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☰ Search Explore Log in Create new account Upload × Chapter 8 Cellular Respiration Dr. Harold Kay Njemanze 8.1 Cellular Respiration 1. Cellular respiration involves various metabolic pathways that break down carbohydrates and other metabolites with the resultant buildup of ATP. 2. Cellular respiration consumes oxygen and produces CO2; because oxygen is required, cellular respiration is aerobic. 3. Cellular respiration usually involves the complete breakdown of glucose into CO2 and H2O. 4. The net equation for glucose breakdown is: C6H12O6 + 6 O2 = 6 CO2 + 6 H2O + energy 5. Cellular respiration releases energy, it is therefore exergonic. 6. Electrons are removed from substrates and received by oxygen, which combines with H+ to become water. 7. Glucose is oxidized and O2 is reduced. 8. The reactions of cellular respiration allow energy in glucose to be released slowly; therefore ATP is produced gradually. 9. The breakdown of glucose yields synthesis of 38net ATP and 40 total ATP. A. NAD+ and FAD 1. Each metabolic reaction in cellular respiration is catalyzed by a specific enzyme. 2. As a metabolite is oxidized, NAD+ (nicotinamide adenine dinucleotide) accepts two electrons and a hydrogen ion (H+); this results in NADH + H+. 3. Electrons received by NAD+ and FAD are high-energy electrons and are usually carried to the electron transport chain. 4. NAD+ is a coenzyme of oxidation-reduction since it both accepts and gives up electrons; thus, NAD+ is sometimes called a redox coenzyme 5. Only a small amount of NAD+ is needed in cells because each NAD+ molecule is used repeatedly. 6. FAD coenzyme of oxidation-reduction can replace NAD+; FAD accepts two electrons and two hydrogen ions to become FADH2. B. Phases of Cellular Respiration 1. Cellular respiration includes THREE phases: A. Glycolysis is the breakdown of glucose in the cytoplasm into two molecules of pyruvate. 1) Enough energy is released for an immediate yield of two ATP. 2) Glycolysis takes place outside the mitochondria and does not utilize oxygen; it is therefore an anaerobic process. Sub a. In the preparatory (prep) reaction, pyruvate enters a mitochondrion and is oxidized to a two-carbon acetyl group and CO2 is removed; this reaction occurs twice per glucose molecule. 1 B. The citric acid cycle: 1) occurs in the matrix of the mitochondrion and produces NADH and FADH2; 2) is a series of reactions that gives off CO2 and produces one ATP; 3) turns twice because two acetyl-CoA molecules enter the cycle per glucose molecule; 4) produces two immediate ATP molecules per glucose molecule. C. The electron transport chain: 1) is a series of carriers in the inner mitochondrial membrane that accept electrons from glucose--electrons are passed from carrier to carrier until received by oxygen; 2) passes electrons from higher to lower energy states, allowing energy to be released and stored for ATP production; 8.2 Outside the Mitochondria: Glycolysis 1. Glycolysis occurs in the cytoplasm outside the mitochondria. 2. Glycolysis is the breakdown of glucose into two pyruvate molecules. 3. Glycolysis is universally found in organisms; therefore, it likely evolved before the citric acid cycle and electron transport chain. 4. Glycolosis can be divided into the energy-investment steps where ATP is used to “jump start” glycolosis, and the energy-harvesting steps, where more ATP is made than used. A. Energy-Investment Steps 1. Glycolysis begins with the activation of glucose with two ATP; the glucose splits into two C3 molecules known as G3P, each of which carries a phosphate group. B. Energy-Yielding Steps 1. Oxidation of G3P occurs by removal of electrons and hydrogen ions. 2. Two electrons and one hydrogen ion are accepted by NAD+, resulting in two NADH; later, when the NADH molecules pass two electrons to the electron transport chain, they become NAD+ again. 3. The oxidation of G3P and subsequent substrates results in four high-energy phosphate groups, which are used to synthesize four ATP molecules; this process is called substrate-level phosphorylation. 4. Two of four ATP molecules produced are required to replace two ATP molecules used in the initial phosphorylation of glucose; therefore there is a net gain of two ATP from glycolysis. 5. Pyruvate enters a mitochondrion (if oxygen is available) and cellular respiration ensues. 6. If oxygen is not available, fermentation occurs and pyruvate undergoes reduction. 2 8.3 Fermentation 1. Fermentation is an anaerobic (i.e., occurs in the absence of oxygen) process which consists of glycolysis plus reduction of pyruvate to either lactate or to alcohol and CO2 (depending on the organism). 2. Animal cell fermentation results in lactate. 3. Bacteria can produce an organic acid like lactate, or an alcohol and CO2. 4. Yeasts produce ethyl alcohol and CO2. 5. NADH passes its electrons to pyruvate instead of to an electron transport chain; NAD+ is then free to return and pick up more electrons during earlier reactions of glycolysis. A. Advantages and Disadvantages of Fermentation 1. Despite a low yield of two ATP molecules, fermentation provides a quick burst of ATP energy for muscular activity. 2. Fermentation products are toxic to cells. a. When blood cannot remove all lactate from muscles, lactate changes pH and causes muscles to fatigue. b. The individual is in oxygen debt because oxygen is needed to restore ATP levels and rid the body of lactate. c. Recovery occurs after lactate is sent to the liver where it is converted into pyruvate; some pyruvate is then respired or converted back into glucose. B. Efficiency of Fermentation 1. Two ATP produced per glucose molecule during fermentation is equivalent to 14.6 kcal. 2. Complete glucose breakdown to CO2 and H2O during cellular respiration represents a potential yield of 686 kcal per molecule. 3. Efficiency of fermentation is 14.6/686 or about 2.1%, far less efficient than complete breakdown of glucose. C. Fermentation Helps Produce Numerous Food Products (Science Focus box) 1. Yeast Fermentation a. Baker’s yeast, Saccharomyces cerevisiae, is added to bread for leavening. The dough rises when yeasts give off CO2. b. Yeasts ferment the carbohydrates of fruit to produce ethyl alcohol in wine, and ferment grains to produce ethyl alcohol in beer. c. The acetic acid bacteria, Acetobacter aceti, spoil wine, and convert the alcohol to acetic acide to produce vinegar. 2. Bacterial Fermentation a. Lactic acid bacteria cause milk to sour and produce yogurt, sour cream, and cheese b. Brine cucumber pickles, sauerkraut, and kimchi are pickled vegetables produced by acid producing fermenting bacteria. 3. Soy Sauce Production 3 a. Yeast and fermenting bacteria are added to soy beans and wheat to produce soy sauce. 8.4 Inside the Mitochondria 1. The next reactions of cellular respiration involve the preparatory reaction, the citric acid cycle, and the electron transport chain. 2. These reactions occur in the mitochondria. 3. A mitochondrion has a double membrane with an intermembrane space (between the outer and inner membrane). 4. Cristae are the inner folds of membrane that jut into the matrix, the innermost compartment of a mitochondrion that is filled with a gel-like fluid. 5. The prep reaction and citric acid cycle enzymes are in the matrix; the electron transport chain is in the cristae. 6. Most of the ATP produced in cellular respiration is produced in the mitochondria; therefore, mitochondria are often called the “powerhouses” of the cell. A. Preparatory Reaction 1. The preparatory reaction connects glycolysis to the citric acid cycle. 2. In this reaction, pyruvate is converted to a two-carbon acetyl group, and is attached to coenzyme A, resulting in the compound acetyl-CoA. 3. This redox reaction removes electrons from pyruvate by a dehydrogenase enzyme, using NAD+ as a coenzyme. 4. This reaction occurs twice for each glucose molecule. 5. CoA carries the acetyl group to the citric acid cycle. 6. The two NADH carry to electrons to the electron transport chain. 7. The CO2 diffuses out of animal cells into blood, transported to lungs, and exhaled. B. Citric Acid Cycle 1. The citric acid cycle occurs in the matrix of mitochondria. 2. The cycle is sometimes called the Krebs cycle, named for Sir Hans Krebs, who described the fundamentals of the reactions in the 1930s. 3. The cycle begins by the addition of a two-carbon acetyl group to a fourcarbon molecule, forming a six-carbon citrate (citric acid) molecule. 4. In the subsequent reactions, at three different times two electrons and one hydrogen ion are accepted by NAD+, forming NADH. 5. At one time, two electrons and one hydrogen ion are accepted by FAD, forming FADH2. 6. NADH and FADH2 carry these electrons to the electron transport chain. 7. Some energy is released and is used to synthesize ATP by substrate-level phosphorylation. 8. One high-energy metabolite accepts a phosphate group and transfers it to convert ADP to ATP. 9. The citric acid cycle turns twice for each original glucose molecule. 4 10. The products of the citric acid cycle (per glucose molecule) are 4 CO2, 2 ATP, 6 NADH and 2 FADH2. 11. Production of CO2 a. The six carbon atoms in the glucose molecule have now become the carbon atoms of six CO2 molecules, two from the prep reaction and four from the citric acid cycle. C. The Electron Transport Chain 1. The electron transport chain is located in the cristae of mitochondria and consists of carriers that pass electrons successively from one to another. 2. NADH and FADH2 carry the electrons to the electron transport system. 3. Members of the Chain a. NADH gives up its electrons and becomes NAD+; the next carrier then gains electrons and is thereby reduced. b. At each sequential redox reaction, energy is released to form ATP molecules. c. Some of the protein carriers are cytochrome molecules, complex carbon rings with a heme (iron) group in the center. 4. Cycling of Carriers a. By the time electrons are received by O2, three ATP have been made. b. When FADH2 delivers electrons to the electron transport system, two ATP are formed by the time the electrons are received by O2. c. Oxygen serves as the terminal electron acceptor and combines with hydrogen ions to form water. 5. The Cristae of a Mitochondrion and Chemiosmosis a. The electron transport chain consists of three protein complexes and two protein mobile carriers that transport electrons. b. The three protein complexes include NADH-Q reductase complex, the cytochrome reductase complex, and the cytochrome oxidase complex; the two protein mobile carriers are coenzyme Q and cytochrome c. c. Energy released from the flow of electrons down the electron transport chain is used to pump H+ ions, which are carried by NADH and FADH2, into intermembrane space. d. Accumulation of H+ ions in this intermembrane space creates a strong electrochemical gradient. e. ATP synthase complexes are channel proteins that serve as enzymes for ATP synthesis. f. As H+ ions flow from high to low concentration, ATP synthase synthesizes ATP by the reaction: ADP + P = ATP. g. Chemiosmosis is the term used for ATP production tied to an electrochemical (H+) gradient across a membrane. h. Once formed, ATP molecules diffuse out of the mitochondrial matrix through channel proteins. 5 i. ATP is the energy currency for all living things; all organisms must continuously produce high levels of ATP to survive. D. Energy Yield From Glucose Metabolism 1. Substrate-Level Phosphorylation a. Per glucose molecule, there is a net gain of two ATP from glycolysis in cytoplasm. b. The citric acid cycle in the matrix of the mitochondria produces two ATP per glucose. c. Thus, a total of four ATP are formed by substrate-level phosphorylation outside of the electron transport chain. 2. ETC and Chemiosmosis a. Most of the ATP is produced by the electron transport chain and chemiosmosis. b. Per glucose, ten NADH and two FADH2 molecules provide electrons and H+ ions to the electron transport chain. c. For each NADH formed within the mitochondrion, three ATP are produced. d. For each FADH2 formed by the citric acid cycle, two ATP are produced. e. For each NADH formed outside mitochondria by glycolysis, two ATP are produced as electrons are shuttled across the mitochondrial membrane by an organic molecule and delivered to FAD. 3. Efficiency of Cellular Respiration a. The energy difference between total reactants (glucose and O2) and products (CO2 and H2O) is 686 kcal. b. An ATP phosphate bond has an energy of 7.3 kcal; 36 to 38 ATP are produced during glucose breakdown for a total of at least 263 kcal. c. This efficiency is 263/686, or 39% of the available energy in glucose is transferred to ATP; the rest of the energy is lost as heat. 8.5 Metabolic Pool 1. In a metabolic pool, substrates serve as entry points for degeneration or synthesis of larger molecules. 2. Degradative reactions (catabolism) break down molecules; they tend to be exergonic. 3. Synthetic reactions (anabolism) build molecules; they tend to be endergonic. A. Catabolism 1. Just as glucose is broken down in cellular respiration, other molecules in the cell undergo catabolism. 