Cellular Respiration: Harvesting Chemical Energy Chapter 9 Cellular Respiration: Harvesting Chemical Energy

Energy flows into an ecosystem as sunlight and leaves as heat Photosynthesis generates O2 and organic molecules, which are used in cellular respiration Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Organic molecules Cellular respiration in mitochondria Fig. 9-2 Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic molecules CO2 + H2O + O2 Cellular respiration in mitochondria Figure 9.2 Energy flow and chemical recycling in ecosystems ATP ATP powers most cellular work Heat energy

Organic compounds possess potential energy as a result of their arrangement of atoms. With the help of enzymes, a cell systematically degrades complex organic molecules that are rich in potential energy to simpler waste products that have less energy. Some of the energy taken out of chemical storage can be used to do work; the rest is dissipated as heat.

The breakdown of organic molecules is exergonic 1. Distinguish between: The breakdown of organic molecules is exergonic Fermentation is a partial degradation of sugars that occurs without O2 Aerobic respiration (most prevalent) consumes organic molecules and O2 and yields ATP Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2 Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy (ATP + heat) 2. Write down the formula for the breakdown of glucose in cellular 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) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

3. How do the catabolic pathways that decompose glucose and other organic fuels yield energy? The relocation of electrons during the chemical reactions of respiration releases energy stored in organic molecules, and this energy ultimately is used to synthesize ATP. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

4. What is a redox reaction? Define the following: 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) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

becomes oxidized (loses electron) becomes reduced (gains electron) Fig. 9-UN1 becomes oxidized (loses electron) becomes reduced (gains electron)

becomes oxidized becomes reduced Fig. 9-UN2 becomes oxidized becomes reduced

Methane (reducing agent) Oxygen (oxidizing agent) Fig. 9-3 Reactants Products becomes oxidized becomes reduced Figure 9.3 Methane combustion as an energy-yielding redox reaction Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water

An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one, just like a ball rolling downhill. A redox reaction that moves electrons closer to oxygen, such as the burning of methane, therefore releases chemical energy that can be put to work. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

5. In general, why are organic molecules that have an abundance of hydrogen atoms excellent fuels? They are excellent fuels because their bonds are a source of “hilltop” electrons. Their energy may be released as these electrons “fall” down an energy gradient when they are transferred to oxygen. In respiration, the oxidation of glucose transfers electrons to a lower energy state, liberating energy that becomes available for ATP synthesis. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

6. Describe the difference between igniting glucose and burning it, and how we break it down in our bodies. Cellular respiration does not oxidize glucose in a single explosive step. Rather, we breakdown glucose in a step-by-step process, releasing energy in small increments rather than in one large quantity. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

7. What is NAD+. What does it become when it is reduced 7. What is NAD+? What does it become when it is reduced? What is its role in cellular respiration? 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

NADH H+ NAD+ + 2[H] + H+ 2 e– + 2 H+ 2 e– + H+ Dehydrogenase Fig. 9-4 2 e– + 2 H+ 2 e– + H+ NADH H+ Dehydrogenase Reduction of NAD+ NAD+ + 2[H] + H+ Oxidation of NADH Nicotinamide (reduced form) Nicotinamide (oxidized form) Figure 9.4 NAD+ as an electron shuttle

NADH passes the electrons to the electron transport chain 8. What is an electron transport chain? Why is it so important in regards to the release of energy? 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(a) Uncontrolled reaction (b) Cellular respiration Fig. 9-5 H2 + 1/2 O2 2 H + 1/2 O2 (from food via NADH) Controlled release of energy for synthesis of ATP 2 H+ + 2 e– ATP Explosive release of heat and light energy ATP Electron transport chain Free energy, G Free energy, G ATP 2 e– 1/2 O2 Figure 9.5 An introduction to electron transport chains 2 H+ H2O H2O (a) Uncontrolled reaction (b) Cellular respiration

9. What element captures electrons at the bottom of the ETC 9. What element captures electrons at the bottom of the ETC. What is formed? Oxygen 10. In summary, during cellular respiration, most electrons travel the following “downhill” route: glucose NADH ETC oxygen

Cellular respiration has three stages: 11. Briefly list and describe the three stages of cellular respiration. Cellular respiration has three stages: Glycolysis (breaks down glucose into two molecules of pyruvate) The citric acid cycle (completes the breakdown of glucose) Oxidative phosphorylation (accounts for most of the ATP synthesis) For each molecule of glucose, between about 36-38 ATP, although 36 is most widely used. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Electrons carried via NADH ATP Substrate-level phosphorylation Fig. 9-6-1 Electrons carried via NADH Glycolysis Glucose Pyruvate Cytosol Figure 9.6 An overview of cellular respiration ATP Substrate-level phosphorylation

Electrons carried via NADH Electrons carried via NADH and FADH2 Fig. 9-6-2 Electrons carried via NADH Electrons carried via NADH and FADH2 Glycolysis Citric acid cycle Glucose Pyruvate Mitochondrion Cytosol Figure 9.6 An overview of cellular respiration ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation

Electrons carried via NADH Electrons carried via NADH and FADH2 Fig. 9-6-3 Electrons carried via NADH Electrons carried via NADH and FADH2 Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis Citric acid cycle Glucose Pyruvate Mitochondrion Cytosol Figure 9.6 An overview of cellular respiration ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation

BioFlix: Cellular Respiration 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 BioFlix: Cellular Respiration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation Enzyme Enzyme Figure 9.7 Substrate-level phosphorylation ADP P Substrate + ATP Product

13. What does the word “glycolysis” mean? Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

14. List and describe the two phases of glycolysis. Glycolysis occurs in the cytoplasm and has two major phases: Energy investment phase The cell spends 2 ATP to get the process rolling Energy payoff phase ATP is produced during substrate-level phosphorylation NAD+ is reduced to NADH by electrons released from the oxidation of glucose Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Energy investment phase Fig. 9-8 Energy investment phase Glucose 2 ADP + 2 P 2 ATP used Energy payoff phase 4 ADP + 4 P 4 ATP formed 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+ Figure 9.8 The energy input and output of glycolysis 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+

15. In the end, where has all the carbon from glucose gone? In the end, all of the carbon originally present in glucose is accounted for in the two molecules of pyruvate. No CO2 is released during glycolysis. Glycolysis occurs whether or not O2 is present If O2 is present, the chemical energy stored in pyruvate and NADH can be extracted by the citric acid cycle and oxidative phosphorylation. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Question 16 16. Glycolysis releases less than a quarter of the chemical energy stored in glucose; most of the energy remains stockpiled in the two molecules of pyruvate. If molecular oxygen is present, the pyruvate enters the mitochondria, where the enzymes of the citric acid cycle complete the oxidation of glucose. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

17. For each acetyl group entering the cycle, please tally the number of NADH, FADH, and ATP molecules that are produce. 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

Figure 9.12 A closer look at the citric acid cycle Acetyl CoA CoA—SH 1 Oxaloacetate Citrate Citric acid cycle Figure 9.12 A closer look at the citric acid cycle

Figure 9.12 A closer look at the citric acid cycle Acetyl CoA CoA—SH 1 H2O Oxaloacetate 2 Citrate Isocitrate Citric acid cycle Figure 9.12 A closer look at the citric acid cycle

Figure 9.12 A closer look at the citric acid cycle Acetyl CoA CoA—SH 1 H2O Oxaloacetate 2 Citrate Isocitrate NAD+ Citric acid cycle NADH 3 + H+ CO2 -Keto- glutarate Figure 9.12 A closer look at the citric acid cycle

Citric acid cycle Succinyl CoA Fig. 9-12-4 Acetyl CoA CoA—SH 1 H2O Oxaloacetate 2 Citrate Isocitrate NAD+ Citric acid cycle NADH 3 + H+ CO2 CoA—SH -Keto- glutarate Figure 9.12 A closer look at the citric acid cycle 4 CO2 NAD+ NADH Succinyl CoA + H+

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

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

Citric acid cycle Succinyl CoA Fig. 9-12-7 Acetyl CoA CoA—SH 1 H2O 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 6 4 CoA—SH FADH2 5 CO2 NAD+ FAD Succinate P P NADH i GTP GDP Succinyl CoA + H+ ADP ATP

