Chapter 10 Cell Respiration PDF

Chapter 10 Cell Respiration Lecture Presentations by Nicole Tunbridge and © 2021 Pearson Education Ltd. Kathleen Fitzpatrick Life Is Work  Living cells require energ...

Chapter 10 Cell Respiration Lecture Presentations by Nicole Tunbridge and © 2021 Pearson Education Ltd. Kathleen Fitzpatrick Life Is Work  Living cells require energy from outside sources to do work  The work of the cell includes assembling polymers, membrane transport, moving, and reproducing  Animals can obtain energy to do this work by feeding on other animals or photosynthetic organisms © 2018 Pearson Education Ltd. Figure 10.1a  Energy flows into an ecosystem as sunlight and leaves as heat  The chemical elements essential to life are recycled  Photosynthesis generates O2 and organic molecules, which are used in cellular respiration  Cells use chemical energy stored in organic molecules to generate ATP, which powers work © 2018 Pearson Education Ltd. Figure 10.1b Concept 10.1: Catabolic pathways yield energy by oxidizing organic fuels  Catabolic pathways release stored energy by breaking down complex molecules  Electron transfer plays a major role in these pathways  These processes are central to cellular respiration © 2018 Pearson Education Ltd. Catabolic Pathways and Production of ATP  The breakdown of organic molecules is exergonic  Fermentation is a partial degradation of sugars that occurs without O2  Aerobic respiration consumes organic molecules and O2 and yields ATP  Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2 © 2018 Pearson Education Ltd.  Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration  Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP + heat) © 2018 Pearson Education Ltd. Redox Reactions: Oxidation and Reduction  The transfer of electrons during chemical reactions releases energy stored in organic molecules  This released energy is ultimately used to synthesize ATP © 2018 Pearson Education Ltd. The Principle of Redox  Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions  In oxidation, a substance loses electrons, or is oxidized  In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced) © 2018 Pearson Education Ltd. Figure 10.UN01 becomes oxidized (loses electron) becomes reduced (gains electron) © 2018 Pearson Education Ltd. Figure 10.UN02 becomes oxidized becomes reduced © 2018 Pearson Education Ltd.  The electron donor is called the reducing agent  The electron receptor is called the oxidizing agent  Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds  An example is the reaction between methane and O2 © 2018 Pearson Education Ltd. Figure 10.3 Reactants Products becomes oxidized Energy becomes reduced Methane Oxygen Carbon dioxide Water (reducing (oxidizing agent) agent) © 2018 Pearson Education Ltd. Oxidation of Organic Fuel Molecules During Cellular Respiration  During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced  Organic molecules with an abundance of hydrogen are excellent sources of high-energy electrons  Energy is released as the electrons associated with hydrogen ions are transferred to oxygen, a lower energy state © 2018 Pearson Education Ltd. Figure 10.UN03 becomes oxidized becomes reduced © 2018 Pearson Education Ltd. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain  In cellular respiration, glucose and other organic molecules are broken down in a series of steps  Electrons from organic compounds are usually first transferred to NAD+, a coenzyme  As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration  Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP © 2018 Pearson Education Ltd. Figure 10.4 NAD+ NADH Dehydrogenase Reduction of NAD+ 2[H] (from food) Oxidation of NADH Nicotinamide Nicotinamide (oxidized form) (reduced form) © 2018 Pearson Education Ltd. Figure 10.4a NAD+ Nicotinamide (oxidized form) © 2018 Pearson Education Ltd. Figure 10.4b NAD+ NADH Dehydrogenase Reduction of NAD+ 2[H] (from food) Oxidation of NADH Nicotinamide Nicotinamide (oxidized form) (reduced form) © 2018 Pearson Education Ltd. Figure 10.UN04 Dehydrogenase © 2018 Pearson Education Ltd.  NADH passes the electrons to the electron transport chain  Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction  O2 pulls electrons down the chain in an energy- yielding tumble  The energy yielded is used to regenerate ATP © 2018 Pearson Education Ltd. Figure 10.5 H2 + ½ O2 2H + ½ O2 Controlled release of 2H + 2e + – energy n chai nsport tra ATP Free energy, G Free energy, G t ro n Elec Explosive ATP release of energy ATP 2 e– ½ O2 2H + H2O H2O (a) Uncontrolled reaction (b) Cellular respiration © 2018 Pearson Education Ltd. The Stages of Cellular Respiration: A Preview  Harvesting of energy from glucose has three stages 1. G ​ lycolysis (breaks down glucose into two molecules of pyruvate) 2. The citric acid cycle (completes the breakdown of glucose) 3. O ​ xidative phosphorylation (accounts for most of the ATP synthesis) © 2018 Pearson Education Ltd. Figure 10.UN05 1. GLYCOLYSIS (color-coded blue throughout the chapter) 2. PYRUVATE OXIDATION and the CITRIC ACID CYCLE (color-coded light orange and dark orange) 3. OXIDATIVE PHOSPHORYLATION: Electron transport and chemiosmosis (color-coded purple) © 2018 Pearson Education Ltd. Figure 10.6_1 Electrons via NADH GLYCOLYSIS Glucose Pyruvate CYTOSOL MITOCHONDRION ATP Substrate-level © 2018 Pearson Education Ltd. Figure 10.6_2 Electrons Electrons via NADH via NADH and FADH2 GLYCOLYSIS PYRUVATE OXIDATION CITRIC ACID Glucose Pyruvate Acetyl CoA CYCLE CYTOSOL MITOCHONDRION ATP ATP Substrate-level Substrate-level © 2018 Pearson Education Ltd. Figure 10.6_3 Electrons Electrons via NADH via NADH and FADH2 GLYCOLYSIS PYRUVATE OXIDATIVE OXIDATION CITRIC PHOSPHORYLATION ACID Glucose Pyruvate Acetyl CoA CYCLE (Electron transport and chemiosmosis) CYTOSOL MITOCHONDRION ATP ATP ATP Substrate-level Substrate-level Oxidative © 2018 Pearson Education Ltd.  The process that generates almost 90% of the ATP is called oxidative phosphorylation because it is powered by redox reactions  A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation © 2018 Pearson Education Ltd. Figure 10.7 Enzyme Enzyme ADP P ATP Substrate Product © 2018 Pearson Education Ltd.  For each molecule of glucose degraded to CO2 and water by respiration, the cell makes up to 32 molecules of ATP © 2018 Pearson Education Ltd.  We can use money as an analogy for cellular respiration:  Glucose is like a larger-denomination bill—it is worth a lot, but it is hard to spend  ATP is like a number of smaller-denomination bills of equivalent value—they can be spent more easily  Cellular respiration cashes in a large denomination of energy (glucose) for the small change of many molecules of ATP © 2018 Pearson Education Ltd. Concept 10.