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7 HOW CELLS RELEASE CHEMICAL ENERGY When Mitochondria Spin Their Wheels OVERVIEW OF ENERGY·RELEASING PATHWAYS Comparison of the Main Types of EnergyReleasing Pathways Overview of Aerobic Respiration THE FIRST STAGE: GLYCOLYSIS SECOND STAGE OF AEROBIC RESPIRATION Acetyl-eoA Formation The Krebs Cycle FERMENTATION PATHWAYS Alcoholic Fermentation Lactate Fermentation ALTERNATIVE ENERGY SOURCES IN THE BODY The Fate of Glucose at Mealtime and In Between Meals Energy From Fats Energy From Proteins PERSPECTIVE ON LIFE THIRD STAGE OF AEROBIC RESPIRATION-THE BIG ENERGY PAYOFF Electron Transfer Phosphorylation Summing Up: The Energy Harvest Interactive Exercises When Mitochondria Spin Their Wheels [pp.106-107] 7.1. 0) OVERVIEW OF ENERGY-RELEASING PATHWAYS [pp.1OS-109] For additional practice, use the interactive vocabulary exercises linked with your BiologyNow CD·ROM. Selected Words: Lufl's syndrome [p.106], Friedreich's ataxia [p.106], anaerobic [p.1OS] Boldfaced Terms [p.107] free radicals _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ [p.ID8] aeroJ:>ic respiration [p.108] glycolysis _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ [p.ID8] pyruvate _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ How Cells Release Chemical Energy 81 [p.l09j Krebs cycle _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ [p.l09jNAD+ _ _ _ _ _ _ _ _ _ _ _ _ _~_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ [p.l09jFAD _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ [p.l09j electron transfer phosphorylation _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ Short Answer 1. What do Luft's syndrome, Friedreich's ataxia, type 1 diabetes, atherosclerosis, amyotrophlc lateral sclerosis, and Parkinson's, Alzheimer's, and Huntington's diseases have in common? [pp.106-lD7] Fill in the Blanks The numbers in this diagram represent information missing in the following narrative, which gives an overview of the three stages of aerobic respiration. [pp.lDS-lD9] 4 I 3 '~y~~ energy input to slart reactions 2 '--,------'1--" \:' e- + H+ " ,-"- 9 I----~J 18r--19 TYPICAL NET ENERGY YIELD:.....!L ATP The initial stage of aerobic respiration is called (2) _ _ _ _ _ _ [p.lDS], and it occurs in the (3) _ _ _ _ _ _ [p.lDS]. Here, with no need for oxygen, enzymes cleave and rearrange a (4) _ _ _ _ _ [p.lDS] molecule into two molecules of (5) _ _ _ _ _ [p.lDS]. During glycolysis, a net yield of (6) _ _ _ _ _ _ (number) [p.lDS] ATP molecules is produced. Once glycolysis ends, though, the energy-releasing pathways differ. Only the aerobic pathway continues in a (7) _ _ _ _ _ _ [p.lDS], where oxygen accepts and removes the electrons that drove the reactions . •By contrast, anaerobic pathways that produce a low-yield of ATP start and end in the cytoplasm. 82 Chapter Seven Aerobic respiration makes the most out of a glucose molecule, commonly (S) (number) [p.109] ATP molecules, or more, are produced. The second stage consists of a cyclic pathway called the (9) [p.109] and a few steps preceding it. Here enzymes break down pyruvate to (10) [p.109] and water. Two coenzymes, abbreviated NAD+ and FAD, ~? serve in glycolysis and the Krebs cycle. When these coenzymes are reduced, we abbreviate them as (11) [p.109] and (12) it produce (13) [p.109]. Krebs cycle reactions and those just preceding (number) [p.109] ATP molecules. The big energy harvest comes in the third stage, after coenzymes give up electrons and hydrogen to electron transfer chains. These chains are the machinery of (14) [p.109]. They create H+ concentration and electric gradients that drive the formation of (15) (number) [p.109] ATP molecules. It is in this final stage of aerobic respiration that so many ATP molecules are produced. As it ends, (16) [p.109] inside the mitochondrion accepts the "spent" (17) [p.109] from the last component of each transport system. At the same time, oxygen (taken into an organism from the atmosphere) also picks up (IS) [p.109] ions and thereby forms (19) [p.109]. Fill in the Blanks Using a glucose molecule as the reactant, complete this equation, which summarizes the degradative pathway known as aerobic respiration: (20) + 0, - > 6 +6 [p.IOS] (21) Supply the appropriate information to state the equation in question 20 for aerobic respiration in words: one molecule of glucose plus six molecules of enzymes) yields molecules of carbon dioxide plus (in the presence of appropriate molecules of metabolic water [p.10S]. Now return to Figure 7.3 [p.109] in the textbook and locate the components of this basic equation. 