Cell Bio 13+14
About how many molecules of ATP are produced by the complete oxidation of glucose to H2O and CO2?
30 Explanation: Glucose is oxidized by glycolysis and the citric acid cycle. A net of two molecules of ATP are produced by glycolysis for every molecule of glucose that enters that pathway. (Although four molecules are produced, two are consumed during the early steps of the pathway to prepare the sugars for the energy-releasing reactions to come.) Additionally, a related molecule, GTP, is produced by the citric acid cycle. However, both of these pathways produce, in addition, activated carriers of electrons, including NADH and FADH2. These activated carriers then donate their electrons to the electron transport chain. The movement of electrons along the electron transport chain releases energy that is harnessed to produce additional ATP. Thus, the operation of the electron transport chain ultimately powers the production of approximately 30 ATP molecules for every molecule of glucose that is oxidized.
How much of the energy that could, in theory, be derived from the breakdown of glucose or fatty acids to H2O and CO2 is captured and used to drive the energetically unfavorable synthesis of ATP?
50% Explanation: Through the production of ATP, the energy derived from the breakdown of sugars and fats is redistributed into packets of chemical energy in a form convenient for use in the cell. In total, nearly 50% of the energy that could, in theory, be derived from the breakdown of glucose or fatty acids to H2O and CO2 is captured and used to drive the energetically unfavorable reaction ADP + Pi → ATP. The remaining energy is released as heat, which in animals helps to keep the body warm. By contrast, a modern combustion engine, such as that of a car, can convert no more than 20% of the available energy in its fuel into useful work.
The pH of the mitochondrial matrix is ___________, which is ___________ than that of the intermembrane space.
7.9; higher Explanation: The pH of the mitochondrial matrix is 7.9, whereas the pH of the intermembrane space is pH 7.2. The higher pH of the matrix is due to the protons being pumped out into the intermembrane space.
What is required for glycolysis to take place?
ADP, NAD+, ATP, Pi Explanation: Central to catabolism is the oxidative breakdown of glucose by the sequential reactions of glycolysis. The reactions take place in the cytosol of most cells, and they do not require the participation of molecular oxygen. Indeed, many anaerobic microorganisms that thrive in the absence of oxygen use glycolysis to produce ATP. Glycolysis requires an initial investment of energy, in the form of ATP, to prepare glucose for the reactions to follow. That early investment is later recouped as glycolysis produces ATP. Glycolysis requires ADP to accept the high-energy phosphate groups produced during the energy-generating steps of the pathway. In step 6, glycolysis creates a high-energy linkage using inorganic phosphate. At the same time, glycolysis requires NAD+ to accept electrons (and a proton) during the oxidation reaction in step 6. The process does not, however, require NADH. It produces NADH.
What provides the fuel to convert CO2 into sugars in chloroplasts?
ATP and NADPH generated in the photosynthetic light reactions Explanation: The reactions of the carbon-fixation cycle take place in the chloroplast stroma and do not require a membrane. The energy and reducing power needed for these reactions come from the ATP and NADPH produced by the photosynthetic light reactions. Because the chloroplast's inner membrane is impermeable to ATP and NADPH, these carriers cannot actually leave the chloroplast. Instead, they are funneled into the carbon-fixation cycle, where they are used to make sugars. The ATP produced by oxidative phosphorylation in mitochondria cannot enter chloroplasts but is used instead to provide fuel for other cellular reactions.
Most of the energy released by oxidizing glucose is saved in the high-energy bonds of what molecules?
ATP and other activated carriers Explanation: 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.
The drug 2,4-dinitrophenol (DNP) makes the mitochondrial inner membrane permeable to H+. The resulting disruption of the proton gradient inhibits the mitochondrial production of ATP.What additional effect would DNP have on the transport of ATP out of the mitochondrial matrix?
ATP export will decrease because its carrier exploits the difference in voltage across the inner membrane. Explanation: The synthesis of ATP is not the only process driven by the proton gradient in mitochondria. Some molecules, such as pyruvate and Pi, are transported into the mitochondrial matrix along with the protons as they move down their electrochemical gradient.Although the transport of ATP out of the matrix is not directly coupled to the flow of protons, it does rely on an intact proton gradient. In this case, the special antiport carrier protein that exports ATP out of the matrix and brings ADP exploits the voltage difference generated by the electrochemical proton gradient. Because the matrix side of the membrane is more negatively charged than the side that faces the intermembrane space, the movement of ATP (which is more negatively charged than ADP) out of the matrix is energetically favorable. Dissipation of this gradient with DNP would not only disrupt ATP synthesis, but would impair the export of ATP from the mitochondrial matrix.
What happens when ATP synthase operates "in reverse" and pumps H+ across a membrane against its electrochemical proton gradient?
ATP is hydrolyzed to form ADP and Pi. Explanation: ATP synthase can either synthesize ATP by harnessing the electrochemical H+ gradient or pump protons against this gradient by hydrolyzing ATP. The direction of operation (and of rotation) at any given instant depends on the net free-energy change (ΔG) for the coupled processes of H+ translocation across the membrane and the synthesis of ATP from ADP and Pi. For example, if the electrochemical proton gradient falls below a certain level, the ΔG for H+ transport into the matrix will no longer be large enough to drive ATP production; instead, ATP will be hydrolyzed by the ATP synthase to rebuild the proton gradient.
Suppose the shaft of ATP synthase that is attached to the rotor were truncated (shortened) such that it no longer extended into the F1 ATPase head. What would be the consequence of this mutation?
ATP would not be produced because the conformation of the F1 ATPase head would not be changed. Explanation: As protons flow down their electrochemical gradient through the rotor in the membrane, the gradient energy is converted into a mechanical energy of rotation of the rotor. Rotation of the rotor drives rotation of the attached shaft, which projects into the head of the F1 ATPase subunit. The rotation of the shaft causes conformational changes in the F1 ATPase, driving the synthesis of ATP. If the shaft is truncated and does not reach into the F1 ATPase head, it will not drive the conformational changes and ATP will no longer be synthesized.
What are the end products of glycolysis?
ATP, NADH, pyruvate Explanation: The oxidative breakdown of glucose begins with the sequential reactions of glycolysis. The term "glycolysis" comes from the Greek glykys, "sweet," and lysis, "splitting." It is an appropriate name, as glycolysis splits a molecule of glucose, which has six carbon atoms, to form two molecules of pyruvate, each of which contains three carbon atoms. Much of the energy released by the breakdown of glucose during glycolysis is used to synthesize ATP from ADP and Pi. The remainder of the useful energy harnessed by the cell during glycolysis is stored in the high-energy electrons of the activated carrier NADH. In total, for every molecule of glucose that enters the pathway, two molecules of ATP and two molecules of NADH are produced. To prepare glucose for the energy-producing chemical rearrangements that ultimately generate pyruvate, two molecules of ATP are consumed during the initial steps of glycolysis. However, this investment of energy is more than recouped in the later steps of glycolysis, when four molecules of ATP are produced. Thus, although ADP is generated in the early steps of glycolysis, it is ultimately converted to ATP. Thus, ADP is not considered a final product of glycolysis. NADPH is a carrier of high-energy electrons that is closely related to NADH. However, this activated carrier operates in anabolic reactions, where it supplies the high-energy electrons needed to synthesize energy-rich biological molecules. The pyruvate produced by glycolysis is ultimately transported from the cytosol—where glycolysis takes place—to the mitochondrial matrix. There, a large enzyme complex removes another carbon from the molecule (in the form of CO2) and produces acetyl CoA. The acetyl group of acetyl CoA is then funneled into the citric acid cycle, where its oxidation will be completed, producing CO2 and H2O. CO2 and H2O are the final products of the complete oxidation of glucose.
The major products of the citric acid cycle are
CO2 and NADH. Explanation: The citric acid cycle produces carbon dioxide and activated carriers from the oxidation of acetyl CoA.
Each molecule of acetyl-CoA entering the citric acid cycle produces two ___________ and four ___________.
