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LectNotes&Reading_NRGGeneration - Energy Generation...

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Unformatted text preview: Energy Generation: Fermentation, Respiration and Photosynthesis Overview Prokaryotic organisms exhibit a truly bewildering array of metabolism, which is one of the keys to the success of these organisms in colonizing almost every environment on this planet. In this lecture, we will look at the diversity of pathways that prokaryotes have for generating energy (i.e. ATP). If you feel you need a basic review, I can recommend Chapter 8, Section 8.1 (pg. 203-206). You should be familiar with the knowledge that ATP and NADH are high-energy carrier molecules of the cell. Fermentation Reading: Section 8.2 (pg. 206-210). Key features of fermentation: 1. ATP is generated through substrate-level phosphorylation only. A high-energy intermediate is generated that can then be used to transfer phosphate to ADP. 2. Oxidation products are reutilized as electron acceptors to balance electron flow. The oxidation of a carbon source such as glucose is coupled to the reduction (i.e. electron flow) to NAD+ to yield NADH. NADH must be re-oxidized to NAD+ so that the energy yielding reactions can continue. To oxidize NADH, an oxidized product, such as pyruvate, is reduced by NADH to yield NAD+. 3. Oxidation of a starting carbon compound is incomplete. Fermented carbon compounds are not completely oxidized to CO2. Thus, less energy is generated than during respiration, which can support complete oxidation. Electron Transfer Systems: Reading: Section 8.3 to 8.5 (pg. 210-222). 1. Electron Transfer Systems are used for both respiration and photosynthesis. 2. Electrons are passed from an electron donor to an electron acceptor. 3. The likelihood that a molecule will serve as an electron acceptor or electron donor is determined by that molecule’s redox potential, E. The more negative the redox potential, the more likely a molecule is to serve as an electron donor, and the more positive the redox potential, the more likely a molecule is to serve as an electron acceptor. Redox potentials are represented in an “electron tower”, and in general, the electrons will move “down the electron tower”. 4. Redox potential, E, directly relates to ∆G. Thus, the greater the difference in E between a donor and acceptor, the greater the energy yield (-∆G). 5. Electron Transport systems are comprised of proteins and cofactors that are imbedded in the membrane and have each a slightly different redox potential. 1 6. The energy derived from electron transfer is used to transport protons from the cytoplasm to the other side of the cytoplasmic membrane. + This creates a higher concentration of H outside than inside the cell, the basis of the proton motive force (PMF). The PMF is composed of a concentration gradient (∆pH) and a charge differential (∆Ψ) across the membrane, which allows the PMF to be useful to cells grown in different environments (e.g. alkaline or high salt). 7. An ETS will be composed of at least 3 parts: a) a substrate oxidoreductase, which is often called a dehydrogenase if it removes H+ from its substrate b) a quinone c) a terminal oxidase Different substrate oxidoreductases and terminal oxidases differ in their ability to pump H + across the membrane. Quinone pumps 2 H . + 8. The ATP synthase uses the PMF to synthesize ATP. + It can also hydrolyze ATP to pump H and generate a PMF. Respiration: Reading: Section 8.6 (pg. 222-228). 1. In the absence of O2, other terminal oxidases are expressed to use alternative terminal e- acceptors. 2. In the absence of organic carbon to generate NADH, other e- donors are used to support an ETS. Photosynthesis (or to be more technically correct Photolysis): Reading: Section 9.2 and 9.3(pg. 235-249). 1. Energy comes from the absorption of light. 2. A chlorophyll molecule absorbs this light causing an e- in the chlorophyll to transfer to a higher energy level. This results in a more negative redox potential for the chlorophyll. Thus, the role of this light is to convert a poor e donor to a good e donor. 3. The chlorophyll molecule is housed by a reaction center that is surrounded by a light absorbing and funneling antenna complex. 4. The high-energy state e- is transferred to an ETS to generate a PMF. In order for chlorophyll to reset after releasing an electron, it must receive an electron. This can come from H20 in the case of Z-pathway of oxygenic photosynthesis, H2S for example in the case of photosystem I (PSI), or cytochrome C and cyclic e flow in the case of photosystem II (PSII). 5. The amount of work that can be accomplished by a photosystem is determined the energy of the light absorbed. 2 PSII of alpha-proteobacteria absorb the longest wavelength of light, which is insufficient to + split a hydrogenated substrate or reduce NADP . PSI of chlorobia absorbs a slightly shorter wavelength of light. This photosystem generates enough energy to split H2S, H2 or an organic + electron donor and to reduce NADP , but it does not generate enough energy to split H20. PSII of oxygenic photosynthetic pathways absorbs the shortest wavelengths of light and is able to spilt H2O. 3 ...
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