7.2. THE FIRST STAGE: GLYCOLYSIS [pp.llO-l11] Selected Words: energy-requiring steps [p.110], energy-releasing steps [p.llO], net yield of glycolysis [p.llO] Boldfaced Terms [p.llO] substrate-level phosphorylation _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ How Cells Release Chemical Energy 83 i; 'i ,i:: Sequence Arrange the steps of glycolysis, the first stage of aerobic respiration, in the correct chronological sequence. Write the letter of step 1 next to 1, the letter of step 2 next to 2, and so on. [pp.llO-lll] a. Each PGAL gives up two electrons and a hydrogen atom to NAD+, forming two NADH. b. Intermediates form; each releases a hydrogen atom and an -OH group. These combine as water. Two molecules of PEP form by the reactions. c. First, one ATP molecule transfers a phosphate group to glucose, then another; atoms are rearranged, because the cell has now invested two ATP molecules already present. d. Each PEP transfers a phosphate group to ADP; once again, two ATP molecules have formed by substrate level phosphorylation. e. Each PGAL also picks up an inorganic phosphate and transfers a pnosphate group to ADP, forming ATP (substrate level pho~phorylation); the original cell investment of two ATP is now paid off. f. In sum, the net energy yield is two ATP for each molecule of glucose entering glycolysis; two molecules of pyruvate are the end product. g. Glucose molecules are present in the cytoplasm. h. The rearranged and phosphorylated glucose splits into two PGAL molecules, each with a tluee-carbon backbone. 1. 2. 3. 4. 5. 6. 7. 8. 7.3. SECOND STAGE OF AEROBIC RESPIRATION [pp.ll2-113] Selected Words: acetyl-CoA [p.1l2], coenzymes [p.1l2], coenzyme A [p.1l2], citric acid cycle [p.1l2] Identification Identify each numbered structure or location in the sketch. [p.1l2] AEROBIC RESPIRATION start (glycolysis) in cytoplasm 1. ___________ membrane 2. ___________ membrane 3. _ _ _ _ _ compartment 4. _ _ _ _ _ compartment 84 Chapter Seven Fill in the Blanks The numbers in this diagram represent information missing in the following narrative about aerobic respiration. Preparatory ~ 5 steps '> o --,..,,~/ 7 .'. ."----..,~.6 11 ;--11 NAO+ 1 00-8 --------- ------------------ Krebs Cycle \\ t/V,<i> GoA .s.······ •. Y"""~ ~ citrate Ii <, f NAO+ 12' -{l .~ \i.-- NAO+ :}:A'DH~' ~~ ~\",,13 12 ;'1 NAD+, iJ~ ~6 /j~ ~ Two (5) ~6 ~ <':'<"/"""" ___ ".",-/>'/'/ ~~~) \\--- '0 i"-. \ i ~ ·s ~ { 10 ·,°':'12- ADP+ phosphate group [pp.112-113] molecules formed by glycolysis leave the cytoplasm and enter a mito- chondrion. In this organelle, the second (preparatory steps) and third (electron transfer phosphorylation) stages of the aerobic pathway are completed. It is during the second stage that glucose is broken down to carbon dioxide and water. Six carbon atoms, three from each pyruvate, enter these reactions. To start, one carbon atom from each pyruvate is removed and attached to oxygen molecules, forming (6) _ _ _ _ __ _ _ _ _ _ _ [pp.112-113] (recall that the Krebs cycle turns twice for each glucose molecule entering glycolysis-all respiration chemistry begins with glycolysis). Next, the two-carbon fragment that is left over joins with (7) (8) [pp.112-113] to make [pp.112-113]. (8) enters the Krebs cycle and transfers its two carbon acetyl group to oxaloacetate; this forms the first intermediate compound, citric acid, or citrate (the Krebs - = cycle is also called the citric acid cycle). Two (9) [pp.112-113] molecules form by substrate-level phosphorylations in the Krebs cycle, and the intermediate compounds get arranged into (10) [pp.112-113]. Cells have only so much (10), which must be regenerated to keep the reactions going. The two A TP molecules formed in the Krebs cycle don't add much to the small yield of two ATP from glycolysis. The ten coenzymes (NADH and FADH2) that gain electrons and hydrogen constitute a big energy payoff for the cell. To summarize these reactions for two turns of the Krebs cycle How Cells Release Chemical Energy 85 and its preparatory stage: two CO2 form, and two NAD+ are reduced to (11) _ _ _ _ _ _ [p.113] in the preparatory stage. In the Krebs cycle, four CO, form, two A TP are generated, and six NAD+ are reduced to (12) [p.113] and two (13) [p.113] are reduced to FADH2 • The original glucose that entered this cycle via glycolysis reactions has lost all of its carbons and can be considered fully consumed, or oxidized, at this stage. 7.4. THIRD STAGE OF AEROBIC RESPIRATION-THE BIG ENERGY PAYOFF [pp.114-115] Selected Words: electron transfer phosphorylation [p.114], electrochemical gradient [p.114], ATP synthases [p.114] Fill in the Blanks The numbers in this diagram represent information missing in the following narrative about electron transport phosphorylation. @ 4~ glycolysis 86 Chapter Seven ;f 2 NAD+ The aerobic pathway's third stage, electron transfer phosphorylation, generates most of the ATP. (1) [p.llS] and (2) deliver electrons and (3) [p.llS] that formed during the first and second stages [p.llS] to electron transfer chains, which are at the membrane dividing a mitochondrion's interior into two compartments. As the electrons are transferred through the chains, they attract H+ (unbound hydrogen) from the (4) are shuttled across the membrane into the (5) [p.llS] compartment. These-ions [p.1l5] compartment. Very soon, the H+ con- centration is greater in the outer compartment than in the inner one. H+ and electric gradients have become established across the membrane. H+ follows the gradients, through the interior of (6) _ _ _ _ __ _ _ _ _ _ _ [p.1l5]. Energy released by the flow drives formation of (7) (8) [p.llS] and unbound (9) [p.1l5] from [p.llS]. Hence the name electron transfer phosphorylation. Short Answer 10. What role does free oxygen play in electron transport phosphorylation? [pp.1l4-ll5] 11. What is the typical yield of ATP from the third stage of aerobic respiration? What is the net ATP harvest from one molecule of glucose? [p.1l5] 7.5. FERMENTATION PATHWAYS [pp.1l6-ll7] Selected Words: Saccharomyces cerevisiae [p.1l6], Lactobacillus [p.1l7] Boldfaced Terms [p.116] alcoholic fermentation _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ [p.117] lactate fermentation _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ How Cells Release Chemical Energy 87 Choice Choose from the following: a. alcoholic fermentation b. lactate fermentation c. applies to both types of fermentation 1. _ _ Glycolysis is the first stage [p.1l6] 2. _ _ Yeasts are famous for their use of this pathway [p.116] 3. _ _ Humans, rabbits, and many other animals use this for a quick fix of energy [p.1l7] 4. _ _ Pyruvate and NADH form, and the net energy yield is two ATP [p.1l6] 5. _ _ Muscle cells use this pathway but not for long; diverting glucose into this pathway would waste too much of its energy for too little ATP [p.1l7] 6. _ _ The final steps simply regenerate NAD+, the coenzyme that assists the breakdown reactions [p.1l6] 7. _ _ Each pyruvate molecule that formed in glycolysis is converted to the intermediate acetaldehyde [p.1l6] 8. _ _ These reactions do not completely degrade glucose to CO2 and H 20 [p.1l6] 9. _ _ Saccharomyces cerevisiae makes bread dough rise [p.1l6] 10. _ _ Produce no more ATP beyond the rather tiny yield from glycolysis [p.1l6] 11. _ _ Drunken birds such as tipsy wild turkeys and robins [p.1l6] 12. _ _ Lactabaccilus and some other bacteria use only this anaerobic pathway [p.1l7] 13. _ _ NADH transfers electrons and hydrogen to this form and thereby converts it to ethanol [p.1l6] 14. _ _ Yields enough energy to sustain many single-celled anaerobic organisms [p.1l6] 15. _ _ The transfer by NADH converts each pyruvate to a three-carbon compound found in animals and other organisms that carryon this process [p.1l7] 7.6. ALTERNATIVE ENERGY SOURCES IN THE BODY [pp.1l8-119] Selected Words: glucose intake [p.1l8], fat reservoir [p.1l8]' fatty acid conversions [p.1l8], fuel of choice for the brain [p.1l9] Choice Choose from the following: a. fatty acids f. glucose c. amino acids b. triglycerides g. glycogen h. acetyl-CoA d. PGAL e. glucose-6-phosphate 1. _ _ At mealtime, a rise in blood concentration of this molecule prompts the pancreas to secrete insulin [p.1l8] 2. _ _ Conversions of these molecules resuit in pyruvate, acetyl-CoA, or one of the intermediates of the Krebs cycle. [p.1l8] 3. _ _ When you aren't eating, the level of this in blood declines [p.1l8] 4. _ _ A conversion to this compound allows cells to trap incoming glucose [p.1l8] 88 Chapter Seven 5. ~~ A storage polysaccharide created by a glucose-6-phosphate pathway [p.1l8] 6. ___ Muscle and liver cells maintain the largest stores of this compound [p.1l8] 7. ___ Makes up 1 percent or so of the body's total energy reserves [p.1l8] 8. ~~ Enzymes split dietary protein into these units [p.1l8] ., 9. ___ An excess of these molecules end up as excess fat [p.1l8] 10. ___ Following removal of amino groups, the carbon backbones may be converted to fats or carbohydrates, or they may enter the Krebs cycle [p.1l9] 11. ~~ Yields more ATP than glucose [p.1l8] 12. ~~ Enzymes in the liver convert glycerol to these molecules [p.1l8] 13. ~~ Muscle cells will not give this up [p.1l8] 14. ~~ Accumulate inside the fat cells of adipose tissue at strategic body regions [p.1l8] 15. ~~ Supply about half of the ATP that muscle, liver, and kidney cells require between meals [p.1l8] 16. ~~ Molecules resulting from enzymes cleaving fatly acid backbones; can enter the Krebs cycle [p.1l8] 7.7. PERSPECTIVE ON LIFE [p.120] Short Answer 1. Through the evolution of life over a very long period of time, photosynthesis and aerobic respiration became "linked" on a global scale. In terms of your knowledge of the two chemical processes, explain what forms this linkage. [p.120] Self-Quiz 'J Are you ready for the exam? Test yourself on key concepts by taking the additional tests linked with your BiologyNow CD-ROM. __ 1. Anaerobic respiration pathways begin and end.in the . [p.108] a. thylakoid of a chloroplast b. plasma membrane of the cell c. inner membrane of the mitochondrion d. cytoplasm e. outer compartment of the mitochondrion _ 2. The first energy-releasing step in glycolysis splits activated glucose into two molecules of . [p.108] a. NAD+ b. PGAL c. ATP d. pyruvate e. PEP How Cells Release Chemical Energy 89 __ 3. The net energy yield of glycolysis is _ _ _ _ _ _ ATP molecules. [pp.108-109j a. three b. four c. thirty-two d.two e. eight __ 4. Pyruvate is regarded as the end product of _ _ _ _ _ . [p.108] a. glycolysis b. acetyl-CoA formation c. fermentation d. electron transfer phosphorylation e. the Krebs cycle __ 5. During which of the follOWing phases of aerobic respiration is ATP produced directly by substrate-level phosphorylation? [p.llO] a. glucose formation b. the Calvin-Benson cycle c. the Krebs cycle d. formation of acetyl-CoA e. electron transfer phosphorylation __ 6. Select the process by which NADH and FADH, transfer electrons along a chain of acceptors to oxygen so as to form water and set up conditions for producing a large number of ATP molecules. [pp.1l4-llS] a. glycolysis b. the Krebs cycle c. acetyl-CoA formation d. fermentation pathways e. electron transfer phosphorylation __ 7. The most efficient means of ATP production during aerobic respiration involves . [pp.ll4-11S] a. concentration of H + and electric gradients across a membrane b. ATP synthases c. formation of ATP in the inner mitochondrial compartment d. NADH and FADH, e. all of the above 90 Chapter Seven - - S. The total number of ATP molecules produced by the complete degradation of one glucose molecule is often thirty-six. Which of the following is not a contribution to that number? [pp.1OS-11S] a. glycolysis produces two ATP molecules b. steps preparatory to the Krebs cycle produce two ATP molecules c. the Krebs cycle produces two ATP molecules d. electron transfer phosphorylation produces thirty-two ATP molecules e. none of the above are correct __ 9. What factor allows fermentation reactions to continue and produce a lo~ yield of ATP when a cell has no oxygen? [p.116] a. the presence of ethanol molecules b. the presence of lactic acid c. ATP itself d. regeneration of the coenzyme NAD+ during glycolysis e. the Krebs cycle - 10. Of the follOwing, which one is not available as an alternative energy source for the human body? [pp.118-ll9] a. fats b. glycogen c. proteins d. carbohydrates e. all of the above are available - 11. The basic reason why we have an accumulation of oxygen in our atmosphere is . [p.120] a. that oxygen produces more oxygen b. cyclic photophosphorylation c. that oxygen is released in electron transfer phosphorylation d. the noncyclic pathway e. the production of ATP Chapter Objectives/Review Questions 1. Define the term free radicals and be able to list at least three age-related mitochondrial disorders. [pp.106-107] 2. Be able to summarize the events that occur during the three stages of aerobic respiration as illustrated in Figure 7.3. [pp.lOS-109] 3. Be able write the general equation for aerobic respiration; then state it in words. [p.10S] 4. Describe the energy-requiring steps of glycolysis. [p.llO] 5. Describe the energy-releasing steps of glycolysis. [p.llO] 6. What is the role of NAD+ in glycolysis? [p.llO] 7. The cell invests (number) ATP molecules to jump-start glycolysis. [p.llO] S. (number) NADH and (number) ATP molecules form during glycolysis, but the net energy yield