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UW-Milwaukee BIOSCI 152 - Microbial Metabolism

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BIO SCI 152 1st Edition Lecture 4Lectures 3 and 4: Microbial MetabolismMetabolism- the sum of all of the chemical reactions that occur in a cellCatabolism- breakdown of complex organic compounds into simpler ones with the release of energy. Anabolism- Synthesis of cell parts from simple chemicals. Requires energy input. Oxidation-Reduction ReactionsOxidation- the removal of electrons (or electrons + protons) from a molecule.Reduction- addition of electrons (or electrons + protons) to a molecule.Oxidation and reduction reactions are usually linked together in ‘Redox’ reactions.Burning a log is ‘oxidation’ of cellulose and 'reduction' of O2. Electrons from cellulose are used to reduce oxygen, forming water.C6H12O6 + 6O2 ---> 6CO2 + 6H2O + heatOrganic molecules generally release energy as they become oxidizedThe catabolic pathways that we will discuss have many redox reactions that release energy in a form that is usable by the cell. (Energy released in small controlled increments instead of in one 'explosion' as with burning of a log)Catabolism involves a series of chemical reactions that often result in oxidation of an energy source, and the formation of ATP (energy 'currency' of cell).Reduced nutrients + ADP + Phosphate----> oxidized end products + ATPThe phosphate bonds of ATP are unstable and can be broken to release energy to drive cellular processes.Another small molecule important in this process is NAD. NAD is a 'coenzyme' (a small molecule that helps an enzyme to do something). NAD carries electrons and protons from one molecule to another. When NAD is reduced (picks up an electron and proton) itis called NADH.Cells make ATP from catabolism of organic compounds by two ways: Fermentation and Respiration.FermentationIn Fermentation synthesis of ATP occurs as a result of chemical reactions in the cytoplasm. This does not use a membrane bound electron transport chain, does not generate a proton gradient across a membrane, and does not require a membrane. The process that is used for ATP generation is called substrate level phosphorylationExample: fermentation of glucose by ‘glycolysis’- (splitting of sugar) produces 2 ATP molecules per glucose moleculeStep I: Glycolysisproduces 2 ATP per glucose molecule produces 2 NADH per glucose molecule1 glucose + 2 ADP + 2 Phosphate + 2 NAD-----> 2 pyruvate + 2 ATP + 2 NADH Step II: production of fermentation end products2 pyruvate + 2 NADH -----> fermentation end products + 2 NADIn the 2nd step of fermentation, NADH is used to reduce pyruvate thus forming productssuch as ethanol or lactic acid. Without this recycling of NAD, glycolysis would stop since there would be no NAD in the oxidized form to allow glycolysis to continue.Also note that the fermentation end products are energy rich compounds. Most of the energy in glucose remains in the fermentation end products.RespirationIn Respiration, synthesis of ATP requires generation of a proton motive force across a membrane. This involves a membrane associated electron transport chain, and a 'terminal electron acceptor' such as O2.glucose + 6 O2 + 38 ADP --------> 6 CO2 + 6 H2O + 38 ATP(most of energy of glucose released; much captured as ATP)step 1. glycolysisstep 2. TCA cycle ('Tricarboxylic acid cycle') also known as 'Krebs cycle' and 'Citric Acid Cycle'step 3. electron transport chain- Electrons pass through an e- transport chain in the membrane, and in the process they pump protons across the membrane. This proton gradient (proton motive force, or 'PMF') can then be used to perform work. (ie to makeATP, or to power the bacterial flagellum, or to transport nutrients, etc.) In aerobic respiration the final electron acceptor (terminal electron acceptor) is O2.O2 + 4e- + 4H+ -----> 2H2OStep 4. ATP synthaseOne way that the PMF is used is to synthesize ATP.This requires a membrane bound enzyme called the ATP synthase (also known as ATPase).As protons pass through the ATP synthase, they cause it to rotate. This brings ADP and phosphate together in the right orientation to react and form ATP.Summary of Fermentation and aerobic Respiration of glucoseFermentation is a set of oxidation-reduction reaction in which some atoms of the energysource (example, glucose) become more oxidized whereas others become more reduced, resulting in the production of a small amount of ATP, and large amounts of fermentation end products such as ethanol or lactic acid. Most of the energy of the compound remains in the fermentation end products.Fermentation of glucose-Does not require O2Does not require Citric Acid Cycle (TCA cycle, Krebs cycle) Does not require electron transport chainDoes not require membrane____________________________________________________In respiration organic material is completely oxidized to CO2, and the electrons are passed through an electron transport chain in the membrane and eventually to a terminal electron acceptor such as O2. A proton gradient across the membrane (proton motive force) is set up during this process, and is used to drive ATP synthesis from ADP and inorganic phosphate.Aerobic Respiration of glucose-.Requires O2 or other added e- acceptorRequires electron transport chain in membraneOften uses TCA cycleNote also: prokaryotes are experts at getting energy from substrates. Without their “bacterial symbionts” (mitochondria, chloroplasts) eukaryotic cells would need to rely oninefficient fermentation for their energy needs and multicellular eukaryotes would not have evolved.Metabolic Diversity of MicroorganismsThus far we have considered catabolism of glucose. All organisms need a source of energy, and a source of carbon, but not all organisms use glucose. Below we describe organisms in terms of their carbon and energy sources.Prefixes used:Energy source: chemoorgano (organic chemicals; that is, chemicals with C-C bonds, such as the sugar glucose)chemolitho (inorganic chemicals; ‘rocks’)photo (light)Carbon source: auto (inorganic C, such as CO2)hetero (organic chemicals)These prefixes are combined followed by the suffix ‘troph’ (to eat) to describe any organism. In the microbial world any of the 6 combinations of prefixes is possibleThe 4 most common classes of organsisms are:Chemoorganoheterotroph- use organic compounds for carbon and energy sourcesChemolithoautotroph- use inorganic molecules for Carbon (CO2) and energy sources (somebacteria, some archaea)Photoautotroph - use light as energy source; CO2 as carbon source (Plants and some


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