across the membrane. As mentioned above, oxidation of two water molecules also releases an additional four protons into the lumen space. Thus water splitting and plastoquinone reduction/re-oxidation result in the generation of eight protons and the creation of a proton gradient across the thylakoid membrane. 

Electron transport and ATP production are coupled to each other by the same mechanism in mitochondria and chloroplasts. In both cases, the coupling depends on the generation of a proton gradient across the inner mitochondrial membrane or across the thylakoid membrane, as the case may be. 

In eukaryotes, oxidative phosphorylation occurs in mitochondria, while photophosphorylation occurs in chloroplasts to produce ATP. Oxidative phosphorylation involves the reduction of O2 to H2O with electrons donated by NADH and FADH2 in all aerobic organisms. After, carbon fuels (nutrients) are oxidized in the citric acid cycle, electrons with electron-motive force is converted into a proton-motive force. Photophosphorylation involves the oxidation of H2O to O2, with NADP as electron acceptor. Therefore, the oxidation and the phosphorylation of ADP are coupled by a proton gradient across the membrane. In both organelles, mitochondria and chloroplast electron transport chains pump protons across a membrane from a low proton concentration region to one of high concentration. The protons flow back from intermembrane to the matrix in mitochondria, and from thylakoid to stroma in chloroplast through ATP synthase to drive the synthesis of adenosine triphosphate. Therefore, the adenosine triphosphate is produced within the matrix of mitochondria and within the stroma of chloroplast. 

---