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

Question 18 Glycolysis and the citric acid cycle produce only 4 ATP molecules per glucose molecule; 2 net ATP from glycolysis and 2 ATP from the citric acid cycle. At this point, molecules of NADH and FADH2 account for most of the energy extracted from the glucose. The electron escorts link glycolysis and the citric acid cycle to the machinery of oxidative phosphorylation, which uses energy released from the electron transport chain to power ATP synthesis. 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 Electrons are transferred from NADH or FADH2 to the electron transport chain 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

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

19. Define the following: ATP synthase – a protein complex (enzyme) that makes ATP from ADP and inorganic phosphate Chemiosmosis – the process in which energy stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work such as the synthesis of ATP. Proton-motive force – the H+ gradient formed across a membrane that is capable of performing 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 30 Figure 9.15 Is the rotation of the internal rod in ATP synthase responsible for ATP synthesis? Number of photons detected (103) 25 20 Sequential trials

EXPERIMENT Magnetic bead Electromagnet Internal rod Sample Catalytic Fig. 9-15a EXPERIMENT Magnetic bead Electromagnet Internal rod Sample Catalytic knob Figure 9.15 Is the rotation of the internal rod in ATP synthase responsible for ATP synthesis? Nickel plate

RESULTS Rotation in one direction Rotation in opposite direction Fig. 9-15b RESULTS Rotation in one direction Rotation in opposite direction No rotation 30 Number of photons detected (x 103) 25 Figure 9.15 Is the rotation of the internal rod in ATP synthase responsible for ATP synthesis? 20 Sequential trials

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+ ADP + P ATP Figure 9.16 Chemiosmosis couples the electron transport chain to ATP synthesis i (carrying electrons from food) H+ 1 Electron transport chain 2 Chemiosmosis Oxidative phosphorylation

During cellular respiration, most energy flows in this sequence: Question 21 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:

24. What are the two mechanisms cells can use to generate ATP when oxygen is not present? In the absence of O2, glycolysis couples with fermentation or anaerobic respiration to produce ATP The distinction between these two is based on whether an ETC is present. 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

In other words, without cellular respiration. 25. Fermentation is a way of harvesting chemical energy without using either oxygen or any electron transport chain. In other words, without cellular respiration. 26. As an alternative to respiratory oxidation or organic nutrients, fermentation is an expansion of glycolysis that allows continuous generation of ATP by the substrate-level phosphorylation of glycolysis. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

In alcohol fermentation, pyruvate is converted to ethanol in two steps 27. Briefly describe the following two processes” In alcohol fermentation, pyruvate is converted to ethanol in two steps The first step releases carbon dioxide from the pyruvate, which is converted to the two-carbon compound acetaldehyde In the second step, acetaldehyde is reduced by NADH to ethanol, regenerating the supply of NAD+ Alcohol fermentation by yeast is used in brewing, winemaking, and baking Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(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 It was thought that lactic acid caused muscle soreness, but recent research suggests that it might be increased levels of potassium ions. 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

29. What is the difference between obligate anaerobes and facultative anaerobes?. 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

30. What is 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

Concept 9.6: 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

Fatty acids are broken down by beta oxidation and yield acetyl CoA 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Citric acid cycle Oxidative phosphorylation Fig. 9-20 Proteins Carbohydrates Fats Amino acids Sugars Glycerol Fatty acids Glycolysis Glucose Glyceraldehyde-3- P NH3 Pyruvate Acetyl CoA Figure 9.20 The catabolism of various molecules from food Citric acid cycle Oxidative phosphorylation

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

You should now be able to: Explain in general terms how redox reactions are involved in energy exchanges Name the three stages of cellular respiration; for each, state the region of the eukaryotic cell where it occurs and the products that result In general terms, explain the role of the electron transport chain in cellular respiration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Distinguish between fermentation and anaerobic respiration Explain where and how the respiratory electron transport chain creates a proton gradient Distinguish between fermentation and anaerobic respiration Distinguish between obligate and facultative anaerobes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings