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Chapter 9 Metabolism is the total of all chemical reactions occurring in the cell. The flow of energy and the participation of enzymes make metabolism possible. Note: The second law of thermodynamics describes the randomness/disorder associated with a system as entropy. As physical and chemical reactions proceed from reactant to product they do so in such a way the entropy of the universe increases. So as energy moves through the various trophic levels the randomness associated with that energy increases. Metabolism can be divided in to two categories: 1. Catabolism – reaction resulting in large, complex higher energy molecules being broken into small, simpler lower energy molecules. a. Some of the energy associated with these molecules is used for work (Free energy – G) b. The remaining energy is released from the system as heat (enthalpy – H) 2. Anabolism – reaction resulting in the formation of a large, complex higher energy molecules from the binding of multiple smaller, less complexed lower energy molecules. a. These process increase the order of a system Chemotrophic microorganisms vary in their energy sources as well as, their electron acceptors. There are three metabolic processes that utilize organic nutrients as electron acceptors (chemoorgnotrophs): 3. Fermentation – a process where the energy substrate is oxidized and degraded without the participation of an exogenous or externally derived electron acceptor ( aka respiration) 4. Aerobic respiration – the final electron acceptor is exogenous/ externally derived oxygen (O2) 5. Anaerobic respiration – the final electron acceptor is an exogenous molecule other than oxygen a. Nitrate b. Sulfate c. Carbon dioxide d. Iron (III) e. Fumerate Electron obtained from inorganic nutrients is accomplished by chemolithotrophs. The final electron acceptor can be any of the following; a. O2 b. Nitrate c. Sulfate Chemoorganotrophic aerobic metabolism 1. Catabolism of an organic molecule a. Protein b. Carbohydrate c. Lipid 2. Aerobic production of energy-transfer and intermediate molecules a. ATP-energy transfer b. NADH and FADH2 electron accepting/donating intermediates c. This process can occur under anaerobic conditions also 3. Carbon nutrients are oxidized to produce more energy-transfer and intermediate molecules Process of Carbohydrate degradation in Microorganisms Microorganisms have several pathways to catobolize glucose and other sugars to pyruvate (stage 1); 1. Glycolysis 2. Pentose phosphate shunt/pathway 3. Entner-Doudoroff pathway I. Glycolytic pathway – this pathway is also referred to as the EmbdenMeyerhof pathway a. This is the most common of the 3 pathways b. Occurs in the Cytoplasmic matrix of both prokaryotic and eukaryotic cells c. In a process called phosphorylation, phosphate groups are added to the 6carbon molecule, raising its free energy to a state that begins the exergonic reaction. d. In the second stage of this process a catabolic reactions releases two 3carbon molecules (Glyceraldeyde-3phosphate and Dihydroxyacetone phosphate) that ultimately produces ATP through an anabolic process call substrate-level phosphorylation e. Produces 2 ATP’s (net) II. Pentose Phosphate Pathway – this pathway is also referred to as the hexose monophosphate pathway a. This pathway can occur simultaneously with either of the other two pathways b. Can occur under aerobic or anaerobic conditions c. Is an amphibolic pathway (both catabolic and anabolic) d. Has different pathways than the other pathways 1) The phosphorylated 6-carbon sugar is converted into a 5carbon sugar (pentose ribulose 5-phosphate 2) Carboxylation occurs (release of carbon dioxide from a carboxly containing compound) 3) NADPH is produced (this compound is normally associated with photosynthesis) e. The 5-carbon compound then enters a variety of pathways resulting in phosphorylated sugars containing between 3 to 7 carbons. There are two unique enzymes responsible for these pathway; 1) Transketolase 2) Transaldolase 3) The result of these processes is the production of 3phosphoryated glucose molecules, a glyceraldehydes 3- phosphate, and 3-carbon dioxide molecules III. f. Functions of this pathway 1) NADPH produced in this pathway serve as a electron source for metabolism 2) The production of 4-carbon sugars (erythose 4-phosphate) is used to synthesis pyridoxal (vitamin B6) 3) The production of the 5-carbon pentose ribose 5-phosphate is need for carbon fixation in photosynthesis 4) Results in the production of ATP’s through the electron transport system. g. Commonly seen in E. coli and Bacillus subtilis h. Produces 1ATP (net) Entner-Doudoroff pathway a. Begins much like glycolysis, but produces pyruvate in a shorter pathway b. Commonly seen in Pseudomonas aeruginosa c. Produces 1 ATP (net), 1NADH, and 1NADPH Fermentation: In the absence of aerobic and anaerobic respiration, NADH is not oxidized by electron transport chains because no external electron acceptor are available, however NADH must be oxidized back into NAD+ to maintain glycolysis or the reaction (glycolysis) will stop. 1. many microbes simply slow down/stop pyruvate dehydrogenase 2. pyruvate, or one of its intermediate, then can serve as an endogenous electron/hydrogen acceptor for NADH 3. this process of NADH reoxidation is call fermentation. Fermentation can lead to the production of ATP, but not as much as aerobic/anaerobic respiration 1. in fermentation the substrate (pyruvate or a dirivative/carbohydrate) is oxidized 2. ATP forms by substrate-level phosphorylation only 3. Oxygen is not needed Types of fermentation: There are 5 main types that we will discuss: 1. Alcohol fermentation 2. Lactic acid fermentation a. homolactic b. heterolactic 3. Mix acid fermentation 4. Butanediol fermentation 5. carbohydrate 1. Alcohol fermentation a. Pyruvate is oxidized to acetaldehyde b. Acetaldehyde is reduced to ethanol c. Seen in fungi, bacteria, algae, and protozoa 2. lactic acid a. pyruvate is reduced to lactate b. occurs in bacteria, algae, molds, protozoa, and animal skeletal muscles c. homolactic fermenters use lactate dehydrogenase to produce lactate only directly from pyruvate( enzyme lactate dehydrogenase) d. heterolactic produce lactate, ethanol, and carbon dioxide (enzyme used is phosphoketolase) i. enterobacteriaceae ii. important to production of wine, cheese and breads 3. Mixed acid a. Pyruvate metabolism results in the production of ethanol, acetic acid, succinic acid, lactic acid, and formic acid b. If formic hydrogenlyase is present then formic acid is degrade to hydrogen and carbon dioxide gases i. Escherichia ii. Salmonella iii. Proteus 4. Butanediol fermentation a. Converts pyruvate to acetone, and finally ethanol b. More ethanol production than other pathways c. Produced by Enterobacter, Serratia, Erwinia, and some Bacillus species.