2. Fat breaks down into glycerol and three fatty acids. a. Glycerol is converted to G3P, a metabolite in glycolysis. b. An 18-carbon fatty acid is converted to nine acetyl-CoA molecules that enter the citric acid cycle. c. Respiration of fat products can produce 108 kcal in ATP molecules; fats are an efficient form of stored energy. 6 3. Amino acids break down into carbon chains and amino groups. a. Hydrolysis of proteins results in amino acids. b. R-group size determines whether carbon chain is oxidized in glycolysis or the citric acid cycle. c. A carbon skeleton is produced in the liver by removal of the amino group, by the process of deamination. d. The amino group becomes ammonia (NH3), which enters the urea cycle and ultimately becomes part of excreted urea. e. The size of the Regroup determines the number of carbons left after deamination. B. Anabolism 1. ATP produced during catabolism drives anabolism. 2. Substrates making up pathways can be used as starting materials for synthetic reactions. 3. The molecules used for biosynthesis constitute the cell’s metabolic pool. 4. Carbohydrates can result in fat synthesis: G3P converts to glycerol, acetyl groups join to form fatty acids. 5. Some metabolites can be converted to amino acids by transamination, the transfer of an amino acid group to an organic acid. 6. Plants synthesize all the amino acids they need; animals lack some enzymes needed to make some amino acids. 7. Humans synthesize 11 of 20 amino acids; the remaining 9 essential amino acids must be provided by the diet. C. The Energy Organelles Revisited 1. Chloroplasts and mitochondria may be related based on their likeness, yet they carry out opposite processes. a. The inner membrane of the chloroplasts forms the thylakoids of the grana. The inner membrane of the mitochondrion forms the convoluted cristae. b. In chloroplasts the electrons passed down the ETC have been energized by the sun. In mitochondria the electrons passed down the ETC have been removed from glucose products. c. In chloroplasts the stroma contains the enzymes of the Calvin cycle. In the mitochondria the matrix contains the enzymes of the citric acid cycle. 2. Flow Of Energy a. Energy flows through organisms. For example, the sun is the energy source for producing carbohydrates in chloroplasts. In the mitochondria, the carbohydrate energy is converted into ATP molecules during cellular respiration. b. Chemicals cycle throughout cells. Mitochondria use carbohydrates and oxygen produced in chloroplasts, and chloroplasts use carbon dioxide and water produced in the mitochondria. 7 Chapter 8: Cellular Respiration Cellular Respiration 1. Write the overall reaction for glucose breakdown and show that it is a redox reaction. 2. Discuss the role of oxidation-reduction enzymes. 3. State the four phases of cellular respiration and tell where each occurs in the cell. 8 Outside the Mitochondria: Glycolysis 4. Contrast the energy-investment step of glycolysis with the energyharvesting steps. 5. Summarize glycolysis by stating the inputs and outputs of the pathway. Fermentation 6. Explain the benefits and drawbacks of fermentation. 9 Inside the Mitochondria 7. Show that glucose products are broken down completely during the preparatory reaction and the citric acid cycle. 8. Give the net gain of substrate level ATP synthesis and NADH as a result of these pathways. 9. Describe how the cristae are organized to produce ATP. 10 Metabolic Pool 10. Show how catabolism of protein and fat utilizes the same pathways as glucose breakdown. 11. Compare and contrast the organization and structure of mitochondria and chloroplasts and how they permit a flow of energy through living things. 11 CELLULAR RESPIRATION STEP 1: GLYCOLYSIS GLUCOSE CCCCCC 2 ATP add 2 phosphates to the glucose, energizing it. Glucose diffuses into the cytoplasm from the blood stream. 2 ATP 2 ADP P–CCCCCC-p (G1phosphate) P–CCC Glucose1,6 phosphate splits, creating 2PGAL molecules. CCC–P (G3phosphate) P P An additional phosphate is added to each PGAL. Each 3C molecule now has a 2nd phosphate added. The NADH & H+ are energy carriers P–CCC-P P – C C C -P High Energy electrons & hydrogen atoms removed from 3C are used to create NADH. 1 NAD + 1 NADH + H+ 2 ADP High energy phosphates removed from 3C are used to make ATP. 2ATP ATP 1 NAD + 1 NADH + H+ 2 ADP 2ATP Glycolysis makes 4ATP(2ATP) net) CCC CCC These 2 Pyruvates finishes glycolysis! 12 SUMMARY Preamble ATP is used for: 1. Chemical work Such as during digestive processes. 