CAMPBELL BIOLOGY TENTH EDITION Reece Urry Cain Wasserman Minorsky Jackson 9 Cellular Respiration and Fermentation Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick 2014 Pearson Education, Inc.
Life Is Work Living cells require energy from outside sources. Some animals, such as the giraffe, obtain energy by eating plants, and some animals feed on other organisms that eat plants. Respiration has three key pathways: glycolysis, the citric acid cycle, and oxidative phosphorylation. Figure 9.1
Overview: Life Is Work Living cells require energy from outside sources. Energy enters most ecosystems as sunlight and leaves as heat. Photosynthesis generates oxygen and organic molecules that the mitochondria of eukaryotes (including plants and algae) use as fuel for cellular respiration. Cells harvest the chemical energy stored in organic molecules and use it to regenerate ATP, the molecule that drives most cellular work. Figure 9.2 2014 Pearson Education, Inc.
Catabolic pathways yield energy by oxidizing organic fuels Catabolic metabolic pathways release the energy stored in complex organic molecules represents potential energy. Enzymes catalyze the systematic degradation of organic molecules that are rich in energy to simpler waste products that have less energy. Some of the released energy is used to do work; the rest is dissipated as heat. Electron transfer plays a major role in these pathways. Several processes are central to cellular respiration and related pathways 2014 Pearson Education, Inc.
Catabolic Pathways and Production of ATP The breakdown of organic molecules is exergonic Fermentation is a partial degradation of sugars that occurs without O 2 Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration Aerobic respiration consumes organic molecules and O 2 and yields ATP is complete degradation of sugar. Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O 2 Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: C 6 H 12 O 6 + 6 O 2 6 CO 2 + 6 H 2 O + Energy (ATP + heat) 686kcal/mole 2014 Pearson Education, Inc.
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 One mole ATP generate G = 7.3 kcal/mol
The Principle of Redox Reactions that result in the transfer of one or more electrons (e ) from one reactant to another are oxidation-reduction reactions, or redox reactions. The loss of electrons from a substance is called oxidation or is oxidized. The addition of electrons to another substance is called reduction or is reduced (the amount of positive charge is reduced). Adding electrons is called reduction because negatively charged electrons added to an atom reduce the amount of positive charge of that atom. 2014 Pearson Education, Inc.
Reducing Agent Oxidizing Agent Xe + Y X + Ye The formation of table salt from sodium and chloride, Na + Cl Na+ + Cl, is a redox reaction. Sodium is oxidized, and chlorine is reduced (its charge drops from 0 to 1). More generally: Xe + Y X + Ye. X, the electron donor, is the reducing agent and reduces Y by donating an electron to it. Y, the electron recipient, is the oxidizing agent and oxidizes X by removing its electron. Redox reactions require both a donor and an acceptor. 2014 Pearson Education, Inc.
The combustion of methane to form water and carbon dioxide: Figure 9.3 When oxygen reacts with the hydrogen from methane to form water, the electrons of the covalent bonds are drawn closer to the oxygen. When oxygen reacts with the carbon from methane to form carbon dioxide, electrons end up farther away from the carbon atom and closer to their new covalent partners, the oxygen atoms, which are very electronegative. In effect, the carbon atom has partially lost its shared electrons. Thus, methane has been oxidized. In effect, each oxygen atom has partially gained electrons, and so the oxygen molecule has been reduced. Oxygen is very electronegative and is one of the most potent of all oxidizing agents. 2014 Pearson Education, Inc.
Organic fuel molecules are oxidized during cellular respiration Respiration, the oxidation of glucose and other molecules in food, is a redox process. In a series of reactions, glucose is oxidized and oxygen is reduced. The electrons lose potential energy along the way, and energy is released. As hydrogen is transferred from glucose to oxygen, the energy state of the electron changes. In respiration, the oxidation of glucose transfers electrons to a lower energy state, releasing energy that becomes available for ATP synthesis. 2014 Pearson Education, Inc.