CO2; activated carriers Explanation: Acetyl CoA is a two-carbon molecule. In the citric acid cycle those carbons are oxidized to two CO2 molecules. In the process of acetyl CoA oxidization, four activated carriers are converted to their reduced forms: 3 NADH and 1 FADH2.
For what reason is cytochrome c oxidase able to pump protons across the inner mitochondrial membrane?
Electron transport drives a conformational change in the protein complex. Explanation: Proton pumping occurs because the transfer of electrons drives allosteric changes in the conformation of cytochrome c oxidase that cause protons to be ejected from the mitochondrial matrix.The complex cycles through a series of three conformations. In one of these conformations, the protein has a high affinity for H+, causing it to pick up an H+ on the matrix side of the membrane. In another conformation, the protein has a low affinity for H+, causing it to release an H+ on the other side of the membrane. The cycle proceeds only in one direction—releasing the proton into the intermembrane space—because one of the steps is driven by allosteric changes in conformation that are driven by the energetically favorable transport of electrons. The same mechanism is thought to be used by the NADH dehydrogenase complex and by many other proton pumps.
When ATP and food molecules such as fatty acids are abundant, which will occur?
Enzymes involved in gluconeogenesis will use energy to produce glucose. Explanation: When energy and food molecules are plentiful, glycolysis is shut down. Under these conditions, the cell does not need to continue to break down glucose to generate energy. Thus, key enzymes in the glycolytic pathway tend to be inhibited by ATP and other metabolites. However, enzymes in the gluconeogenesis pathway tend to be regulated in the opposite direction. For example, the products of ATP hydrolysis—ADP and AMP—indicate that energy reserves are low. These molecules stimulate glycolysis. At the same time, AMP inhibits gluconeogenesis. Thus, when ATP is abundant, gluconeogenesis is favored. Under these conditions, cells produce glucose that can be converted into glycogen and stored for later use, when energy is depleted and food is scarce. Neither glycolysis nor gluconeogenesis can operate in reverse. Both pathways include enzymes whose reactions proceed in one direction so strongly that they are effectively irreversible. Because the activities of these enzymes are regulated by similar metabolites but in opposite directions, both pathways would not be inactivated under the same conditions. Nor will they ever proceed simultaneously. To do so would be hugely wasteful, as it would allow cells to synthesize and break down metabolites in a futile cycle, consuming energy for no reason.
Determine whether the following statement is true or false: The oxygen consumed during the oxidation of glucose in animal cells is returned to the atmosphere as CO2.
False Explanation: A common misconception about the citric acid cycle is that the oxygen atoms required to make CO2 from the acetyl groups entering the citric acid cycle are supplied by atmospheric O2. In fact, these oxygen atoms come from water (H2O). Three molecules of H2O are split as they enter each turn of the citric acid cycle, and their oxygen atoms are ultimately used to make CO2. The O2 that we breathe is actually reduced to H2O by the electron transport chain; it does not form the CO2 that we exhale.
After an overnight fast, most of the acetyl CoA entering the citric acid cycle is derived from what type of molecule?
Fatty Acids Explanation: Fats serve as a major reservoir of fuel in the body. Most animal species possess specialized fat-storing cells called adipocytes. In response to hormonal signals, fatty acids can be released from these depots into the bloodstream for other cells to use as required. Such a need arises after a period of not eating. Even a normal overnight fast results in the mobilization of fat: in the morning, most of the acetyl CoA that enters the citric acid cycle is derived from fatty acids rather than from glucose. After a meal, however, most of the acetyl CoA entering the citric acid cycle comes from glucose derived from food. Fasting cells can also mobilize glucose that has been stored in the form of glycogen. However, fat is a far more important storage material than glycogen, in part because the oxidation of a gram of fat releases about twice as much energy as the oxidation of a gram of glycogen. Amino acids are not used as a form of energy storage. A cell will metabolize its own proteins only under desperate circumstances, not after a regular overnight fast.
How do fermentation reactions in oxygen-starved muscle cells and anaerobically grown yeast cells differ?
Fermentation in muscle cells produces lactate and in yeast produces ethanol plus CO2. Explanation: For most animal and plant cells, glycolysis is only a prelude to the third and final stage of the breakdown of food molecules, in which large amounts of ATP are generated in mitochondria by oxidative phosphorylation, a process that requires oxygen. However, for many anaerobic microorganisms, which can grow and divide in the absence of oxygen, glycolysis is the principal source of ATP. Certain animal cells also rely on ATP produced by glycolysis when oxygen levels fall, such as in skeletal muscle cells during vigorous exercise.Such energy-yielding pathways, which break down sugar in the absence of oxygen, are called fermentations. In these anaerobic conditions, fermentation converts the pyruvate made by glycolysis into products that can be excreted from the cell: lactate in muscle cells, for example, or ethanol and CO2 in the yeast cells used in brewing and breadmaking. In both processes, the NADH gives up its electrons and is converted back to the NAD+ required to maintain the reactions of glycolysis.
In the electron transport chain, the oxygen atoms in O2 become part of which molecule?
H2O Explanation: 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.
Experiments performed by Hans Krebs in the 1930s revealed that the set of reactions that oxidize food molecules and produce CO2 occur in a cycle. In one experiment, Krebs exposed pigeon muscles to malonate, a compound that inhibits succinate dehydrogenase—the enzyme that converts succinate to fumarate, indicated in red in the linear representation of the reactions of the citric acid cycle (below). Which of the following observations, made in malonate-treated muscle, led Krebs to believe that this set of reactions is cyclical?
If fumarate were added, succinate would accumulate. Explanation: Malonate blocks the conversion of succinate to fumarate. But if the reactions were linked in a cycle, the addition of any compound "downstream" of the blocked reaction would cause an accumulation of succinate—the substrate just before the blockage. If fumarate were added, for example, succinate would accumulate. It would be carried through the downstream reactions until it produced oxaloacetate, which would then combine with the carbons from acetyl CoA to produce citrate at the start of the cycle. Citrate would then continue on until it was converted to succinate, at which point malonate would block its further conversion to fumarate. Thus, succinate would accumulate. Krebs found that addition of fumarate, malate, or oxaloacetate would result in an accumulation of succinate in muscles treated with the inhibitor malonate. If citrate were added to the tissue preparation, succinate would accumulate; but this would occur even if the pathway were linear. So this result would not reveal the cyclical nature of the pathway. If succinate were added to the preparation, nothing more would happen because malonate would block its further conversion to fumarate. An accumulation of oxaloacetate would never be seen because any oxaloacetate produced from malate during the final step of the pathway would be consumed at the start of the pathway. Finally, regardless of whether the pathway is linear or cyclical, oxygen is not produced by any reaction in the citric acid cycle.
You have joined a lab that studies the metabolic pathway shown below. Just recently, an inhibitor for one of the enzymes, enzyme 3, has become available and the head of the lab wants you to use this inhibitor to determine whether the pathway is linear (as shown) or circular, with another enzyme (not shown) converting F back to A. You have access to all the molecules. What result suggests that the pathway may be circular rather than linear?
Inhibition of enzyme 3 and addition of extra metabolite D leads to buildup of C. Explanation: Blocking of enzyme 3 prevents C from being converted to D. In either a linear or a circular pathway, adding extra A or B leads to C buildup. In a linear pathway, adding extra D or E leads to buildup of F, but in a circular pathway adding extra D or E leads to buildup of C because F converts to A. Experiments similar to this determined that the citric acid cycle was a cycle, with oxaloacetate helping to produce citrate.
What is true of the antenna complex of a photosystem?
It captures light energy. Explanation: Each photosystem consists of a set of antenna complexes, which capture light energy, and a reaction center, which converts that light energy into chemical energy. In an antenna complex, hundreds of chlorophyll molecules are arranged so that the light energy captured by one chlorophyll molecule can be transferred to a neighboring chlorophyll molecule in the network. In this way, energy jumps randomly from one chlorophyll molecule to the next—either within the same antenna or in an adjacent antenna. At some point, this wandering energy will be passed to a chlorophyll dimer called the special pair, located in the reaction center. The special pair holds its electrons at a slightly lower energy than do the other chlorophyll molecules, so when energy is accepted by this special pair, it becomes effectively trapped there.
What does the pyruvate dehydrogenase complex do?
It converts pyruvate into acetyl CoA in the mitochondrial matrix. Explanation: In aerobic metabolism in eukaryotic cells, the pyruvate produced by glycolysis is actively pumped into the mitochondrial matrix. There, it is rapidly decarboxylated by a giant complex of three enzymes, called the pyruvate dehydrogenase complex. The pyruvate dehydrogenase complex contains multiple copies of three enzymes—pyruvate dehydrogenase, dihydrolipoyl transacetylase, and dihydrolipoyl dehydrogenase. This enzyme complex removes a CO2 from pyruvate to generate NADH and acetyl CoA. The latter is produced when the acetyl group derived from pyruvate is linked to coenzyme A (CoA).In the next stage in cell respiration, the acetyl group in acetyl CoA is donated to the citric acid cycle in the mitochondrial matrix, where it will be fully oxidized to CO2.The action of the pyruvate dehydrogenase complex produces NADH; thus, it consumes NAD+ rather than regenerating it. The enzyme complex, which operates in the mitochondrial matrix, oxidizes pyruvate, producing acetyl CoA and CO2. The complete oxidation of the remaining acetyl carbons to CO2 takes place during the citric acid cycle.
What happens to the ATP produced during stage 1 of photosynthesis?
It is consumed within the chloroplast to produce glyceraldehyde 3-phosphate. Explanation: The chloroplast's inner membrane is impermeable to the ATP and NADPH that are produced in the stroma during stage 1 of photosynthesis. These molecules are therefore funneled into the carbon-fixation cycle, where they are used to drive the reactions that produce glyceraldehyde 3-phosphate. For every three molecules of CO2 that enter the cycle, one molecule of glyceraldehyde 3-phosphate is ultimately produced, at the expense of nine molecules of ATP and six molecules of NADPH, which are consumed in the process. Glyceraldehyde 3-phosphate, the three-carbon sugar that is the final product of the cycle, provides the starting material for the synthesis of the many other sugars and other organic molecules that the plant needs. Glyceraldehyde 3-phosphate can also be stored in the chloroplast in the form of starch or fat. When the plant is in need of energy, this stored starch and fat can be broken down to sugars and fatty acids, which are exported to the cytosol to help support the metabolic needs of the plant. These metabolites can be further broken down by glycolysis and the citric acid cycle, ultimately leading to the production of ATP by oxidative phosphorylation in mitochondria. Plants use this ATP to power a huge variety of metabolic reactions, just as animal cells and other nonphotosynthetic organisms do.
In terms of energy production, what is true of cell respiration?
It is more efficient than a gasoline-powered engine. Explanation: Although the biological oxidation of glucose to CO2 and H2O consists of many interdependent steps, the overall process of cell respiration is remarkably efficient. Almost half of the total energy that could be released by burning sugars or fats is captured and stored in the phosphate bonds of ATP during cell respiration. The ΔGo for the complete oxidation of glucose, for example, is -2867 kJ/mole. In biological systems, this reaction yields approximately 30 molecules of ATP, and the hydrolysis of ATP in cells liberates from 46 to 54 kJ/mole of usable energy, depending on conditions (such as the ATP/ADP ratio). If we say, on average, ATP hydrolysis yields 50 kJ/mole of energy, then 30 ATP would provide 1500 kJ/mole of energy—more than half of that available from the oxidation of glucose to CO2 and H2O. Electric motors and gasoline engines operate at about 10-20% efficiency. If cells operated at this efficiency, an organism would have to eat voraciously just to maintain itself. Moreover, because the wasted energy is liberated as heat, large organisms (including humans) would need far better mechanisms for cooling themselves. It is hard to imagine how animals could have evolved without the elaborate yet economical mechanisms that allow cells to extract a maximum amount of energy from food.
The electron-transport chain in mitochondria accepts high-energy electrons directly from which molecule?
NADH Explanation: The generation of ATP is powered by the flow of electrons that are derived from the burning of carbohydrates, fats, and other foodstuffs during glycolysis and the citric acid cycle. These "high-energy" electrons are provided by activated carriers generated during these two sets of catabolic reactions, with the majority being churned out by the citric acid cycle that operates in the mitochondrial matrix. NADH produced during glycolysis can also donate electrons to the electron-transport chain, but it must first be imported into the mitochondrial matrix. Because the mitochondrial inner membrane is impermeable to NADH, this transport requires energy. Thus, NADH produced during glycolysis ultimately yields less net energy than NADH generated by the citric acid cycle, which takes place in the mitochondrial matrix.
The citric acid cycle produces which activated carriers that transfer high-energy electrons to the electron-transport chain?
NADH and FADH2 The citric acid cycle produces reduced forms of two activated carriers, NADH and FADH2. These electron carriers transfer a hydride ion, two electrons, and one proton to the electron-transport chain complexes.
Antimycin A is a piscicide (fish poison) used to manage fisheries and kill invasive species. Antimycin A blocks the transfer of electrons through the cytochrome b-c1 complex. What components of the electron transport chain are bound to high-energy electrons after treating a mitochondrion with antimycin A?
NADH and the NADH dehydrogenase complex are bound to high-energy electrons while O2 and the cytochrome c oxidase complex are not. Explanation: The electron transport chain will be blocked at complex III (cytochrome b-c1), so any complexes after this point (cytochrome c oxidase) will not become reduced or bound to the high-energy electrons. O2 will also remain oxidized as O2 and not reduced to H2O. The complex before complex III (NADH dehydrogenase) will still be reduced and bound to high-energy electrons. The NADH will remain as reduced NADH since it will no longer be able to continue to pass electrons to complex I because complex I is already bound to electrons.
Most of the energy for the synthesis of ATP comes from which molecule?
NADH produced by the citric acid cycle Explanation: Much of the energy carried by NADH and FADH2 is ultimately converted into the bond energy of ATP. The six NADH molecules produced in the mitochondrial matrix during the citric acid cycle (three per turn; two turns per original molecule of glucose oxidized) pass their high-energy electrons to the NADH dehydrogenase complex—the first complex in the chain. As the electrons pass from one enzyme complex to the next, they promote the pumping of protons across the inner mitochondrial membrane. In this way, each NADH molecule provides enough net energy to generate about 2.5 molecules of ATP--for a total of 15. FADH2 molecules, of which two are produced by two turns of the citric acid cycle, pass their electrons to a membrane-embedded mobile carrier further down the respiratory chain than do NADH; they thus promote the pumping of fewer protons and provide enough energy for only 1.5 molecules of ATP each.
In the electron-transport chain in chloroplasts, which molecule serves as the final electron acceptor?
NADP+ Explanation: In the first stage of photosynthesis, an electron-transport chain in the thylakoid membrane harnesses the energy of electron transport to pump protons into the thylakoid space; the resulting proton gradient then drives the synthesis of ATP by ATP synthase. This electron-transport process differs from that of oxidative phosphorylation in terms of where its high-energy electrons come from—and where they wind up. During oxidative phosphorylation, high-energy electrons are donated by NADH produced during glycolysis and the citric acid cycle and they are ultimately passed along to molecular oxygen. In photosynthesis, the high-energy electrons donated to the photosynthetic electron-transport chain come from a molecule of chlorophyll that has absorbed energy from sunlight. And they are donated not to O2 but to NADP+, to produce NADPH.
Which is the most permeable membrane of the mitochondrion?
Outer membrane Explanation: The mitochondrion has a smooth outer membrane that contains large porin proteins that allow for the passage of all molecules up to a certain size. The inner membrane is highly folded and contains the enzymes of the electron-transport chain.
What is true for eukaryotic cells?
Sugars and fats are both converted to acetyl CoA in the mitochondria. Explanation: In aerobic metabolism in eukaryotic cells, the pyruvate produced by glycolysis is actively pumped into the mitochondrial matrix. There, it is rapidly decarboxylated by a giant complex of enzymes, called the pyruvate dehydrogenase complex. The products of this set of reactions are CO2 (a waste product), NADH, and acetyl CoA. In addition to sugar, which is broken down during glycolysis, fat is a major source of energy for most nonphotosynthetic organisms, including humans. Like the pyruvate derived from glycolysis, the fatty acids derived from fat are also converted into acetyl CoA in the mitochondrial matrix. Some amino acids are also transported from the cytosol into the mitochondrial matrix, where they can be converted into acetyl CoA or one of the other intermediates of the citric acid cycle. Thus, in the eukaryotic cell, the mitochondrion represents the center toward which all energy-yielding catabolic processes lead, whether they begin with sugars, fats, or proteins.
What does it mean for a bond to be "high energy," such as the bonds between phosphate groups in ATP?
The hydrolysis of the bond is energetically favorable. Explanation: The bond is not particularly strong or hard to break, and it is chemically identical to other phosphate bonds. However, due to the chemical configuration of the molecule, and conditions inside the cell, hydrolysis of a high-energy phosphate bond releases a large amount of energy. The two outermost phosphate groups in ATP are held to the rest of the molecule by "high-energy" phosphoanhydride bonds and are readily transferred to other organic molecules—or to water. A hydrolysis reaction is essentially the transfer of the phosphate group to water, yielding ADP and inorganic phosphate (Pi). The large negative ΔGº of the ATP hydrolysis reaction arises from a number of factors. Release of the terminal phosphate group removes an unfavorable repulsion between adjacent negative charges; in addition, the inorganic phosphate ion (Pi) that is released is stabilized by favorable hydrogen-bond formation with water. Although the ΔGº of this reaction is -30.5 kJ/mole, its ΔG inside cells is much more negative because the ratio of ATP to the products ADP and Pi is kept so high. Inside a cell, this hydrolysis of the terminal phosphate of ATP yields between 46 and 54 kJ/mole of usable energy.
Protons are pumped across the mitochondrial inner membrane as electrons are transferred through the mitochondrial electron transport chain. Which of the following statements about proton pumping are correct?
The mitochondria use the proton gradient to synthesize ATP. The pH inside the mitochondrial matrix is higher than in the intermembrane space. The NADH dehydrogenase, cytochrome b-c1, and cytochrome oxidase complexes all pump protons across the membrane. Explanation: The three complexes—NADH dehydrogenase, cytochrome b-c1, and cytochrome c oxidase—all pump protons from the mitochondrial matrix to the intermembrane space. This leads to a lower pH in the intermembrane space. The proton gradient established by the electron transport chain complexes is then used by ATP synthase in the mitochondrial membrane to synthesize ATP. In this way, the energy in the high-energy electrons held by NADH is converted to chemical energy in ATP.
In eukaryotic cells, why must metabolism be tightly regulated?
The substrates involved in metabolic reactions can be used by a number of different enzymes. Explanation: The various products and intermediates of metabolic reactions are often substrates for multiple enzymes. Recall, for example, the number of intermediates from the citric acid cycle that can act as precursors for many important organic molecules, including amino acids, nucleic acids, sugars, and lipids. Thus, depending on conditions, a cell must decide whether to route key metabolites into anabolic or catabolic pathways—in other words, whether to use them to build other molecules or burn them to provide immediate energy. To balance the activities of these competing yet interrelated reactions—and to allow organisms to adapt swiftly to changes in food availability or energy expenditure—an elaborate network of control mechanisms regulates and coordinates the activity of the enzymes that catalyze the myriad metabolic reactions that go on in a cell.
How do enzymes maximize the energy harvested from the oxidation of food molecules?
They allow the stepwise oxidation of food molecules, which releases energy in small amounts. Explanation: 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.
How do the high-energy electrons of activated carriers contribute to forming the high-energy phosphate bonds of ATP?
