Chapter 13 Smartworks
You are packing for a hiking trip during which you'll be burning a lot of calories with physical activity. You want to pack as efficiently as possible since you need to carry a tent and all your food. You can get the most calories out of 5kg of food if it is in the form of ________.
Fat The oxidation of a gram of fat releases about twice as much energy (9 calories versus 4 calories) as the oxidation of a gram of glycogen. Fats contain mostly highly reduced C-H bonds, whereas carbohydrates are already partially oxidized.
Although the citric acid cycle itself does not use O2, it requires a functioning electron transport chain (which uses O2) in order to regenerate which molecule for further use in the citric acid cycle? a) NAD+ b) NADH c) FADH2 d) ATP e) ADP
a) NAD+ Although the citric acid cycle accounts for about two-thirds of the total oxidation of carbon compounds in most cells, none of its steps use molecular oxygen. The cycle, however, requires O2 to proceed because the NADH generated passes its high-energy electrons to an electron transport chain in the inner mitochondrial membrane, and this chain uses O2 as its final electron acceptor. Oxygen thus allows NADH to hand off its high-energy electrons, regenerating the NAD+ needed to keep the citric acid cycle going. The same is true for regenerating the FAD needed to accept electrons from the citric acid cycle to produce FADH2. None of the reactions in the citric acid cycle involve ATP or ADP.
Where does the oxidative (oxygen-dependent) stage of the breakdown of food molecules occur in a eukaryotic cell? a) mitochondrion b) endoplasmic reticulum c) Golgi apparatus d) cytosol
a) mitochondrion The oxidative stage of the breakdown of food molecules takes place entirely in the mitochondrion. The citric acid cycle, which requires oxygen to proceed, occurs in the mitochondrial matrix. And oxidative phosphorylation, which consumes a large amount of oxygen, takes place on the inner mitochondrial membrane. The breakdown of sugars such as glucose begins with glycolysis, which takes place in the cytosol. However, this process can occur in the absence of molecular oxygen.
When nutrients are plentiful, plants can store glucose as what? a) starch b) glucose 6-phosphate c) glycogen and starch d) fats e) glycogen
a) starch Plants convert some of the sugars they make through photosynthesis during daylight into fats and into starch, a branched polymer of glucose very similar to glycogen, which is the form in which glucose is stored in animal cells. An abundance of glucose 6-phosphate can trigger the storage of glucose, as it indicates a surplus of food molecules in the cell. It, however, is not a form of storage for glucose. Glucose is a sugar, so it is not stored as a fat.
The NADH generated during glycolysis and the citric acid cycle feeds its high-energy electrons to which of the following? a) the electron transport chain b) H2O c) ADP d) the citric acid cycle e) FAD
a) the electron transport chain The NADH produced during glycolysis (and the NADH and FADH2 generated by the citric acid cycle) transfers its high-energy electrons to the electron transport chain. This series of electron carriers is embedded in the inner mitochondrial membrane in eukaryotic cells (and in the plasma membrane of aerobic prokaryotes). As the electrons pass through the series of electron acceptor and donor molecules that form the chain, they fall to successively lower energy states. At specific sites in the chain, the energy released is used to drive protons (H+) across the inner membrane, from the mitochondrial matrix to the intermembrane space. This movement generates a proton gradient across the inner membrane, which serves as a source of energy (like a battery) that can be tapped to drive a variety of energy-requiring reactions. The most prominent of these reactions is the phosphorylation of ADP to generate ATP. At the end of the transport chain, the electrons are added to molecules of O2, and the resulting reduced oxygen molecules immediately combine with protons from the surrounding solution to produce H2O. The electrons have now reached their lowest energy level.
The ethanol in wine and beer is produced from metabolic reactions carried out by the yeast Saccharomyces cerevisiae. Since it is of great commercial value, researchers have studied factors that influence ethanol production. To maximize ethanol yield, which environmental factor should be limiting? a) sunlight b) oxygen c) glucose d) carbon dioxide
b) oxygen During glycolysis, NAD+ is reduced to NADH. In the presence of oxygen, NADH donates electrons to the electron-transport chain (ETC) in the inner mitochondrial membrane. Oxygen serves as the final electron acceptor in the ETC. If oxygen is not available, yeast carry out fermentation, and NAD+ is regenerated when NADH donates electrons to pyruvate, leading to ethanol production.
Most of the energy released by oxidizing glucose is saved in the high-energy bonds of what molecules? a) GDP and other activated carriers b) H2O and CO2 c) ATP and other activated carriers d) ADP and other activated carriers e) O2
c) ATP and other activated carriers Much of the energy released by an energetically favorable reaction, such as the oxidation of a food molecule, must be stored temporarily before it can be used by cells to fuel energetically unfavorable reactions, such as the synthesis of all the other molecules needed by the cell. In most cases, the energy is stored as chemical bond energy in a set of activated carriers, small organic molecules that contain one or more energy-rich covalent bonds. Activated carriers store energy in an easily exchangeable form, either as a readily transferable chemical group or as readily transferable ("high-energy") electrons. The most important activated carriers are ATP and two molecules that are close chemical cousins, NADH and NADPH.
What happens to the energy captured during glycolysis and the citric acid cycle by the activated carriers NADH and FADH2. a) It is passed to an electron transport chain that uses it to produce oxygen b) It is used to drive biosynthetic reactions. c) It is passed to an electron transport chain that uses it to generate a proton gradient across the inner mitochondrial membrane. d) It is passed to ADP to form ATP. e) It is passed to an electron transport chain that uses it to oxidize food molecules.
c) It is passed to an electron transport chain that uses it to generate a proton gradient across the inner mitochondrial membrane. In the final stage in the oxidation of food molecules, called oxidative phosphorylation, the chemical energy captured by the activated carriers produced during glycolysis and the citric acid cycle is used to generate ATP. NADH and FADH2 transfer their high-energy electrons to the electron transport chain—a series of electron carriers embedded in the inner mitochondrial membrane in eukaryotic cells (and in the plasma membrane of aerobic prokaryotes). As the electrons pass through the series of electron acceptor and donor molecules that form the chain, they fall to successively lower energy states. At specific sites in the chain, the energy released is used to drive protons (H+) across the inner membrane, from the mitochondrial matrix to the intermembrane space. This movement generates a proton gradient across the inner membrane, which serves as a source of energy (like a battery) that can be tapped to drive a variety of energy-requiring reactions—most importantly, the phosphorylation of ADP to generate ATP on the matrix side of the inner membrane.
In eukaryotic cells, what is the final electron acceptor in the electron transport chain? a) ATP b) NAD+ c) O2 d) FADH2 e) CO2
c) O2 In oxidative phosphorylation, the chemical energy captured by the activated carriers produced during glycolysis and the citricacid cycle is used to generate ATP. NADH and FADH2 transfer their high-energy electrons to the electron transport chain—a series of electron carriers embedded in the inner mitochondrial membrane in eukaryotic cells. As the electrons pass through the series of electron acceptor and donor molecules that form the chain, the energy they release is harnessed to produce ATP. At the end of the transport chain, the electrons are added to molecules of O2 to produce water. The electrons have now reached their lowest energy level, and all the available energy has been extracted from the food molecule being oxidized. Oxidative phosphorylation allows cells to extract energy from food with great efficiency. And it cannot take place in the absence of oxygen, the final acceptor of electrons from the electron transport chain.
In the electron transport chain, the oxygen atoms in O2 become part of which of the following molecules? a) CO2 b) ATP c) NADH d) H2O e) glucose (C6H12O6)
d) H2O The electron transport chain donates electrons to O2. This action reduces the oxygen, which combines with protons to produce H2O. CO2 is produced by the citric acid cycle, not by the electron transport chain. Its oxygen comes from water, not O2. Likewise, NADH is generated during the citric acid cycle and donates its electrons to the electron transport chain. It does not interact directly or combine with O2. Although the movement of electrons along the electron transport chain yields energy that is ultimately used to drive the synthesis of ATP, ATP does not interact with the electron transport chain or with molecular oxygen. Glucose is a food molecule whose oxidative breakdown ultimately fuels the synthesis of ATP. In the process, glucose is oxidized to CO2. It is not directly involved in electron transport and does not receive oxygen from the electron transport chain.
CO2 is released in which steps of the citric acid cycle, as shown below? (6) a) Steps 2 and 4 b) Steps 1 and 8 c) Steps 1 and 5 d) Steps 3 and 4 e) Steps 2, 3, and 4
d) Steps 3 and 4 The steps of the citric acid cycle that release CO2 are those in which an intermediate loses a carbon. In step 3, a six-carbon substrate (isocitrate) is converted into a five-carbon product (α-ketoglutarate). The lost carbon is released as CO2. In step 4, the five-carbon α-ketoglutarate reacts with a molecule of coenzyme A to yield the four-carbon succinyl CoA. Again, the missing carbon is accounted for by CO2, which is released in this step. In the first step of the citric acid cycle, the two-carbon acetyl group is fed into the cycle and combined with oxaloacetate. Step 2 is an isomerization reaction that converts citrate to isocitrate. In step 5, an inorganic phosphate displaces the CoA from succinyl CoA, forming a high-energy phosphate linkage to succinate. This phosphate is then passed to GDP to form GTP. In the final step of the cycle, oxaloacetate is regenerated by the enzyme malate dehydrogenase. Because the citric acid cycle catalyzes the complete oxidation of a two-carbon acetyl group to CO2, only two molecules of CO2 will be produced.
How do enzymes maximize the energy harvested from the oxidation of food molecules? a) They guarantee that each reaction involved in the oxidation of food molecules proceeds in just one direction. b) They allow what would otherwise be an energetically unfavorable oxidation reaction to occur. c) They allow oxidation reactions to take place without an input of activation energy. d) They allow the stepwise oxidation of food molecules, which releases energy in small amounts. e) They allow a larger amount of energy to be released from food molecules such as glucose.
d) They allow the stepwise oxidation of food molecules, which releases energy in small amounts. If food molecules, such as glucose, were oxidized by burning in a fire, the energy contained in those molecules would be released all at once. This quantity is too large to be captured by any carrier molecule, so all of this energy would be released as heat. In a cell, enzymes catalyze the breakdown of sugars via a series of small steps, in which a portion of the free energy released is captured by the formation of activated carriers—most often ATP and NADH. These activated carriers can then be made available to do useful work for the cell. Enzymes lower the activation energy barrier that must be surmounted by the random collision of molecules to allow these reactions. However, an initial input of energy is still required even for enzyme-catalyzed reactions. The complete oxidation of glucose to CO2 and H2O is energetically highly favorable. The ΔG° of the reaction is -2880 kJ/mole. Thus, the total free energy released by the complete oxidative breakdown of glucose is 2880 kJ/mole. This amount of energy is exactly the same for the enzyme-catalyzed reactions or for the direct burning of the sugar in a fire. Although the overall oxidation of glucose is energetically favorable, many of the reactions involved in the process are readily reversible and can occur in either direction, depending on the relative concentrations of the participating molecules.
Which two-carbon molecule enters the citric acid cycle? a) pyruvate b) carbon dioxide c) citrate d) acetyl CoA e) oxaloacetate
d) acetyl CoA In the first step of the citric acid cycle, acetyl CoA donates a two-carbon acetyl group to oxaloacetate to form citrate. These carbons are then oxidized to produce CO2. Pyruvate, which is generated during glycolysis from the splitting of glucose, has three carbons. It is decarboxylated in the mitochondrial matrix to produce acetyl CoA. Oxaloacetate accepts a two-carbon acetyl group from acetyl CoA to form citrate, which is then oxidized by the cycle.
Under anaerobic conditions, which metabolic pathway regenerates the supply of NAD+ needed for glycolysis? a) acid cycle b) breakdown of amino acids c) breakdown of fats d) fermentation e) formation of acetyl CoA
d) fermentation NADH has a special role as an intermediate in the catabolic system of reactions that generate ATP through the oxidationof food molecules, including sugars such as glucose, or amino acids or fats. Such oxidation reactions are accompanied by reductions. Thus, as the food molecules are oxidized, electrons are transferred to NAD+, which is thereby reduced to NADH. The formation of acetyl CoA is carried out by an enzyme complex that oxidizes pyruvate to form acetyl CoA and CO2. In the process, electrons are transferred to NAD+ to generate NADH. The citric acid cycle oxidizes the acetyl group of acetyl CoA to CO2, generating NADH in the process. Fermentation reactions take the pyruvate produced during glycolysis and convert it into lactate or ethanol. To support this chemical conversion, NADH gives up its electrons, thereby producing NAD+. Without this replenishment of NAD+, glycolysis could not continue.
Useful energy is obtained by cells when sugars derived from food are broken down by which processes? a) gluconeogenesis, the citric acid cycle, and oxidative phosphorylation b) gluconeogenesis, fermentation, and oxidative phosphorylation c) glycolysis, the citric acid cycle, and gluconeogenesis d) glycolysis, the citric acid cycle, and oxidative phosphorylation e) glycolysis, the Calvin cycle, and oxidative phosphorylation
d) glycolysis, the citric acid cycle, and oxidative phosphorylation For most animal and plant cells, sugars such as glucose are broken down into CO2 and water by glycolysis and the citric acid cycle. These processes, in addition to generating some ATP of their own, produce activated carriers that provide energy that is used to drive the synthesis of large amounts of ATP by oxidative phosphorylation. Gluconeogenesis is a process in which glucose is synthesized, so it should not be included in the list of processes that break down sugar. Gluconeogenesis is effectively the reverse of glycolysis. Fermentation reactions can break down sugars in the absence of oxygen. But they would not be coupled with oxidative phosphorylation, as suggested in the responses, as this process requires a large input of oxygen. The Calvin cycle is a process by which sugars are synthesized during photosynthesis in plant cells. Again, this process produces sugars rather than breaking them down.
In what form do plant and animal cells store fat? a) glycogen b) phospholipids c) starch d) triacylglycerol e) nitroglycerin
d) triacylglycerol Both animal and plant cells store fats in the form of fat droplets. In animals, these droplets accumulate in special fat-storing cells called adipocytes. In plants, they are found in chloroplasts along with starch granules. Starch is a polymer made of glucose; it is a molecule used by plants to store sugars. In animal cells, these sugars are stored in a related polymer called glycogen. In both plant and animal cells, fats are stored as triacylglycerol, a hydrophobic molecule that consists of three fatty acid chains attached to a molecule of glycerol. The triacylglycerol in plants and animals differs only in the types of fatty acids that predominate: plant oils contain unsaturated fatty acids (with one or more double bonds) and animal fats are saturated. Phospholipids are the main constituent of cell membranes in both animal and plant cells. These amphipathic molecules form lipid bilayers and not fat droplets; thus, they are not a form of fat storage. Nitroglycerin is an explosive compound manufactured by mixing glycerol with concentrated sulfuric and nitric acids. In diluted form, it can be used to treat chest pain and heart conditions. It is not produced by plants or animals and does not represent a stored form of fat.
What occurs in the first step of the citric acid cycle? a) CO2 is released. b) Two molecules of acetyl CoA combine to form oxaloacetate. c) ATP is consumed. d) NADH is produced. e) A two-carbon molecule is combined with a four-carbon molecule to form citrate.
e) A two-carbon molecule is combined with a four-carbon molecule to form citrate. In the initial reaction of the citric acid cycle, a two-carbon acetyl group combines with a four-carbon oxaloacetate molecule to form the six-carbon citrate after which the cycle is named. CO2 is given off only after those carbons have been oxidized. And NADH is produced only after oxidation of those carbons releases energy that can be captured by NAD+. The citric acid cycle does not require an initial investment of energy in the form of ATP, as does glycolysis.
Which of the following processes generates the largest number of ATP molecules? a) fermentation b) citric acid cycle c) gluconeogenesis d) glycolysis e) electron transport chain
e) electron transport chain For most animal and plant cells, the breakdown of glucose by glycolysis is only a prelude to the third and final stage of the oxidation of food molecules, in which large amounts of ATP are generated in mitochondria by oxidative phosphorylation, a process that requires the consumption of oxygen. Glycolysis and the citric acid cycle, which break down glucose into CO2 and water, each produce a small amount of ATP. More importantly, they generate activated carriers, such as NADH and FADH2. These activated carriers donate their high-energy electrons to the electron transport chain in the inner mitochondrial membrane. The movement of these electrons along the chain ultimately drives the synthesis of large amounts of ATP. Fermentation reactions break down sugar molecules in the absence of oxygen. In the process, they produce a small amount of ATP. Aerobic respiration, which requires molecular oxygen, produces a much larger yield of ATP. Gluconeogenesis consumes ATP, using its energy to produce glucose.
When food is plentiful, animals can store glucose as what? a) acetyl CoA b) glucose 6-phosphate c) glycogen or starch d) starch e) glycogen
e) glycogen In animal cells, glucose is stored in the form of glycogen, a branched polymer of glucose. This large polysaccharide is stored as small granules in the cytoplasm of many animal cells, but mainly in liver and muscle cells. The synthesis and degradation of glycogen occur by separate metabolic pathways, which can be rapidly and coordinately regulated to suit an organism's needs. Plants convert some of the sugars they make through photosynthesis during daylight into fats and into starch, a branched polymer of glucose very similar to animal glycogen. Acetyl CoA is an activated carrier, not a storage molecule. Glucose 6-phosphate is a glycolytic intermediate that, when abundant, can trigger the storage of glucose, but it is not itself a stored form of glucose.
What does the term "gluconeogenesis" refer to? a) the breakdown of glucose during glycolysis b) the transport of glucose across a cell membrane c) the breakdown of glucose during fermentation d) the release of glucose from molecules such as glycogen e) the synthesis of glucose from small organic molecules such as pyruvate
e) the synthesis of glucose from small organic molecules such as pyruvate Animals need an ample supply of glucose. Active muscles need glucose to power their contraction, and brain cells depend almost exclusively on glucose for energy. During periods of fasting or intense physical exercise, the body's glucose reserves get used up faster than they can be replenished from food. One way to increase available glucose is to synthesize it from pyruvate by a process called gluconeogenesis—a term that means "to create new glucose." Gluconeogenesis is, in many ways, a reversal of glycolysis: it builds glucose from pyruvate, whereas glycolysis breaks down glucose and produces pyruvate. Glucose can also be released by glycogen breakdown, but this is a different process from gluconeogenesis. The transport of glucose across a membrane is simply called glucose transport!
