In the presence of O2, pyruvate enters the mitochondrion Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules In the presence of O2, pyruvate enters the mitochondrion Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

CYTOSOL MITOCHONDRION NAD+ NADH + H+ 2 1 3 Acetyl CoA Pyruvate Fig. 9-10 CYTOSOL MITOCHONDRION NAD+ NADH + H+ 2 1 3 Acetyl CoA Figure 9.10 Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the citric acid cycle Pyruvate Coenzyme A CO2 Transport protein

The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Pyruvate CO2 NAD+ CoA NADH + H+ Acetyl CoA CoA CoA Citric acid cycle 2 Fig. 9-11 Pyruvate CO2 NAD+ CoA NADH + H+ Acetyl CoA CoA CoA Citric acid cycle 2 CO2 Figure 9.11 An overview of the citric acid cycle FADH2 3 NAD+ FAD 3 NADH + 3 H+ ADP + P i ATP

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Citric acid cycle Succinyl CoA Fig. 9-12-8 Acetyl CoA CoA—SH NADH +H+ 1 H2O NAD+ 8 Oxaloacetate 2 Malate Citrate Isocitrate NAD+ Citric acid cycle NADH 3 7 + H+ H2O CO2 Fumarate CoA—SH -Keto- glutarate Figure 9.12 A closer look at the citric acid cycle 4 6 CoA—SH FADH2 5 CO2 NAD+ FAD Succinate P NADH i GTP GDP Succinyl CoA + H+ ADP ATP

Concept 9.4: 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 For the Cell Biology Video ATP Synthase 3D Structure — Side View, go to Animation and Video Files. For the Cell Biology Video ATP Synthase 3D Structure — Top View, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 O2, forming H2O Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 9.13 Free-energy change during electron transport NADH 50 2 e– NAD+ FADH2 2 e– FAD Multiprotein complexes  40 FMN FAD Fe•S  Fe•S Q  Cyt b Fe•S 30 Cyt c1 IV Free energy (G) relative to O2 (kcal/mol) Cyt c Cyt a Cyt a3 20 Figure 9.13 Free-energy change during electron transport e– 10 2 (from NADH or FADH2) 2 H+ + 1/2 O2 H2O

The electron transport chain generates no ATP 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 The chain’s function is to break the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

INTERMEMBRANE SPACE H+ Stator Rotor Internal rod Cata- lytic knob ADP Fig. 9-14 INTERMEMBRANE SPACE H+ Stator Rotor Internal rod Figure 9.14 ATP synthase, a molecular mill Cata- lytic knob ADP + P ATP i MITOCHONDRIAL MATRIX

Number of photons detected (103) Fig. 9-15 EXPERIMENT Magnetic bead Electromagnet Internal rod Sample Catalytic knob Nickel plate RESULTS Rotation in one direction Rotation in opposite direction No rotation Figure 9.15 Is the rotation of the internal rod in ATP synthase responsible for ATP synthesis? 30 Number of photons detected (103) 25 20 Sequential trials

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Electron transport chain 2 Chemiosmosis Fig. 9-16 H+ H+ H+ H+ Protein complex of electron carriers Cyt c V Q   ATP synthase  2 H+ + 1/2O2 H2O FADH2 FAD NADH NAD+ Figure 9.16 Chemiosmosis couples the electron transport chain to ATP synthesis ADP + P ATP i (carrying electrons from food) H+ 1 Electron transport chain 2 Chemiosmosis Oxidative phosphorylation

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 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 9-17 CYTOSOL Electron shuttles span membrane MITOCHONDRION 2 NADH or 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 Glycolysis Oxidative phosphorylation: electron transport and chemiosmosis 2 Pyruvate 2 Acetyl CoA Citric acid cycle Glucose + 2 ATP + 2 ATP + about 32 or 34 ATP Figure 9.17 ATP yield per molecule of glucose at each stage of cellular respiration About 36 or 38 ATP Maximum per glucose:

Most cellular respiration requires O2 to produce ATP Concept 9.5: Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen Most cellular respiration requires O2 to produce ATP Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions) In the absence of O2, glycolysis couples with fermentation or anaerobic respiration to produce ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Anaerobic respiration uses an electron transport chain with an electron acceptor other than O2, for example sulfate Fermentation uses phosphorylation instead of an electron transport chain to generate ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Two common types are alcohol fermentation and lactic acid fermentation 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Animation: Fermentation Overview In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2 Alcohol fermentation by yeast is used in brewing, winemaking, and baking Animation: Fermentation Overview Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 9-18 Figure 9.18 Fermentation 2 ADP + 2 Pi 2 ATP Glucose Glycolysis 2 Pyruvate 2 NAD+ 2 NADH 2 CO2 + 2 H+ 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation 2 ADP + 2 Pi 2 ATP Glucose Glycolysis Figure 9.18 Fermentation 2 NAD+ 2 NADH + 2 H+ 2 Pyruvate 2 Lactate (b) Lactic acid fermentation

(a) Alcohol fermentation Fig. 9-18a 2 ADP + 2 P 2 ATP i Glucose Glycolysis 2 Pyruvate 2 NAD+ 2 NADH 2 CO2 + 2 H+ Figure 9.18a Fermentation 2 Acetaldehyde 2 Ethanol (a) Alcohol fermentation

In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, 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 when O2 is scarce Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(b) Lactic acid fermentation Fig. 9-18b 2 ADP + 2 P 2 ATP i Glucose Glycolysis 2 NAD+ 2 NADH + 2 H+ 2 Pyruvate Figure 9.18b Fermentation 2 Lactate (b) Lactic acid fermentation

Fermentation and Aerobic Respiration Compared Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration Cellular respiration produces 38 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Ethanol or lactate Citric acid cycle Fig. 9-19 Glucose Glycolysis CYTOSOL Pyruvate O2 present: Aerobic cellular respiration No O2 present: Fermentation MITOCHONDRION Ethanol or lactate Acetyl CoA Figure 9.19 Pyruvate as a key juncture in catabolism Citric acid cycle

The Evolutionary Significance of Glycolysis Glycolysis occurs in nearly all organisms Glycolysis probably evolved in ancient prokaryotes before there was oxygen in the atmosphere Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings