HOW CELLS RELEASE CHEMICAL ENERGY

8 HOW CELLS RELEASE CHEMICAL ENERGY INTRODUCTION Chapter 8 looks at the various ways that cells can extract energy from food. Both aerobic and anaerobic mechanisms are covered, but a major emphasis of the chapter is aerobic respiration. FOCAL POINTS The introductory section [p.122110oks at the problems that arise when mitochondria do not function properly. Figure 8.3 rp.125j provides an overview of aerobic respiration. Reaction pathway of glycolysis is presented in a detailed flow chart [p.1271. Figure 8.5 Ip.128j provides an overview of aerobic respiration in the mitochondria. Figure 8.6 [p.129) looks at the second stage ofaerobic respiration, acetyl-coa formation, and the Krebs cycle. Figure 8.7 lp.j 30J details the electron transfer pathway. Figure 8.8 lp.13l) summarizes aerobic respiration. Figures 8.9Ip.I32) looks at two of the many fermentation pathways seen in living organisms. Figure 8.12 [p. I 35j shows how various foods in your diet can enter the various stages of aerobic respiration. INTERACTIVE EXERCISES Impacts, Issues: When Mitochondria Spin Their Wheels [p.122] 8.1. OVERVIEW OF CARBOHYDRATE BREAKDOWN PATHWAYS [pp.124-1251 Selected Words: mitochondria [p.i221, Luft's syndrome Ip.122), Friedreich's ataxia [p.122), anaerobic rp.124 I, coenzyme's lp.l251, NAD+ [p.1251, FAD [p.1251, Krebs cycle rp.1251, electron transfer phosphorylation [p.125] Boldfaced, Page-Referenced Terms Ip.1241 aerohic respiration ~ [p.124j glycolysis.~_~ ~ Ip.124j pyruvate.! 82 Chapter Eight

[p.124] fermentation [p.124j aerobic Ip.1241 anaerobic-respiration Short Answer [p.122] I. Why are mitochondria so crucial to normal 2. Compare and contrast aerobic respiration and anaerobic fermentation. Fill-in-the-Blanks Ipp.124-1251 Most organisms make (3) by (4) which breaks down organic molecules such as carbohydrates. The (5) pathway uses oxygen while the (6) pathwily docs not. Both of these pathways begin with the same reaction, (7). This reaction converts one molecule of (8) to two molecules of (9). Aerobic respiration ends in the (10), while anaerobic fermentation ends in the (II). Aerobic respiration yields about (12) molecules ofatp per glucose, while fermentation ends with a net yield of ( 13) molecules of A TP. During aerobic respiration, glycolysis is followed by the (14). In order to begin the (14), pyruvate must be converted to (15) Hydrogen ions and electrons released by these reactions are picked lip by (16) and (17) The hydrogen ions and clectrons are then delivered to the (18), the third stage of the aerobic pathway. Operation of the electron transfer chain allows for the formation of many molecules of (19) L How Cells Release Chemical Energy 83

Completion [p.12s] 20. For each of the three stages of aerobic respiration, list all of the molecules that go into the reaction and all of the molecules that come out of the reaction. Refer to Figure 8.3 in your text. 8.2. GLYCOLYSIS-GLUCOSE BREAKDOWN STARTS [pp.126-127] Selected Words: energy-requiring steps [p.127], energy-generating steps [p.1271. PGAL (phosphoglyceraldehyde) [p.l26]. NADH Ip.J26]. net energy yield ]p.l261 Boldfaced, Page-Referenced Term [p.126] substrate-level phosphorylation Fill-in-the-Blallks [p.127j Refer to the diagram of glycolysis to fill in the following blanks. (I) I recall Ch.7J organisms Cilll synthesize and stockpile energy-rich carbohydrates and other food molecules ti'olll inorganic raw materials for later use. When used as an energy source, (2) is partially broken down by the glycolytic pathway; at the end of this process some of its stored energy remains in two (3) molecules. Some of the energy ofglucose is released during the breakdown reactions and used in forming the energy carrier (4) and the reduced coenzyme (5). These reactions take place in the cytoplasm. Glycolysis begins with two phosphate groups being transferred to (6) from two (7) molecules. The addition of two phosphate groups to (6) energizes it and causes it to become unstable and split apart. forming two molecules of (8). Each (8) gains one (9) group from the cytoplasm, then ( 10) atoms and electrons from each PGAL are transferred to NAJ)'. changing this coenzyme to NADH. At the same time. two (II) l11olecules form by substrate-level phosphorylation; the cell's energy investment is paid on: One ( 12) ~_l11olecule is released from each 2-PGA as a waste product. The resulting intermcdiates arc nlliler unstable; each gives lip a( n) (13) group to ATP. Once again, two (14) molecules IHlve formed by ( 15) phosphorylation. for each ( 16) molecule entering glycolysis, the net t'llcrgy yield is two ATP molecules th,lt the cdl Cilll use anytime In do work. The end products of glycolysis arc two molecules oft 17), each with ;ten) (I R) -carbon backbone. 84 Chapter Eight

Sequence [p.127] Arrange the following events of the glycolysis pathway in correct chronological sequence. Write the letter of the first step next to 19, the letter ofthe second step next to 20, and so on. 19. A. The..first two A TPs form by substrate-level phosphorylation; the cell's energy debt is paid off 20. B. Diphosphorylated glucose (fructose-i, 6 21. bisphosphate) molecules split to form two 22. PGAL; this is the first energy-releasing step 23. C. Two 3-carbon pyruvate molecules form as the 24. end products ofglycolysis D. Glucose is present in the cytoplasm 25. E. Two more ATPs form by substrate-level 1 26. phosphorylation, the cell gains A TP; net yield of ATP from glycolysis is two ATPs f, ~ F. The cell invests two A TPs; one phosphate group r ~ ( is attached to each end ofthe gltu;ose molecule (fructose-i, 6-bisphosphate). G. Two PGAL gain two phosphate groups from the cytoplasm H. Hydrogen atoms and electrons from each PGAL are transferred to NAD', reducing this carrier to NADH 8.3. SECOND STAGE OF AEROBIC RESPIRATION [pp.128-129] 8.4. AEROBIC RESPIRATION'S BIG ENERGY PAYOFF [pp.130-131] Selected Words: acetyl-coa [p.128], oxaloacetate [p.129], citrate [p.1291. NADH [p.129]. FAD [p.129j, FADH2 [p.l29], electron transfer phosphorylation [p.130), ATP synthases Ip.130j, final electron acceptor [p.130] How Cells Release Chemical Energy 85

Fill-in-the-Blanks [pp.12s-131] Ifsufficient oxygen is present, the end product ofglycolysis enters a preparatory step, (1) formation. This step converts pyruvate into (1), the molecule that enters the (2) cycle. This is followed by (3) phosphorylation. During these three processes, a total of (4) and (5) (energy-carrier molecules) are typically generated. In the preparatory conversions prior to the Krebs cycle and within the Krebs cycle, the food molecule fragments are further broken down into molecules of(6). During these reactions, hydrogen atoms (7) (with their, are stripped from the fragments and transferred to the coenzymes (8) and (9) Labeling [p.128] In exercises 10-14, identify the structure or location in the top diagram; in exercises J5-1S, identify the chemical substance involved in the lower diagram. In exercise 19, name the metabolic pathway depicted. 10. of mitochondrion 10 11 12 13 14 II. of mitochondrion 12. of mitochondrion 13. of mitochondrion 14. IS. 16. 17. IS. 19. pyruvate from cytoplasm (inner membrane) OUTER COMPARTMENT 86 Chapter Eight

Fill-in-the-Blanks [p.130] ( During (20) NADH and FADH2 deliver their electrons into an electron transfer chain. The harnessed energy from the electrons is used to (21) hydrogen ions across the (22) membrane ofthe mitochondria. A hydrogen ion gradient forms as the hydrogen ions move from the (23) to the (24) compartment, The hydrogen ions move from the (23) through a d.annel protein called (25) ' The flow ofthe hydrogen ions results in the product jon of (26) molecules ofatp. Once the hydrogen ions have moved back into the inner compartment and the electrons have moved through the electron transfer chain. they are picked up by (27) to form (28)! 8.5. ANAEROBIC ENERGY-RELEASING PATHWAYS [pp.132-133] 8.6. THE TWITCHERS [p.133j Selected Words: ethanol [p.132], Saccharomyces cerevisiae [p.132], Lactobacillus rp.133], slow-twitch muscle fibers [p.133), fast-twitch muscle fibers [p.133] Boldfaced, Page-Referenced Terms [p.132j alcoholic fermentation [p. J33 J lactate fermentation,,,, L How Cells Release Chemical Energy 87

Labeling [p.) 351 Identify the process or substance indicated in the illustration. 6. 7. 8. 9. 10. II. 12. urea How Cells Release Chemical Energy 89 L

Choice lpp.134-135] For questions 13-27, refer to the text and Figure 8.12; choose from the following: a. glucose b. glucose-6-phosphate c. glycogen d. fatty acids e. triglycerides f. PGAL g. acetyl-coa h. amino acids i. glycerol j. proteins 13. Fats that are broken down between meals or during exercise as alternatives to glucose 14. Used between meals when free glucose supply dwindles; enters glycolysis after conversion IS. Its breakdown yields much more ATP than does glucose breakdown 16. Absorbed in large amounts immediately following a meal 17. Represents only I percent or so of the total stored energy in the body 18. Following removal of amino groups, the carbon backbones may be converted to fats or carbohydrates or they may enter the Krebs cycle 19. On the average, represents 78 percent ofthe body's stored food 20. Between meals liver cells can convert it back to free glucose and release it 21. Amino groups undergo conversions that produce urea, a nitrogen-containing waste product excreted in urine 22. Converted in the liver to PGAL, a key intermediate ofglycolysis 23. Accumulate inside the fat cells of adipose tissues, at strategic points under the skin 24. A storage polysaccharide produced from glucose-6-phosphate following food intake that exceeds cellular energy demand (and increases ATP production to inhibit glycolysis) 25. Building blocks of the compounds that represent 21 percent ofthe body's stored food 26. A product resulting from enzymes cleaving circulating fatty acids; enters the Krebs cycle SELF-TEST I. Glycolysis would quickly halt if the process 3. When glucose is used as an energy ran out which serves as the source, the largest amount of ATP is hydrogen and electron acceptor. Ip.126] generated by the portion of the a. NADP' entire respiratory process. [1'1'.130-1311 b. ADP a. glycolytic pathway c. NAD' b. ilcetyl-coa formation d. Hp c. Krebs cycle d. electron transfer phosphorylation 2. The ultimate electron acceptor in aerobic respiration is. 11'.130 I 4. The process by which about 10 percent of a. NADH the energy stored in a sligar molecule is h. carbon dioxide (CO.) released as it is converted into two small c. oxygen ( 0') - organic-add molecules is " d. ATP Ip.1261 <1. photolysis b. glycolysis c. fermentation 90 Chapter Eight

5. During which of the following phases of 7. What is the name of the process by which respiration is ATP produced directly by reduced NADH transfers electrons along a substrate-level phosphorylation? [p.127 J chain of acceptors to oxygen so as to form a. Glycolysis water and in which the energy released b. Krebs cycle along the way is used to generate ATP? c. Both a and b [p.130] a. Glycolysis 6. ATP production by electron transfer b. Acetyl-CoA formation phosphorylation involves c. The Krebs cycle [p.no) d. Electron transfer phosphorylation a. H+ concentration and electron gradients across a membrane 8. Pyruvate can be regarded as the end b. ATP synthases product of. [pp.126-127] c. both a and b a. glycolysis d. neither a nor b b. acetyl-coa formation c. fermentation d. the Krebs cycle CHAPTER OBJECTIVES/REVIEW QUESTIONS 1. No matter what the source of energy may be, organisms must convert it to, a form ofchemical energy that can drive metabolic reactions. [p.124) 2. The main energy-releasing pathway is respiration. [p.124} 3. By the end ofthe second stage ofaerobic respiration, which includes the cycle, has been completely degraded to carbon dioxide and water. [pp.125, 129} 4. Which stage ofaerobic respiration has the highest yield ofa TP? [pp.130-131 J 5. Explain, in general terms, the role ofoxygen in aerobic respiration. [p.125 J 6. Explain the purpose served by the cell investing two ATP molecules into the chemistry ofglycolysis. [p.126] 7. Consult Figure 8.6 in the main text. Relate the events that happen during acetyl-coenzyme A formation and explain how the process ofacetyl-coa formation relates glycolysis to the Krebs cycle. [p.129) 8. Explain how the flow ofhydrogen ions through the mitochondrion membrane accounts for the production of ATP molecules. [p.no] 9. Briefly describe the process ofelectron transfer phosphorylation by stating what reactants are needed apd what the products are. State how many ATP molecules are produced through operation ofthe transport system. [p.i 31] 10. List some environments where very little oxygen is present and where anaerobic organisms might be found. [p.132] 11. List the main anaerobic energy-releasing pathways and the examples oforganisms that use them. [pp.132-133 J 12. Describe what happens to pyruvate in anaerobic organisms. Then explain the necessity for pyruvate to be converted to a fermentation product. [pp.132-133] 13. List some sources ofenergy (other than glucose) that can be fed into the respiratory pathways. rpp.134-135 J INTEGRATING AND APPLYING KEY CONCEPTS 1. What problems might humans and other organisms experience if their mitochondria were defective? 2. Human skeletal muscle has both slow-twitch and fast-twitch fibers. Where in the body would you expect each type to predominate? 3. How is the "oxygen debt" experienced by runners and sprinters related to aerobic respiration and fermentation in humans? 4. Predict what your body would do if you switched to a diet of 100 percent protein. How Cells Release Chemical Energy 91, L