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate  Glycolysis (“sugar splitting”) breaks down glucose into two molecules of pyruvate  Glycolysis occurs in the cytoplasm and has two major phases  Energy investment phase  Energy payoff phase  Glycolysis occurs whether or not O2 is present © 2018 Pearson Education Ltd. Figure 10.UN06 CITRIC OXIDATIVE PYRUVATE GLYCOLYSIS ACID PHOSPHORYL- OXIDATION CYCLE ATION ATP © 2018 Pearson Education Ltd. Figure 10.8 Energy Investment Phase Glucose 2 ATP used 2 ADP + 2 P Energy Payoff Phase 4 ADP + 4 P 4 ATP formed 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Net Glucose 2 Pyruvate + 2 H2O 4 ATP formed – 2 ATP used 2 ATP 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+ © 2018 Pearson Education Ltd. Figure 10.9a GLYCOLYSIS: Energy Investment Phase Glyceraldehyde 3-phosphate (G3P) ATP Glucose Fructose ATP Fructose Glucose 6-phosphate 6-phosphate 1,6-bisphosphate ADP ADP Isomerase 5 Hexokinase Phosphogluco- Phospho- Aldolase Dihydroxyacetone isomerase fructokinase 1 4 phosphate (DHAP) 2 3 © 2018 Pearson Education Ltd. Figure 10.9aa_1 GLYCOLYSIS: Energy Investment Phase Glucose © 2018 Pearson Education Ltd. Figure 10.9aa_2 GLYCOLYSIS: Energy Investment Phase ATP Glucose Glucose 6-phosphate ADP Hexokinase 1 © 2018 Pearson Education Ltd. Figure 10.9aa_3 GLYCOLYSIS: Energy Investment Phase ATP Glucose Fructose Glucose 6-phosphate 6-phosphate ADP Hexokinase Phosphogluco- isomerase 1 2 © 2018 Pearson Education Ltd. Figure 10.9ab_1 GLYCOLYSIS: Energy Investment Phase Fructose 6-phosphate © 2018 Pearson Education Ltd. Figure 10.9ab_2 GLYCOLYSIS: Energy Investment Phase Fructose ATP Fructose 6-phosphate 1,6-bisphosphate ADP Phospho- fructokinase 3 © 2018 Pearson Education Ltd. Figure 10.9ab_3 GLYCOLYSIS: Energy Investment Phase Glyceraldehyde 3-phosphate (G3P) Fructose ATP Fructose 6-phosphate 1,6-bisphosphate ADP Isomerase 5 Phospho- Aldolase Dihydroxyacetone fructokinase 4 phosphate (DHAP) 3 © 2018 Pearson Education Ltd. Figure 10.9b GLYCOLYSIS: Energy Payoff Phase 2 ATP 2 H2O 2 ATP 2 NADH 2 ADP 2 NAD+ + 2 H+ 2 ADP 2 2 2 2 2 Triose Phospho- Phospho- Enolase Pyruvate phosphate 2 P glycerokinase glyceromutase kinase Glycer- dehydrogenase i 9 aldehyde 1,3-Bisphospho- 7 3-Phospho- 8 2-Phospho- Phosphoenol- 10 Pyruvate 6 3-phosphate glycerate glycerate glycerate pyruvate (PEP) (G3P) © 2018 Pearson Education Ltd. Figure 10.9ba_1 GLYCOLYSIS: Energy Payoff Phase Glyceraldehyde 3-phosphate (G3P) Isomerase 5 Aldolase Dihydroxyacetone 4 phosphate (DHAP) © 2018 Pearson Education Ltd. Figure 10.9ba_2 GLYCOLYSIS: Energy Payoff Phase Glyceraldehyde 2 NADH 3-phosphate (G3P) 2 NAD+ + 2 H+ 2 Triose phosphate 2 Pi Isomerase dehydrogenase 5 1,3-Bisphospho- Aldolase 6 glycerate Dihydroxyacetone 4 phosphate (DHAP) © 2018 Pearson Education Ltd. Figure 10.9ba_3 GLYCOLYSIS: Energy Payoff Phase 2 ATP Glyceraldehyde 2 NADH 3-phosphate (G3P) 2 NAD+ + 2 H+ 2 ADP 2 2 Triose Phospho- phosphate 2 Pi glycerokinase Isomerase dehydrogenase 1,3-Bisphospho- 7 3-Phospho- 5 6 Aldolase Dihydroxyacetone glycerate glycerate 4 phosphate (DHAP) © 2018 Pearson Education Ltd. Figure 10.9bb_1 GLYCOLYSIS: Energy Payoff Phase 2 3-Phospho- glycerate © 2018 Pearson Education Ltd. Figure 10.9bb_2 GLYCOLYSIS: Energy Payoff Phase 2 H2O 2 2 2 Phospho- Enolase glyceromutase 9 8 3-Phospho- 2-Phospho- Phosphoenol- glycerate glycerate pyruvate (PEP) © 2018 Pearson Education Ltd. Figure 10.9bb_3 GLYCOLYSIS: Energy Payoff Phase 2 H2O 2 ATP 2 ADP 2 2 2 2 Phospho- Enolase Pyruvate glyceromutase kinase 9 8 10 3-Phospho- 2-Phospho- Phosphoenol- Pyruvate glycerate glycerate pyruvate (PEP) © 2018 Pearson Education Ltd. Concept 10.3: After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules  In the presence of O2, pyruvate enters a mitochondrion (in eukaryotic cells), where the