from the process is (number) ATP molecules. [p.llO] 9. For every glucose molecule entering glycolyis, two molecules are produced as an end product. [p.llO] 10. Describe exactly where in the mitochondria the second and third stages of aerobic respiration occur. [p.112] 11. For one glucose molecule, how many molecules of carbon dioxide and acetyl-CoA are produced in the preparatory steps? [pp.1l2-113] 12. For one pyruvate molecule during two turns of the Krebs cycle, (number) molecules of NADH, (number) molecules of FADH" (number) of ATP molecules, and (number) molecules of CO, are formed. [p.1l3] 13. During electron transport phosphorylation, electron transfer chains and ATP synthases are involved to produce (number) molecules of ATP. [p.llS] 14. The total net harvest of ATP, involving all stages of aerobic respiration, is _ _ _ _ __ =--:-::___ (number) molecules of ATP. [p.llS] 15. Be able to summarize the biochemistry of alcoholic fermentation and lactate fermentation and tell what types of organisms these processes occur in. [pp.1l6-117] 16. What keeps both types of fermentation operational is the regeneration of ; this allows even a low yield of energy production to provide a quick fix of energy for cells lacking oxygen. [p.1l6] 17. Summarize the metabolic processing of glucose at and between meals. [p.llS] 18. Explain how the body metabolizes fats and proteins when glucose is not available. [pp.llS-1l9] 19. Briefly explain how the pathways of photosynthesis and respiration are biochemically linked on a global scale. [p.l20] .- Media Menu Review Questions Questions 1-3 are drawn from the following Infotrac College Edition article: "My Personal Challenge." Jackleen Armstrong. The Exceptional Parent, August 1995. 1. Jackleen Armstrong suffers from a disease known as process is characterized by dysfunction in skeletal _ _ _ _ __ 2. List th~ characteristics of her disease, as told by teenager Jackleen,Armstrong. . This disease 3. What is the name of the medicine used by Jackleen that does give her some relief? How Cells Release Chemical Energy 91 1 :1 Questions 4-10 are drawn from the following Infotrac College Edition article: "Mitochondria: Cellular Energy Co.: Researchers Strive to Keep the Energy Pipeline Open in the Face of Damaging Cellular Insults." Amy Adams. The Scientist, June 24, 2002. 4. S. 6. 7. 8. that impair mitochondrial proteins, encoded by both and mitochondrial genes, cause widespread disturbances in muscle, nerve, kidney, and other high-energy cells. Several cellular insults, including mutations, toxins, and lack of , can induce mitochondrial failure, but the end result is the same: too little to meet the cell's demands. Neurons are particularly susceptible to damage from low levels of ATP, owing to their high-energy needs and almost exclusive reliance on for energy. With milder mutations that impair mitochondrial proteins, the diseases proceed as a _ _ _ _ __ process. As long as there is minimal ATP energy, the person may be impaired but not ill. In these cases. the disease may not appear until later in life, as in and diseases. Researchers discovered the link between and Parkinson disease in 1982 when symptoms very much like Parkinson disease developed in drug addicts. The heroin they had injected contained a compound called which is metabolized into a strong inhibitOl' of the first protein complex in the electron transport chain. This led researchers to track down _ _ _ _ __ _ _ _ _ _ _ mutations in patients with Parkinson disease. A known complex I inhibitor called was administered to rats. The rats exhibited symptoms similar to those of Parkinson disease. They also had nerve cell in the substantia nigra, the same brain region affected in patients with Parkinson disease. Protein were also found that Were similar to those found in people who died from Parkinson disease. It is not known why cells in the substantia nigra of the brain are so affected by complex I mutations. One possibility is that the synthesis and degradation of the used by these cells prodUCE H 20, as a by-product. It is suspected that many cellular defects associated with Parkinson disease result from the damage to critical proteins and the harmful mutations caused by long-term exposure to . If the cause is enigmatic, the effect is clear: the cell is unable to function and o· 9. 10. , Integrating and Applying Key Concepts 1. Evaluate this statement: Every atom in your body has first passed through the cell(s) of a photosyn- thetic organism. 92 Chapter Seven