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CHAPTER 13 HOW CELLS OBTAIN ENERGY FROM FOOD 2009 Garland Science Publishing The Breakdown and Utilization of Sugars and Fats 13-1 Glycolysis is an anaerobic process used to catabolize glucose. What does it mean for this process to be anaerobic? (a) No oxygen is required. (b) No oxidation occurs. (c) It takes place in the lysosome. (d) Glucose is broken down by the addition of electrons. 13-2 Which of the following stages in the breakdown of the piece of toast you had for breakfast generates the most ATP? (a) the digestion of starch to glucose (b) glycolysis (c) the citric acid cycle (d) oxidative phosphorylation 13-3 The advantage to the cell of the gradual oxidation of glucose during cellular respiration compared with its combustion to CO2 and H2O in a single step is that ________________. (a) more free energy is released for a given amount of glucose oxidized (b) no energy is lost as heat (c) energy can be extracted in usable amounts (d) more CO2 is produced for a given amount of glucose oxidized 13-4 The final metabolite produced by glycolysis is ___________. (a) acetyl CoA (b) pyruvate (c) 3-phosphoglycerate (d) glyceraldehyde 3-phosphate 13-5 Glycolysis generates more stored energy than it expends. What is the net number of activated carrier molecules produced in this process (number and type of molecules produced minus the number of those molecules used as input)? (a) 6 ATP, 2 NADH (b) 4 ATP, 4 NADH (c) 2 ATP, 2 NADH (d) 4 ATP, 2 NADH 13-6 Which of the following steps or processes in aerobic respiration include the production of carbon dioxide? (a) breakdown of glycogen (b) glycolysis (c) conversion of pyruvate to acetyl CoA (d) oxidative phosphorylation 13-7 In step 4 of glycolysis, a six-carbon sugar (fructose 1,6-bisphosphate) is cleaved to produce two three-carbon molecules (dihydroxyacetone phosphate and glyceraldehyde 3phosphate). Which enzyme catalyzes this reaction? (a) aldolase (b) phosphoglucose isomerase (c) enolase (d) triose phosphate isomerase 13-8 On a diet consisting of nothing but protein, which of the following is the most likely outcome? (a) loss of weight because amino acids cannot be used for the synthesis of fat (b) muscle gain because the amino acids will go directly into building muscle (c) tiredness because amino acids cannot be used to generate energy (d) excretion of more nitrogenous (ammonia-derived) wastes than with a more balanced diet 13-9 Figure Q13-9 represents a cell lining the gut. Draw numbered labeled lines to indicate exactly where inside a cell the following processes take place. Figure Q13-9 1. 2. 3. 4. 5. 6. 7. glycolysis citric acid cycle conversion of pyruvate to activated acetyl groups oxidation of fatty acids to acetyl CoA glycogen breakdown release of fatty acids from triacylglycerols oxidative phosphorylation 13-10 Fill in the spaces in the table below. For steps 1, 4, 5, and 8, name the correct substrates, enzyme, or products. For all the other steps, name the enzyme and draw the missing structure. 13-11 Which of the following processes do not take place in the mitochondria? (a) citric acid cycle (b) conversion of pyruvate to activated acetyl groups (c) oxidation of fatty acids to acetyl CoA (d) glycogen breakdown 13-12 Which reaction does the enzyme phosphoglucose isomerase catalyze? (a) glucose → glucose 6-phosphate (b) fructose 6-phosphate → fructose 1,6-bisphosphate (c) glucose 6-phosphate → fructose 6-phosphate (d) glucose → glucose 1-phosphate 13-13 What purpose does the phosphorylation of glucose to glucose 6-phosphate by the enzyme hexokinase serve as the first step in glycolysis? (a) It helps drive the uptake of glucose from outside the cell. (b) It generates a high-energy phosphate bond. (c) It converts ATP to a more useful form. (d) It enables the glucose 6-phosphate to be recognized by phosphofructokinase, the next enzyme in the glycolytic pathway. 13-14 A. B. C. How does the generation of ATP by oxidative phosphorylation differ from ATP generation by substrate-level phosphorylation? What catabolic process uses substrate-level phosphorylation, and how many ATP molecules are generated in this way in the reaction pathway? Where does oxidative phosphorylation take place, and what other processes are required for this to occur? 