2. Transport work Such as transporting substances across membranes. 3. Mechanical work Such as beating of cilia and muscle contraction. Cellular respiration produces ATP. It is an exergonic reaction. Delta G = 2870 KJ/mol or 686 kcal/mol. Cells recycle the ATP they use for work. Cellular respiration transfers energy stored in food to molecules of ATP. ATP is a nucleotide with high energy phosphate bonds that the cells hydrolyze for energy to drive other endergonic reactions. The terminal phosphate group from ATP contains all the energy. This phosphate is transferred to other compounds in a process called phosphorylation. The molecule acquiring the phosphate is said to be phosphorylated. Redox reactions. Oxidation-reduction reactions= chemical reactions which involve a partial or complete transfer of electrons from one reactant to another. This is called Redox Rx for short. Oxidation= partial or complete loss of electrons. Reduction= partial or complete gain of electrons. This reaction requires both a donor and an acceptor. So when one reactant is oxidized the other is reduced. 13 • Examples of redox reactions becomes oxidized (loses electron) Na + Na+ Cl + Cl– becomes reduced (gains electron) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Oxidation CO2 + 2H2O CH4 + 2O2 Reduction Oxidation Xe + Y X Reduction Where: X = substance being oxidized It is a reducing agent. Since it reduces y. Y = substance being reduced. It is an oxidizing agent. Since it oxidizes x 14 + Ye Cellular respiration as a Redox process. Apply this rule to cellular respiration. Oxidation C6H12O6 + 602 6CO2+ 6H2O+Energy Reduction THE PROCESS OF CELLULAR RESPIRATION- HARVESTING CELLULAR ENERGY • Energy – Flows into an ecosystem as sunlight and leaves as heat Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic CO2 + H2O + O2 Cellular molecules respiration in mitochondria ATP powers most cellular work Figure 9.2 Heat energy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 15 • An overview of cellular respiration Electrons carried via NADH and FADH2 Electrons carried via NADH Cytosol Mitochondrion ATP Figure 9.6 Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle Glycolsis Pyruvate Glucose Substrate-level phosphorylation ATP ATP Substrate-level phosphorylation Oxidative phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings C6H12O6 + 6O2 Organic compound + oxygen (food) 6CO2 + 6H2O + ATP Carbon dioxide + water + energy Cellular respiration actually involves THREE steps: 1. Glycolysis…….1b…..Connecting Step 2. Krebs Cycle 3. Electron Transport Chain/Oxidative Phosphorylation 1. Glycolysis; This is a catabolic pathway Involves 10 steps. Each step requires specific enzymes. The first five steps is called the energy-investment phase (because ATP is used) while the last five steps are called the energy- yielding phase. Oxygen is not used. It is therefore considered to be anaerobic One molecule of glucose produces 2-3C molecules called pyruvate. Needs an investment of 2 ATP to start reaction. A total of 4 ATP molecules are produced. A net of 2 ATP molecules are produced. 2 NADH and 2 H ions are also produced. 16 This takes place in the cytosol of the cytoplasm, of ALL cell types(prokaryotes and Eukaryoyes) Glucose (6C) NADH ATP 3C 3C Pyruvic Acid 1b. Connecting Step: 2 pyruvates enter the mitochondrial matrix. The 2 pyruvates are converted to 2 acetyl coenzyme A. No ATP is produced. 2 CO2 are produced. 2 NADH are produced and 2 H+ are produced. Oxygen is not used. It is therefore considered to be anaerobic This takes place in the mitochondrial matrix. 2. Krebs Cycle; also known as the Citric Acid Cycle or the Tricarboxylic Acid Cycle. This is a catabolic pathway Completes glucose oxidation by breaking down pyruvate derivative (acetyl CoA) into carbon dioxide. For every glucose molecule, Krebs cycle goes around twice. -One for each acetyl Co A. Each step requires a specific enzyme. Oxygen is not used. 2 ATPs- 1(one) from each cycle is produced. 4 CO2-2 from each cycle are produced. 8 NADH-4 from each cycle are produced. 2 FADH2-1 from each cycle are produced. This takes place in the cytoplasm (Prokaryotes) But takes place in the mitochondrial matrix (Eukaryotes). 