oxidation of glucose is completed © 2018 Pearson Education Ltd. Oxidation of Pyruvate to Acetyl CoA  Before the citric acid cycle can begin, pyruvate must be converted to acetyl coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle  This step is carried out by a multienzyme complex that catalyzes three reactions 1. Oxidation of pyruvate and release of CO2 2. Reduction of NAD+ to NADH 3. Combination of the remaining two-carbon fragment and coenzyme A to form acetyl CoA © 2018 Pearson Education Ltd. Figure 10.UN07 CITRIC OXIDATIVE PYRUVATE GLYCOLYSIS ACID PHOSPHORYL- OXIDATION CYCLE ATION © 2018 Pearson Education Ltd. Figure 10.10 MITOCHONDRION CYTOSOL Coenzyme A CO2 1 3 2 NAD+ NADH + H+ Acetyl CoA Pyruvate Transport protein © 2018 Pearson Education Ltd. The Citric Acid Cycle  The citric acid cycle, also called the Krebs cycle, completes the breakdown of pyruvate to CO2  The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn © 2018 Pearson Education Ltd.  The citric acid cycle has eight steps, each catalyzed by a specific enzyme  The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate  The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle  The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain © 2018 Pearson Education Ltd. Figure 10.UN08 CITRIC OXIDATIVE PYRUVATE GLYCOLYSIS ACID PHOSPHORYL- OXIDATION CYCLE ATION ATP © 2018 Pearson Education Ltd. Figure 10.11 PYRUVATE OXIDATION Pyruvate (from glycolysis, 2 molecules per glucose) CO2 NAD + CoA NADH + H+ Acetyl CoA CoA NADH + H+ CoA NAD+ CITRIC ACID 2 CO2 CYCLE FADH2 2 NAD+ FAD 2 NADH + 2 H+ ADP + P i ATP © 2018 Pearson Education Ltd. Figure 10.11a PYRUVATE OXIDATION Pyruvate (from glycolysis, 2 molecules per glucose) CO2 NAD+ CoA NADH + H+ Acetyl CoA CoA © 2018 Pearson Education Ltd. Figure 10.11b Acetyl CoA CoA NADH + H+ CoA NAD+ CITRIC ACID 2 CO2 CYCLE FADH2 2 NAD+ FAD 2 NADH + 2 H+ ADP + P i ATP © 2018 Pearson Education Ltd. Figure 10.12a Acetyl CoA CoA-SH H2O 1 Oxaloacetate 2 Citrate Isocitrate © 2018 Pearson Education Ltd. Figure 10.12b Isocitrate NAD+ NADH 3 + H+ CO2 CoA-SH α-Ketoglutarate 4 CO2 NAD+ NADH + H+ Succinyl CoA © 2018 Pearson Education Ltd. Figure 10.12c Fumarate 6 CoA-SH FADH2 5 FAD Succinate Pi GTP GDP Succinyl CoA ADP ATP © 2018 Pearson Education Ltd. Figure 10.12d NADH + H+ NAD+ Oxaloacetate 8 Malate 7 H2O Fumarate © 2018 Pearson Education Ltd. Concept 10.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis  Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food  These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation © 2018 Pearson Education Ltd. The Pathway of Electron Transport  The electron transport chain is in the inner membrane (cristae) of the mitochondrion  Most of the chain’s components are proteins, which exist in multiprotein complexes  Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O  Electron carriers alternate between reduced and oxidized states as they accept and donate electrons © 2018 Pearson Education Ltd.  Electrons are transferred from NADH or FADH2 to the electron transport chain  Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O2  The electron transport chain generates no ATP directly  It breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts © 2018 Pearson Education Ltd. Figure 10.UN09 CITRIC OXIDATIVE PYRUVATE GLYCOLYSIS ACID PHOSPHORYL- OXIDATION CYCLE ATION ATP © 2018 Pearson Education Ltd. Figure 10.13 NADH (least electronegative) 50 2 e– NAD+ Complexes I-IV FADH2 each consist of 2 e– FAD multiple proteins Free energy (G) relative to O2 (kcal/mol) 40 FMN I with electron Fe S Fe S II carriers. Q III Cyt b 30 Fe S Cyt c1 IV Cyt c Cyt a Electron transport Cyt a3 20 chain 10 2 e– 2 H+ + ½ O2 0 (most electronegative) H2O © 2018 Pearson Education Ltd. Figure 10.13a NADH (least electronegative) 50 2 e– NAD+ Free energy (G) relative to O2 (kcal/mol) FADH2 Complexes I-IV each consist of 2 e– FAD multiple proteins 40 I with electron FMN II carriers. Fe S Fe S Q III Cyt b Fe S 30 Cyt c1 IV Cyt c Cyt a Electron transport chain Cyt a3 20 10 2 e– © 2018 Pearson Education Ltd. Figure 10.13b 30 Cyt c1 IV Free energy (G) relative to O2 (kcal/mol) Cyt c Cyt a Cyt a3 20 10 2 e– 2 H+ + ½ O2 0 (most electronegative) H2O © 2018 Pearson Education Ltd. Chemiosmosis: The Energy-Coupling Mechanism  The energy released as electrons are passed down the electron transport chain is used to pump H+ from the mitochondrial matrix to the intermembrane space  H+ then moves down its concentration gradient back across the membrane, passing through the protein complex ATP synthase © 2018 Pearson Education Ltd.  H+ moves into binding sites on the rotor of ATP synthase, causing it to spin in a way that catalyzes phosphorylation of ADP to ATP  This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work © 2018 Pearson Education Ltd. Figure 10.14 H+ Stator INTERMEMBRANE SPACE Rotor Internal rod Catalytic knob ADP + Pi ATP MITOCHONDRIAL MATRIX © 2018 Pearson Education Ltd.  Certain electron carriers in the electron transport chain accept and release H+ along with the electrons  In this way, the energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis  The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work © 2018 Pearson Education Ltd. Figure 10.15 H+ ATP Protein H+ H+ synthase complex H+ of electron Cyt c carriers IV Q I III II 2 H+ + ½ O2 H2O FADH2 FAD NADH NAD+ ADP + P i ATP (carrying electrons from food) H+ 1 Electron transport chain 2 Chemiosmosis Oxidative phosphorylation © 2018 Pearson Education Ltd. Figure 10.15a H+ H+ Protein H+ complex of electron Cyt c carriers IV Q I III II 2 H+ + ½ O2 H2O FADH2 FAD NADH NAD+ (carrying electrons from food) 1 Electron transport chain © 2018 Pearson Education Ltd. Figure 10.15b ATP synthase H+ ADP + P i ATP H+ 2 Chemiosmosis © 2018 Pearson Education Ltd. An Accounting of ATP Production by Cellular Respiration  During cellular respiration, most energy flows in this sequence: glucose → NADH → electron transport chain → proton-motive force → ATP  About 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 32 ATP  The rest of the energy is lost as heat © 2018 Pearson Education Ltd. Figure 10.16 Electron shuttles MITOCHONDRION span membrane 2 NADH or CYTOSOL 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 GLYCOLYSIS PYRUVATE OXIDATIVE OXIDATION CITRIC PHOSPHORYLATION ACID Glucose 2 Pyruvate 2 Acetyl CoA CYCLE (Electron transport and chemiosmosis) + 2 ATP + 2 ATP + about 26 or 28 ATP About Maximum per glucose: 30 or 32 ATP © 2018 Pearson Education Ltd. Figure 10.16a Electron shuttles span membrane 2 NADH or 2 FADH2 2 NADH GLYCOLYSIS Glucose 2 Pyruvate + 2 ATP © 2018 Pearson Education Ltd. Figure 10.16b 2 NADH 6 NADH 2 FADH2 PYRUVATE CITRIC OXIDATION ACID 2 Acetyl CoA CYCLE + 2 ATP © 2018 Pearson Education Ltd. Figure 10.16c 2 NADH or 2 FADH2 2 NADH 6 NADH 2 FADH2 OXIDATIVE PHOSPHORYLATION (Electron transport and chemiosmosis) + about 26 or 28 ATP © 2018 Pearson Education Ltd. Figure 10.16d About Maximum per glucose: 30 or 32 ATP © 2018 Pearson Education Ltd.  