In marine systems, nitrogen is fixed primarily by a single genus of cyanobacteria, Trichodesmium. These organisms are unusual in that they fix N2 and produce O2 from photosynthesis within the same cell at the same time. The nitrogenase in Trichodesmium is presumed to contain molybdenum and a lot of iron, as it does in other cyanophytes. Because of the massive iron requirement of the nitrogenase enzyme, some scientists have hypothesized that N2-fixation in oceans may be limited globally by iron. Two Trichodesmium strains are presently in culture in Cebic laboratories, and the genes for several of the subunits of the multimeric nitrogenase complex have been sequenced.
Algae are a very diverse group of predominantly aquatic photosynthetic organisms that account for almost 50% of the photosynthesis that takes place on Earth. Algae have a wide range of antenna pigments to harvest light energy for photosynthesis giving different types of algae their characteristic colour. Early work done with algae contributed much to what is presently known about the carbon dioxide fixation pathway and the light harvesting reactions. The processes of photosynthesis in algae and higher plants are very similar. From among the three types of carbon dioxide‐concentrating mechanisms known in photosynthetic organisms, two types are found in different types of algae. Algae are proposed to play a role in the global carbon cycle by helping remove excess carbon dioxide from the environment. Recently, algae are recognized as a promising biodiesel source due to its efficient absorption and conversion of solar energy into chemical energy.
Photosynthesis - PRE-ORDER | IRON BEAN Games
If one plastoquinol simply delivered its electrons and protons and was done, there would be two protons delivered per plastoquinol. That's one proton output per electron that was input.A second plastoquinol would do exactly the same thing. There would be four protons output for four electrons initially input. That's still one proton output per electron that was input.Figure PS5.4. Diagram showing protons pumped per electron consumed, with the addition of the Q-loop. Inputs are shown in red and outputs in blue. Recycled elements are provided in green. If, instead, one electron is recycled each time, then every second plastoquinol leads to the delivery of an extra pair of protons. That's because in picking up the recycled electrons, a plastoquinone has had to travel back to the stroma side of the membrane and pick up two more protons. Overall, that means six protons are delivered for four electron input, or 1.5 protons output per electron input. Since the proton gradient is what is generating the ATP, then by increasing the number of protons pumped per electron coming in, efficiency is increased. The efficiency advantage is even greater than that described above. The Q-loop is thought to double the amount of protons pumped by cytochrome b6f. Explain why. The electron transport chain through cytochrome b6f is rather short. The electron is first passed to a "high-potential FeS cluster", or Rieske cluster. It differs from most FeS clusters in that two of the amino acid residues that bind it in the protein are histidines, rather than the usual cysteines. Rieske clusters usually have higher reduction potentials than other FeS clusters. They were one of the last in a series of FeS clusters encountered in the electron transport chain in oxidative phosphorylation, and their more positive reduction potential was needed to keep the electron transport chain moving in the right direction.Figure PS5.5. Coordination environment of the iron atoms in a Rieske FeS cluster. The reduction potential of a Rieske FeS cluster is generally more positive than that of a regular FeS cluster, in which the Fe2S2 core is held in place by four cysteines. Explain why. The final stop for the electron is a small, soluble protein, plastocyanin. The electron carrier in plastocyanin is not an iron, but rather a copper atom. The copper ion, which can toggle between Cu+ and Cu2+, is held in place by two histidines, a methionine, and a cysteine. The plastocyanin transports the electron through the lumen to the next complex, photosystem I. Figure PS5.6. Coordination environment of the copper ion in plastocyanin. Problem PS5.3.Use periodic trends to suggest a reason why a copper atom appears later in the electron transport chain, whereas several iron atoms appeared earlier in the chain. Problem PS5.4.a) What would be the overall charge on the coordination complex in plastocyanin, assuming the Cu(II) state?b) What would be the overall charge on the coordination complex in plastocyanin, assuming the Cu(I) state?c) Based on charge considerations alone, how would you expect the reduction potential of plastocyanin to compare to the Rieske FeS cluster? 1. X-ray crystal structures: Kurisu, G.; Zhang, H.; Smith, J.L.; Cramer, W.A. Structure of the cytochrome b6f complex of oxygenic photosynthesis: tuning the cavity. Science 2003, 302, 1009-1014. Images obtained via RCSB Protein Data Bank (1VF5).
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