13-15 Which of the following cells rely exclusively on glycolysis to supply them with ATP? (a) anaerobically growing yeast (b) aerobic bacteria (c) skeletal muscle cells (d) plant cells 13-16 In anaerobic conditions, skeletal muscle produces _____________. (a) lactate and CO2 (b) ethanol and CO2 (c) lactate only (d) ethanol only 13-17 Anaerobically growing yeast further metabolizes the pyruvate produced by glycolysis to CO2 and ethanol as part of a series of fermentation reactions. A. What other important reaction occurs during this fermentation step? B. Why is this reaction (i.e., the answer to part A) essential for the anaerobically growing cell? 13-18 The first energy-generating steps in glycolysis begin when glyceraldehyde 3-phosphate undergoes an energetically favorable reaction in which it is simultaneously oxidized and phosphorylated by the enzyme glyceraldehyde 3-phosphate dehydrogenase to form 1,3bisphosphoglycerate, with the accompanying conversion of NAD+ to NADH. In a second energetically favorable reaction catalyzed by a second enzyme, the 1,3bisphosphoglycerate is then converted to 3-phosphoglycerate, with the accompanying conversion of ADP to ATP. Which of the following statements is true? (a) The reaction glyceraldehyde 3-phosphate → 1,3-bisphosphoglycerate should be inhibited when levels of NADH fall. (b) The δG° for the oxidation of the aldehyde group on glyceraldehyde 3-phosphate to form a carboxylic acid is more negative than the δG° for ATP hydrolysis. (c) The high-energy bond to the phosphate group in glyceraldehyde 3-phosphate contributes to driving the reaction forward. (d) The cysteine side chain on the enzyme is oxidized by NAD+. 13-19 The simultaneous oxidation and phosphorylation of glyceraldehyde 3-phosphate forms a highly reactive covalent thioester bond between a cysteine side chain (reactive group – SH) on the enzyme (glyceraldehyde 3-phosphate dehydrogenase) and the oxidized intermediate (see arrow in Figure Q13-19A). If the enzyme had a serine (reactive group – OH) instead of a cysteine at this position, which could form only a much lower-energy bond to the oxidized substrate (see arrow in Figure Q13-19B), how might this new enzyme act? Figure Q13-19 (a) (b) (c) (d) It would oxidize the substrate and phosphorylate it without releasing it. It would oxidize the substrate but not release it. It would phosphorylate the substrate on the 2 position instead of the 1 position. It would behave just like the normal enzyme. 13-20 In the absence of oxygen, yeast cells can switch to a completely anaerobic metabolism called fermentation. Which of the following is a final product of fermentation in yeast? 13-21 Pyruvate must move from the cytosol into the mitochondria, where it oxidized to form CO2 and acetyl CoA by the pyruvate dehydrogenase complex. How many different enzymes and what total number of polypeptides, respectively, are required to perform this oxidation process in the mitochondrion? (a) 1; 60 (b) 3; 3 (c) 3; 26 (d) 3; 60 13-22 In the absence of oxygen, mammalian cells will endure for a short time using a process called fermentation. Which of the following is a final product of fermentation in mammalian cells? 13-23 In the reaction cycle involved in the oxidation of pyruvate, what are the advantages of having three enzyme activities contained in a single large complex instead of having three smaller and physically independent enzymes? 13-24 Glycolysis and the citric acid cycle comprise two different sets of oxidation reactions. The reaction sequence for glycolysis is linear, whereas the reaction sequence for the citirc acid cycle forms a circle. How does this difference in the arrangement of reactions influence the rare of these processes when an excess amount of a single intermediate is added? 13-25 Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. During glycolysis, glucose molecules are broken down to yield CO2 and H2O. B. The cleavage of fructose 1,6-bisphosphate yields two molecules of glyceraldehyde 3-phosphate. C. D. E. F. Anaerobic respiration is not the same as fermentation, as only the former requires an electron-transport chain. When subjected to anaerobic conditions, glycolysis in mammalian cells continues and causes a buildup of pyruvate in the cytosol. The pyruvate dehydrogenase complex catalyzes three different, but linked, enzymatic reactions. Amino acids can be transported into the mitochondria and converted into acetyl CoA. 13-26 The reaction cycle that uses acetyl CoA to generate electron carrier molecules needed in the electron-transport chain is important for powering the cell. Which of the names below is not one of those commonly used to describe this reaction cycle? (a) tricarboxylic acid cycle (b) Krebs cycle (c) oxaloacetic acid cycle (d) citric acid cycle 13-27 Two molecules of CO2 are produced after the completion of a single citric acid reaction cycle. Where does the required oxygen come from? (a) water (b) phosphates (c) molecular oxygen (d) acetyl CoA 13-28 Fatty acids can easily be used to generate energy for the cell. Which of the following fatty acids will yield more energy? Explain your answer. (a) CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH=CH-COOH (b) CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH (c) CH3-CH=CH-CH2-CH2-CH2-CH2-CH=CH-COOH (d) CH3-CH2-CH2-CH2-CH2-CH2-CH2-COOH 13-29 For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase may be used more than once. Oxidative phosphorylation is a process that occurs in the __________________ of mitochondria. It requires an electron-transport chain that operates on the high-energy electrons taken from the activated carrier molecules __________________ and __________________ that are produced by glycolysis and the citric acid cycle. These electrons are transferred through a series of molecules, and the energy released during these transfers is used to generate a gradient of __________________, or __________________. Because their concentration is much __________________ outside than inside the mitochondria, the flow of __________________, or __________________, down the concentration gradient is energetically very __________________ and can thus be coupled to the production of ATP from ADP. Thus, oxidative phosphorylation refers to the oxidation of __________________ and __________________ molecules and the phosphorylation of __________________. Without this process, the yield of ATP from each glucose molecule would be __________________ decreased. ADP ATP cytosol electrons FADH2 favorable glucose GTP H+ higher inner membrane lower matrix moderately NAD+ NADH Pi protons severely slightly unfavorable 13-30 The citric acid cycle is outlined in Figure Q13-30. Some of these reactions produce small molecules. Select from the list below to fill in the empty boxes. Keep in mind that some choices may be used more than once and others not used at all. Figure Q13-30 1. ATP 2. ADP 3. GTP 4. GDP 5. NAD+ 6. NADH 7. FADH 8. FADH2 13-31 In step 3 of the citric acid cycle, the oxidation of isocitrate and the production of CO2 are coupled to the reduction of NAD+, generating NADH and an α-ketoglutarate molecule. In the isocitrate molecule shown in Figure Q13-31, which carbon is lost as CO2 and which is converted to a carbonyl carbon? a) b) c) d) 4 and 6 6 and 5 5 and 4 6 and 4 Figure Q13-31 13-32 In step 4 of the citric acid cycle, the reduction of NAD+ to NADH is coupled to the generation of CO2 and the formation of a high-energy thioester bond. What molecule provides the sulfhydryl group necessary to form the thioester bond? (a) pyruvate (b) acetyl CoA (c) CoA (d) cysteine side chain in the catalytic pocket 13-33 In step 4 of the citric acid cycle, the reduction of NAD+ to NADH is coupled to the generation of CO2 and the formation of a high-energy thioester bond. The energy of the thioester bond is harnessed in step 5. What is the energy used for? (a) to generate a molecule of GTP (b) to generate a molecule of ATP (c) to generate a proton gradient (d) to generate a molecule of NADH 13-34 In the final step of the citric acid cycle, oxaloacetate is regenerated through the oxidation of malate and coupled with the production of what other molecule? (a) FADH (b) NADH (c) GTP (d) CO2 How We Know: Unraveling the Citric Acid Cycle 13-35 The oxygen-dependent reactions required for cellular respiration were originally thought to occur in a linear pathway. By using a competitive inhibitor for one enzyme in the pathway, investigators discovered that these reactions occur in a cycle. What compound served as the inhibitor? (a) malonate (b) (c) (d) malate fumarate succinate 13-36 The oxygen-dependent reactions required for cellular respiration were originally thought to occur in a linear pathway. By using a competitive inhibitor for one enzyme in the pathway, investigators discovered that these reactions occur in a cycle. Which enzyme was inhibited? (a) aconitase (b) isocitrate dehydrogenase (c) malate dehydrogenase (d) succinate dehydrogenase 13-37 The oxygen-dependent reactions required for cellular respiration were originally thought to occur in a linear pathway. By using a competitive inhibitor for one enzyme in the pathway, investigators discovered that these reactions occur in a cycle. Which product in the reaction pathway builds up when the inhibitor is added? (a) citrate (b) succinate (c) fumarate (d) malate 13-38 The oxidative reactions of cellular respiration were the focus of intense study in the 1930s. These reactions are represented in a linear pathway, as they were thought to occur. Each product is designated as a lettered compound (A through H) in Figure Q13-38. Figure Q13-38 A. B. C. What was the first observation that Krebs made when he added malonic acid to the minced muscle samples, and what was his conclusion about how and where it was acting in the reactions he was studying? What happens when the malonate block is introduced and subsequently compound A is added in excess? What is the result if compound G is added after the block, instead of A? How did Krebs attempt to reconcile these two results? What additional observation led Krebs to hypothesize that what was previously thought to be a linear sequence of reactions is actually a cyclic sequence of reactions? How did this idea further explain the earliest observations that the addition of any single compound in the pathway greatly increases oxygen uptake by the muscle tissue? 13-39 Do you expect the cell to produce more ATP from one glucose molecule or from one fatty acid molecule? Explain your answer. 13-40 Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. The proteins of the electron-transport chain remove a pair of high-energy electrons from the cofactors NADH and FADH2, after which the electrons move across the inner mitochondrial membrane to maintain the voltage gradient. B. Gluconeogenesis is a linear reaction pathway that the cell employs to generate glucose from pyruvate and is exactly the reverse of the reactions in the glycolytic pathway. C. With respect to the amount of energy stored in molecules of the body, 6 g of glycogen is the equivalent of 1 g of fat. D. Glycogen phosphorylase cleaves glucose monomers from the glycogen polymer, phosphorylating it at the same time so that it can be fed unchanged into the glycolytic pathway. 13-41 In the final stage of the oxidation of food molecules, a gradient of protons is formed across the inner mitochondrial membrane, which is normally impermeable to protons. If cells were exposed to an agent that causes the membrane to become freely permeable to protons, which of the following effects would you expect to observe? (a) The ratio of ATP to ADP in the cytoplasm would fall. (b) NADH would build up. (c) Carbon dioxide production would cease. (d) The consumption of oxygen would fall. 13-42 Pyruvate is an important metabolic intermediate that can be converted into several other compounds, depending on which enzyme is catalyzing the reaction. Which of the following cannot be produced from pyruvate in a single enzyme-catalyzed reaction? (a) lactate (b) oxaloacetate (c) citrate (d) alanine 13-43 For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once. A carbon atom in a CO2 molecule in the atmosphere eventually becomes a part of one of the enzymes that catalyzes glycolysis in one of your cells. The CO2 first enters a cell in a corn leaf, where photosynthesis fixes the carbon to make it part of a sugar molecule; this travels from the leaf to an ear of corn, where it is stored as part of a polysaccharide __________________ molecule in the corn seed. You then eat a corn chip made from the corn seed. You digest the corn seed, and the free __________________ travels in your bloodstream, eventually being taken up by a liver cell and stored as __________________. When required, this storage molecule breaks down into glucose 1-phosphate, which enters the glycolytic pathway. Glycolysis produces __________________, which is converted into acetyl CoA, which enters the __________________. Several intermediates in this process can provide the carbon skeleton for the production of __________________, which are then incorporated into the enzymes that catalyze steps in glycolysis. amino acids carbon fixation citric acid cycle fatty acid fermentation galactose glucose glycogen insulin lactate nucleotides oxidative phosphorylation pyruvate starch triacylglycerol 13-44 When glucose is being used up and not replaced from food intake, the blood sugar level can be maintained by synthesizing it from smaller molecules such as pyruvate or lactate. This process is called gluconeogenesis. Which organ is principally responsible for supplying glucose to the rest of the body when glucose reserves are low? (a) liver (b) pancreas (c) spleen (d) gall bladder 13-45 Step 3 in glycolysis requires the activity of phosphofructokinase to convert fructose 6phosphate into fructose 1,6-bisphosphate. Which of the following molecules is an allosteric inhibitor of this enzyme? (a) Pi (b) AMP (c) ADP (d) ATP 13-46 The conversion of fructose 1,6-bisphosphate is catalyzed by a fructose 1,6bisphosphatase and is one of the final steps in gluconeogenesis. Which of the following molecules is an allosteric activator of this enzyme? (a) Pi (b) AMP (c) ADP (d) ATP 13-47 Which of the following polymer of glucose is used as a vehicle to store energy reserves in animal cells? (a) glucagon (b) glycogen (c) starch (d) glycerol 13-48 The intermediates of the citric acid cycle are constantly being depleted because they are used to produce many of the amino acids needed to make proteins. The enzyme pyruvate carboxylase converts pyruvate to oxaloacetate to replenish these intermediates. Bacteria, but not animal cells, have additional enzymes that can carry out the reaction acetyl CoA + isocitrate → oxaloacetate + succinate. Which of the following compounds will not support the growth of animal cells when used as the major source of carbon in food, but will support the growth of nonphotosynthetic bacteria? (a) pyruvate (b) glucose (c) fatty acids (d) fructose 13-49 Pyruvate can be converted into many other molecules by various biosynthetic and metabolic pathways, which makes it a central hub in the regulation of cellular metabolism. Which of the following molecules is not made from pyruvate? (a) oxaloacetate (b) ethanol (c) lactate (d) NADH 13-50 In humans, glycogen is a more useful food storage molecule than fat because _____________________. (a) a gram of glycogen produces more energy than a gram of fat (b) it can be utilized to produce ATP under anaerobic conditions, whereas fat cannot (c) it binds water and is therefore useful in keeping the body hydrated (d) for the same amount of energy storage, glycogen occupies less space in a cell than does fat