There are three reasons why the number of ATP is not known exactly 1. Photophosphorylation and the redox reactions are not directly coupled; the ratio of NADH to ATP molecules is not a whole number 2. ATP yield varies depending on whether electrons are passed to NAD+ or FAD in the mitochondrial matrix 3. The proton-motive force is also used to drive other kinds of work © 2018 Pearson Education Ltd. Concept 10.5: Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen  Most cellular respiration depends on electronegative oxygen to pull electrons down the transport chain  Without oxygen, the electron transport chain will cease to operate  In that case, glycolysis couples with anaerobic respiration or fermentation to produce ATP © 2018 Pearson Education Ltd.  Anaerobic respiration uses an electron transport chain with a final electron acceptor other than oxygen, for example, sulfate  Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP © 2018 Pearson Education Ltd. Types of Fermentation  Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis  Two common types are alcohol fermentation and lactic acid fermentation © 2018 Pearson Education Ltd.  In alcohol fermentation, pyruvate is converted to ethanol in two steps  The first step releases CO2 from pyruvate  The second step produces NAD+ and ethanol  Alcohol fermentation by yeast is used in brewing, winemaking, and baking © 2018 Pearson Education Ltd. Figure 10.17a 2 ADP + 2 P i 2 ATP Glucose GLYCOLYSIS 2 Pyruvate 2 NAD+ 2 NADH 2 CO2 + 2 H+ NAD+ REGENERATION 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation © 2018 Pearson Education Ltd.  In lactic acid fermentation, pyruvate is reduced by NADH, forming NAD+ and lactate as end products, with no release of CO2  Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt  Human muscle cells use lactic acid fermentation to generate ATP during strenuous exercise when O2 is scarce © 2018 Pearson Education Ltd. Figure 10.17b 2 ADP + 2 P i 2 ATP Glucose GLYCOLYSIS 2 NAD+ 2 NADH + 2 H+ 2 Pyruvate NAD+ REGENERATION 2 Lactate (b) Lactic acid fermentation © 2018 Pearson Education Ltd. Comparing Fermentation with Anaerobic and Aerobic Respiration  All use glycolysis (net ATP = 2) to oxidize glucose and harvest the chemical energy of food  In all three, NAD+ is the oxidizing agent that accepts electrons during glycolysis © 2018 Pearson Education Ltd.  The processes have different mechanisms for oxidizing NADH to NAD+:  In fermentation, an organic molecule (such as pyruvate or acetaldehyde) acts as a final electron acceptor  In cellular respiration, electrons are transferred to the electron transport chain  Cellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule © 2018 Pearson Education Ltd.  Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2  Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration  In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes © 2018 Pearson Education Ltd. Figure 10.18 Glucose Glycolysis CYTOSOL Pyruvate No O2 present: O2 present: Fermentation Aerobic cellular respiration MITOCHONDRION Ethanol, Acetyl CoA lactate, or other products CITRIC ACID CYCLE © 2018 Pearson Education Ltd. The Evolutionary Significance of Glycolysis  Glycolysis is an ancient process  Early prokaryotes likely used glycolysis to produce ATP before O2 accumulated in the atmosphere  Used in both cellular respiration and fermentation, it is the most widespread metabolic pathway on Earth  This pathway occurs in the cytosol so does not require the membrane-bound organelles of eukaryotic cells © 2018 Pearson Education Ltd.