The main energy foods, carbohydrates and fats, are reservoirs of electrons associated with hydrogen. These molecules are stable because of the barrier of activation energy. Without this barrier, a food molecule like glucose would combine almost instantaneously with O2. If activation energy is supplied by igniting glucose, it burns in air, releasing 686 kcal (2,870 kj) of heat per mole of glucose (about 180 g). This reaction cannot happen at body temperatures. Instead, enzymes within cells lower the barrier of activation energy, allowing sugar to be oxidized in a series of steps. 2014 Pearson Education, Inc.
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 the coenzyme NAD + (nicotinamide adenine dinucleotide) 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 2014 Pearson Education, Inc.
NAD+ trap electrons from glucose NAD+ functions as the oxidizing agent in many of the redox steps during the breakdown of glucose. Dehydrogenase enzymes strip two hydrogen atoms from the substrate (glucose), thus oxidizing it. The enzyme passes two electrons and one proton to NAD+. The other proton is released as H+ to the surrounding solution. By receiving two electrons and only one proton, NAD+ has its charge neutralized when it is reduced to NADH. Each NADH molecule formed during respiration represents stored energy. 2011 Pearson Education, Inc.
Figure 9.4 NAD + 2 e + 2 H+ Dehydrogenase 2 e + H + NADH H + Nicotinamide (oxidized form) 2[H] (from food) Reduction of NAD + Oxidation of NADH H + Nicotinamide (reduced form)
Figure 9.5 H 2 + ½ O 2 2 H + ½ O 2 2 H + + 2 e Controlled release of energy Free energy, G Explosive release Free energy, G 2 e ATP ATP ATP 2 H + ½ O 2 H 2 O H 2 O (a) Uncontrolled reaction (b) Cellular respiration
Electrons extracted from glucose and stored in NADH transferred to oxygen Each NADH molecule formed during respiration represents stored energy. This energy is tapped to synthesize ATP as electrons fall down an energy gradient from NADH to oxygen. Cellular respiration uses an electron transport chain to break the fall of electrons to O2 into several steps. Electrons released from food are shuttled by NADH to the top higher-energy end of the chain. At the bottom lower-energy end, oxygen captures the electrons along with H+ to form water. 2014 Pearson Education, Inc.
The electron transport chain consists of several molecules primarily proteins built into the inner membrane of a mitochondrion of eukaryotic cells and the plasma membrane of aerobically respiring prokaryotes. In summary, during cellular respiration, most electrons travel the following downhill route: glucose NADH electron transport chain oxygen. The cell uses the energy released from this stepwise electron fall to regenerate ATP Electron transfer from NADH to oxygen is an exergonic reaction: G = 53 kcal/mol 2014 Pearson Education, Inc.
The Stages of Cellular Respiration: A Preview Cellular respiration has three stages:- Glycolysis breaks down glucose into two molecules of pyruvate. Glycolysis occurs in the cytosol. The Citric Acid Cycle or Kreb s cycle completes the breakdown of glucose. The citric acid cycle occurs in the mitochondrial matrix of eukaryotic cells or in the cytoplasm of prokaryotes. Oxidative Phosphorylation the electron transport chain accepts electrons from the first 2 stages and accounts for most of the ATP synthesis because it is powered by redox reactions. Oxidative phosphorylation occurs at the inner membrane of the mitochondrion. 2014 Pearson Education, Inc.
Cellular Respiration-3 stages Electrons via NADH Electrons via NADH and FADH 2 Glucose GLYCOLYSIS Pyruvate PYRUVATE OXIDATION Acetyl CoA CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION (Electron transport and chemiosmosis) CYTOSOL MITOCHONDRION ATP Substrate-level phosphorylation Figure 9.6 ATP Substrate-level phosphorylation ATP Oxidative phosphorylation
Substrate-level phosphorylation Figure 9.7 In substrate level phosphorylation an enzyme transfers a phosphate group from a substrate molecule to ADP. Some ATP is also formed directly during glycolysis and the citric acid cycle by substrate-level phosphorylation, in which an enzyme transfers a phosphate group from an organic substrate molecule to ADP, forming only 4 ATP.
The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration For each molecule of glucose degraded to CO 2 and water by respiration, the cell makes up to 32 molecules of ATP 2014 Pearson Education, Inc.
Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Glycolysis ( splitting of sugar ) breaks down 6-carbon glucose into two molecules of 3-carbon pyruvate Glycolysisis a 10 step process that occurs in the cytoplasm and has two major phases: 1. In the energy investment phase, the cell spends ATP. 2. In the energy payoff phase, this investment is repaid with interest. ATP is produced by substrate-level phosphorylation, and NAD+ is reduced to NADH by electrons released by the oxidation of glucose. No CO2 is produced during glycolysis. Glycolysis occurs +/- oxygen. 2014 Pearson Education, Inc.
Energy Investment Phase Figure 9.8 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 H 2 O Net Glucose 4 ATP formed 2 ATP used 2 NAD + + 4 e + 4 H + 2 2 2 Pyruvate + 2 H 2 O ATP NADH + 2 H +
Figure 9.9a Glycolysis: Energy Investment Phase Glucose ATP Glucose 6-phosphate Fructose 6-phosphate ADP Hexokinase 1 Step 1 Glucose enters the cell and is phosphorylated by hexokinase, which transfers a P group from ATP to glucose to form Glucose-6-P. Phosphoglucoisomerase 2 Step 2 Phosphoglucoisomerase converts glucose-6- phosphate to its isomer,fructose-6- phosphate
Figure 9.9b Glycolysis: Energy Investment Phase Fructose 6-phosphate ATP ADP Fructose 1,6-bisphosphate Step 3 Fructokinase transfers a P group from ATP tofructose-6- phosphate. With P groups on opposite sides, fructose-1,6- biphosphate is ready to be split in half. Phosphofructokinase Aldolase 4 3 Dihydroxyacetone phosphate Isomerase 5 Glyceraldehyde 3-phosphate To step 6 So far 2 molecules of ATP have been used.
Step 4 Aldose cleaves fructose 1,6-biphosphate into two different 3 carbon sugars: Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde-3-phosphate (G3-P) Step 5 Isomerase catalyzes the reversible conversion between the 3-carbon sugar isomers. This reaction never reaches equilibrium because glyceraldehyde-3-phosphate is used as quickly as it is formed as a substrate in the subsequent step in glycolysis.
Figure 9.9c Glycolysis: Energy Payoff Phase 2 NADH 2 NAD + + 2 H + 2 2 ATP 2 ADP 2 Triose phosphate dehydrogenase 6 2 P i 1,3-Bisphosphoglycerate Phosphoglycerokinase 7 3-Phosphoglycerate 2 molecules of NADH have been produced and 2 molecules of ATP have been generated.
Step 6 Triose phosphate dehydrogenase catalyzes 2 sequential reactions while holding glyceraldehyde-3-phosphate in its active site. The sugar is oxidized by the transfer of - and H+ to NAD+. This is a very exergonic redox reaction. The enzyme uses the released energy to add a P group to the oxidized substrate. The 2 1,3-bisphosphoglycerate molecules produced have very high potential energy. Step 7 Phosphoglycerokinase The P group added to make 1,3-bisphosphoglycerate in the previous step is transferred to ADP in an exergonic reaction, with 2 ATP produced. So the energy made available by oxidation in step 6 has been used to generate ATP.
igure 9.9d 2 more molecules of ATP have been generated Pyruvate is the final product of the 10 step process of glycolysis and the initial substrate of the citric acid cycle (if O2 is present) Glycolysis: Energy Payoff Phase 2 2 2 H 2 O 2 2 ADP 2 ATP 2 3-Phosphoglycerate Phosphoglyceromutase Step 8 Phosphoglyceromutase relocates the remaining P group to change 3-phosphoglycerate to 2-phosphoglycerate. 8 2-Phosphoglycerate Enolase 9 Step9 Phosphoenolpyruvate (PEP) Enolase catalyzes a double bond formation in 2-phosphoglycerate yielding phosphoenolpyruvate(pep). The electron arrangement of PEP allows it to have high potential energy. This high potential energy allows step 10 to occur. Pyruvate kinase 10 Step 10 Pyruvate Pyruvate Kinase Transfers a P group from the 2 PEP molecules formed in step 9 to generate 2 ATP from 2 ADP (substrate-level phosphorylation).glucose is oxidized to Pyruvate
Energy Produced by Glycolysis 2 ATP invested in energy investment phase (1 and 3 steps) 4 ATP produced in energy payoff phase (7 and 10 steps). Net gain 2 ATP Additional energy is stored as 2 NADH (6 step) which can be used to make ATP by oxidative phosphorylation if O 2 is present. 2014 Pearson Education, Inc.
The citric acid cycle completes the energyyielding oxidation of organic molecules In the presence of O 2, pyruvate enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA, which links the glycolysis to citric acid cycle This step is carried out by a multienzyme complex that catalyses three reactions 2014 Pearson Education, Inc.
Pyruvate to Acetyl CoA is an intermediate step process Figure 9.10 Pyruvate enters the mitochondrion via active transport. Pyruvate s carboxyl group is removed and released as CO2 within the mitochondrion. The remaining 2 carbon fragment is oxidized, forming acetate. The extracted - are transferred to NAD+, and energy is stored as NADH. Coenzyme A (CoA), a sulfur containing compound derived from B vitamins, is attached to acetate by an unstable bond form Acetyl CoA. It enters the citric acid cycle. 2014 Pearson Education, Inc.
The Citric Acid Cycle The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix 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 cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH 2 per cycle but it run twice so everything will be double in number. The NADH and FADH 2 produced by the cycle relay electrons extracted from food to the electron transport chain Copyright 2014 Pearson Education, Inc.,
Figure 9.11 Pyruvate CO 2 NAD + CoA NADH + H + Acetyl CoA CoA CoA The citric acid cycle oxidizes organic fuel derived from pyruvate run twice, finally generating 2 ATP, 6 NADH, and 2 FADH2. The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain. Citric acid cycle 2 CO 2 FADH 2 FAD 3 NAD + 3 NADH + 3 H + ADP + P i ATP
Fig.9.12a Acetyl CoA CoA-SH Oxaloacetate 1 Citrate Citric acid cycle Step 1 -Acetyl CoA adds its acetyl group to oxaloacetate to produce citrate.
Fig.9.12b Acetyl CoA CoA-SH 1 H 2 O Oxaloacetate 2 Citrate Isocitrate Citric acid cycle Step 2 -Citrate is converted to its isomer, isocitrate by the removal of one H2O molecule and the addition of another H2O molecule.
Fig.9.12c Acetyl CoA CoA-SH 1 H 2 O Oxaloacetate 2 Citric acid cycle Citrate Isocitrate NAD + NADH 3 + H + CO 2 α-ketoglutarate Step 3 -Isocitrate is oxidized (reducing NAD+ to NADH) and the resulting compound loses a CO2 molecule to form α-ketoglutarate.
Fig.9.12d Acetyl CoA CoA-SH 1 H 2 O Step 4 -Another CO2 is lost and the resulting compound is oxidized. This reduces NAD+ to NADH. CoA is then attached to the resulting molecule by an unstable bond to form SuccinylCoA. Oxaloacetate Citric acid cycle Citrate 2 CoA-SH 4 Isocitrate NAD + NADH 3 + H + CO 2 α-ketoglutarate NAD + CO 2 Succinyl CoA NADH + H +
Fig.9.12e Step 5 -A P group displaces the CoA. Then the P group is transferred to guanosine diphosphate (GDP) forming GTP. GTP can be used to make an ATP molecule or GTP can power cellular work. Succinate is produced when the P group is transferred to GDP. Acetyl CoA Oxaloacetate 1 Citric acid cycle CoA-SH CoA-SH Citrate 2 CoA-SH 4 H 2 O Isocitrate NAD + NADH 3 + H + CO 2 α-ketoglutarate 5 NAD + CO 2 Succinate ADP GTP GDP P i Succinyl CoA NADH + H + ATP
Fig.9.12f Step 6-2 hydrogens are transferred to FAD (flavin dinucleotide), forming FADH2 and oxidizing succinate to form fumarate. Acetyl CoA Oxaloacetate 1 Citric acid cycle CoA-SH Citrate 2 H 2 O Isocitrate NAD + NADH 3 + H + CO 2 Fumarate CoA-SH α-ketoglutarate 6 CoA-SH 4 FADH 2 FAD 5 NAD + CO 2 Succinate GTP GDP ADP P i Succinyl CoA NADH + H + ATP
Fig.9.12g Step 7- H2O is added, converting fumarate to malate. Acetyl CoA 1 CoA-SH H 2 O Oxaloacetate 2 H 2 O 7 Malate Citric acid cycle Citrate Isocitrate NAD + NADH 3 + H + CO 2 Fumarate CoA-SH α-ketoglutarate 6 CoA-SH 4 FADH 2 FAD 5 NAD + CO 2 Succinate GTP GDP ADP P i Succinyl CoA NADH + H + ATP