3. Electron Transport Chain and Oxidative Phosphorylation/Chemiosmosis Synthesis The ETC is made of electron carrier molecules embedded in the inner mitochondrial membrane. Except for ubiquinone (Q) most of the carrier molecules are proteins and are tightly bound to prosthetic groups (nonprotein cofactors). The inner mitochondrial membrane has a chain of electron transport proteins. NADH and FADH2 are coenzymes that have high energy electrons. These high energy electrons are passed through the chain of electron transport proteins. Finally they will be passed on to oxygen. This is called the final electron acceptor. Oxygen will then combine with 2 hydrogen ions to form water. The energy released during this transfer of electrons through the electron transport chain is used to pump H+ from the mitochondrial matrix into the inner membrane space. Now there is H+ gradient established between the intermembrane space (greater) and the matrix (lesser) 17 H+ therefore will move from intermembrane space back to the matrix through a special protein complex located in the inner mitochondrial membrane called an ATP synthase. This flow of H+ will activate this enzyme and subsequently produce 6H2O and 34 ATPs. Produces 90% of the ATP. This is the chemiosmotic synthesis of ATP. This step uses oxygen. Most of the ATP of the body is produced here. Takes place in the cell membrane (Prokaryotes) Takes place in the inner mitochondrial membrane (Eukaryotes). Oxidative phosphorylation This is ATP production that is coupled to the exergonic transfer of electrons from food to oxygen. A small amount of ATP is produced directly by the reactions of glycolysis and Krebs cycle. This mechanism of producing ATP is called substrate level phosphorylation. Substrate level phosphorylation This is ATP production by direct enzymatic transfer of phosphate from an intermediate substrate in catabolism to ADP. Glycolysis Is a catabolic reaction during which 6-C Glucose is split into 2-3Csugars called pyruvates? This reaction may take place in the presence or absence of oxygen.But generally considered to be anaerobic. Energy investment phase The cell uses ATP to phosphorylate the intermediates of glycolysis. 2 ATP molecules are invested Energy yielding phase 2- 3C intermediates are oxidized. For each glucose molecule entering glycolysis a net gain of ATP is produced. 2 ATP molecules are therefore yielded. Heme group Prosthetic group composed of four organic rings surrounding a single iron atom. Cytochrome Type of protein molecule that contains a Heme prosthetic group and functions as an electron carrier in the electron transport chain of mitochondria and chloroplast. 18 FMN= Flavin mononucleotide NAD= Nicotinamide Adenine Dinucleotide BREAKDOWN OF ATP PRODUCED: Glycolysis: 4 ATP are produced/2 net ATP None from connecting step Krebs cycle: 2 ATP are produced Electron Transport Chain (chemiosmosis): 34 ATP are produced TOTAL----38 (40) ATPs are produced.………………………………… INTERFERENCE: A number of poisons may interfere with the chemiosmosis mechanism. There are 3 types (classes) of respiratory poisons: a. cyanide These types of poisons will block the passage of electrons from cytochrome a3 to oxygen. As a result no protons are pumped and no ATP is made b. Inhibitors These types of poisons inhibit ATP synthesis. An example are antibiotics e.g. oligomycin c. uncouplers These types of poisons short circuit the proton current by making the lipid bilayer leaky to H ions. This will in turn make it impossible to establish a proton gradient and so no ATP will be made. Metabolic Processes Food can be oxidized under different conditions. 1. Aerobic The breakdown of food in the presence of oxygen. 2. Anaerobic The breakdown of food in the absence of oxygen. 3. Fermentation Anaerobic breakdown of food. 4. Facaultative Breakdown of food in the presence or absence of oxygen. Organisms may be classified based on the effect oxygen has on their growth and metabolism. 1. STRICT OBLIGATE AEROBES Organisms that require oxygen for growth development. example, humans and Bacillus bacteria 19 2. STRICT OBLIGATE ANAEROBES Organisms that only grow in the absence of oxygen and are in fact poisoned by it. Example, Bacteroides fragilis and Clostridium botulinum 3. FACULTATIVE ANAEROBES Organisms that may grow in the presence or absence of oxygen. Examples include yeast, muscle cells and bacteria. Many yeasts, enteric bacteria, such as the Gram-negative Escherichia coli, and skin-dwelling Gram-positive halophiles, like Staphylococcus, are facultative anaerobes 20 Download 1. Science 2. Biology 3. 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