Fig.9.11h Step 8- Malate is oxidized to oxaloacetate which can reenter the cycle for another round. NAD+ is reduced to NADH. H 2 O NAD + NADH 7 + H + 8 Malate Acetyl CoA Oxaloacetate 1 Citric acid cycle CoA-SH Citrate 2 H 2 O Isocitrate NAD + NADH 3 + H + CO 2 Fumarate CoA-SH α-ketoglutarate 6 CoA-SH 4 FADH 2 FAD 5 NAD + CO 2 Succinate GTP GDP ADP P i Succinyl CoA NADH + H + ATP
During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis Following glycolysis and the citric acid cycle, NADH and FADH 2 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 2014 Pearson Education, Inc.
The Pathway of Electron Transport The electron transport chain is in the cristae of the mitochondrion Most of the chain s components are proteins, which exist in multiprotein complexes The carriers alternate reduced and oxidized states as they accept and donate electrons Electrons drop in free energy as they go down the chain and are finally passed to O 2, which combine with H of NADH or FADH2 to form H 2 O 2014 Pearson Education, Inc.
Figure 9.13 50 2 e NADH NAD + FADH 2 Free energy (G) relative to O 2 (kcal/mol) 40 30 20 10 FMN Fe S I Q 2 e Fe S Cyt b FAD II Fe S III Cyt c 1 Multiprotein complexes Cyt c Cyt a 2 e (originally from NADH or FADH 2 ) IV Cyt a 3 0 2 H + + 1 / 2 O 2 H 2 O
Electrons are transferred from NADH or FADH 2 to the electron transport chain Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O 2 The electron transport chain generates no ATP The chain s function is to break the large free-energy drop from food to O 2 into smaller steps that release energy in manageable amounts 2014 Pearson Education, Inc.
Chemiosmosis: The Energy-Coupling Mechanism Electron transfer in the electron transport chain causes proteins to pump H + from the mitochondrial matrix to the intermembrane space H + then moves back across the membrane, passing through channels in ATP synthase ATP synthase uses the exergonic flow of H + to drive phosphorylation of ATP This is an example of chemiosmosis, the use of energy in a H + gradient to drive cellular work 2014 Pearson Education, Inc.
Figure 9.14 INTERMEMBRANE SPACE Rotor H + Stator Internal rod Catalytic knob ADP + P i MITOCHONDRIAL MATRIX ATP
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 protonmotive force, emphasizing its capacity to do work 2014 Pearson Education, Inc.
Figure 9.15 Protein complex of electron carriers H + H + Cyt c H + H + ATP synthase I Q III IV II FADH 2 FAD 2 H + + ½ O 2 H 2 O (carrying electrons from food) NADH NAD + 1 Electron transport chain 2 Oxidative phosphorylation ADP + P i H + ATP Chemiosmosis
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 38 ATP There are several reasons why the number of ATP is not known exactly 2014 Pearson Education, Inc.
Figure 9.16 CYTOSOL Electron shuttles span membrane 2 NADH 2 NADH or 2 FADH 2 2 NADH 6 NADH 2 FADH 2 MITOCHONDRION GLYCOLYSIS Glucose 2 Pyruvate PYRUVATE OXIDATION 2 Acetyl CoA CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION (Electron transport and chemiosmosis) + 2 ATP + 2 ATP + about 26 or 28 ATP Maximum per glucose: 1NADH makes 3ATP and 1FADH2 makes 2ATP About 30 or 32 ATP 10NADH = 10X3ATP=30ATP and 2FADH2 = 2X2ATP= 4ATP. Finally 28ATP+4ATP= 32ATP in eukaryotes but prokaryotes has more 30+4ATP= 34ATP by oxidative phosphorylation. Only 2ATP from glycolysis and 2ATP from citric acid cycle. Total #ATP generate by eukaryotes 2+2+32 =36ATP and by prokaryote 2+2+34=38ATP.
Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen Most cellular respiration requires O 2 to produce ATP Glycolysis can produce ATP with or without O 2 (in aerobic or anaerobic conditions) In the absence of O 2, glycolysis couples with fermentation or anaerobic respiration to produce ATP Anaerobic respiration uses an electron transport chain with an electron acceptor other than O 2, for example sulfate or nitrate Fermentation uses substrate level phosphorylation instead of an electron transport chain to generate ATP 2014 Pearson Education, Inc.
Two 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. 2014 Pearson Education, Inc.
In alcohol fermentation, pyruvate is converted to ethanol in two steps The first step releases CO 2 The second step produces ethanol Alcohol fermentation by yeast is used in brewing, winemaking, and baking 2014 Pearson Education, Inc.
Figure 9-17a 2 ADP + 2 P i 2 ATP Glucose Glycolysis 2 NAD + 2 NADH + 2 H + 2 Pyruvate 2 CO 2 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation
In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO 2 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 when O 2 is scarce 2014 Pearson Education, Inc.
Figure 9-17 b 2 ADP + 2 P i 2 ATP Glucose Glycolysis 2 NAD + 2 NADH + 2 H + 2 Pyruvate 2 Lactate (b) Lactic acid fermentation
Comparing Fermentation with Anaerobic and Aerobic Respiration All use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food In all three, NAD + is the oxidizing agent that accepts electrons during glycolysis The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O 2 in cellular respiration Cellular respiration produces 36 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule 2014 Pearson Education, Inc.
Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O 2 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 2014 Pearson Education, Inc.
Figure 9.18 Glucose CYTOSOL Glycolysis Pyruvate No O 2 present: Fermentation O 2 present: Aerobic cellular respiration Ethanol, lactate, or other products Acetyl CoA MITOCHONDRION CITRIC ACID CYCLE
The Evolutionary Significance of Glycolysis Ancient prokaryotes are thought to have used glycolysis long before there was oxygen in the atmosphere Very little O 2 was available in the atmosphere until about 2.7 billion years ago, so early prokaryotes likely used only glycolysis to generate ATP Glycolysis is a very ancient process Glycolysis occurs in nearly all organisms 2014 Pearson Education, Inc.
Glycolysis and the citric acid cycle connect to many other metabolic pathways Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways
The Versatility of Catabolism Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration Glycolysis accepts a wide range of carbohydrates Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle 2014 Pearson Education, Inc.
Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA) Fatty acids are broken down by beta oxidation and yield acetyl CoA An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate 2014 Pearson Education, Inc.
Proteins Carbohydrates Fats Amino acids Sugars Glycerol Fatty acids GLYCOLYSIS Glucose Glyceraldehyde 3- P NH 3 Pyruvate Acetyl CoA Figure 9.19 CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION
Biosynthesis (Anabolic Pathways) The body uses small molecules to build other substances These small molecules may come directly from food, from glycolysis, or from the citric acid cycle 2014 Pearson Education, Inc.
Regulation of Cellular Respiration via Feedback Mechanisms Feedback inhibition is the most common mechanism for control If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway 2014 Pearson Education, Inc.
Figure 9.20 Glucose GLYCOLYSIS Fructose 6-phosphate Phosphofructokinase AMP Stimulates Inhibits Fructose 1,6-bisphosphate Inhibits ATP Pyruvate Acetyl CoA Citrate CITRIC ACID CYCLE Oxidative phosphorylation
Inputs Glucose Glycolysis Outputs 2 Pyruvate + 2 ATP + 2 NADH Inputs Outputs 2 Pyruvate 2 Acetyl CoA 2 Oxaloacetate Citric acid cycle 2 ATP 6 CO2 8 2 NADH FADH2 Overview Cellular Respiration