They are used by the electron-transport chain to make a proton gradient. Explanation: Activated carriers transfer their high-energy electrons to the electron-transport chain to form a proton gradient across the mitochondrial inner membrane. The proton gradient is then used to power ATP synthesis as they flow down their electrochemical gradient through ATP synthase.
Your friends are on a low-fat, high-carbohydrate diet, which they claim will prevent fat accumulation within their bodies. They eat tons of pasta and bread without worrying about calorie count. What can you correctly say to your friends about their potential to accumulate lipids on their low-fat diet?
They will accumulate fats because cells can convert glycolytic metabolites into lipids. Explanation: After a meal, any excess glucose is stored as glycogen or fat. Cells can readily convert sugars to fats through glycolytic and citric acid cycle intermediates.
Under which conditions will ATP synthase produce ATP?
When there is a higher concentration of protons in the intermembrane space than in the matrix. Explanation: The respiratory enzyme complexes in the electron transport chain use the energy released by electron transfer to pump protons across the inner mitochondrial membrane, from the matrix into the intermembrane space. This pumping of protons generates an H+ gradient—or pH gradient—across the inner membrane. In most cells, this electrochemical proton gradient is then used to drive the synthesis of ATP from ADP and Pi. The device that makes this possible is ATP synthase.The part of the protein that catalyzes the phosphorylation of ADP is shaped like a lollipop head that projects into the mitochondrial matrix; it is penetrated by a central stalk that is attached to a transmembrane H+ carrier. The passage of protons through the carrier causes the carrier and its stalk to spin rapidly, like a tiny motor. As the stalk rotates, it rubs against proteins in the enzyme's stationary head, altering their conformation and causing them to produce ATP. In this way, a mechanical deformation gets converted into the chemical-bond energy of ATP.
Which of the following drives the production of ATP from ADP and Pi by ATP synthase?
a proton (H+) gradient Explanation: Membrane-based systems use the energy stored in an electrochemical proton (H+) gradient to synthesize ATP. This proton gradient is generated by the movement of electrons along a membrane. When the protons flow back across the membrane, down their electrochemical gradient, their energy can be harnessed to do work. ATP synthase uses the energy stored in the proton gradient to produce ATP.
In photosynthesis, what drives the generation of ATP by ATP synthase?
a proton gradient across the thylakoid membrane Explanation: a proton gradient across the thylChemiosmotic coupling drives ATP production during oxidative phosphorylation and photosynthesis. Both membrane-based processes make use of the energy stored in a proton gradient to synthesize ATP. During photosynthesis, energy captured by a chlorophyll-containing photosystem excites electrons, which are then passed on to a photosynthetic electron-transport chain. One of the electron carriers in this chain, called the cytochrome b6-f complex, uses some of this energy to pump protons across the thylakoid membrane. This carrier resembles the cytochrome c reductase complex of mitochondria. As in mitochondria, an ATP synthase embedded in the membrane then uses the energy of the electrochemical proton gradient to produce ATP.
Fatty acids can be used to produce energy by conversion to ___________ in the ___________ of the cell.
acetyl CoA; mitochondria Explanation: Fatty acids are broken down to acetyl CoA in the mitochondria. This acetyl CoA can then enter the citric acid cycle to produce energy.
The proton gradient that drives ATP synthesis during photosynthesis is generated by which of the following?
an electron carrier that pumps protons out of the stroma into the thylakoid space Explanation: To produce ATP and NADPH, plant cells make use of two photosystems that work in series. When the first photosystem (which, paradoxically, is called photosystem II) absorbs light energy, its reaction center passes electrons to a mobile electron carrier called plastoquinone, which is part of the photosynthetic electron-transport chain. This carrier transfers the high-energy electrons to a proton pump called cytochrome b6-f complex. Like the proton pumps in the mitochondrial inner membrane, cytochrome b6-f complex uses the movement of electrons to generate an electrochemical proton gradient. In this case, the pump extracts protons from water in the chloroplast stroma and pumps them into the thylakoid space. The resulting electrochemical proton gradient then drives the production of ATP by an ATP synthase located in the thylakoid membrane. Electrons excited by photosystem I reach an energy level high enough to be used to reduce NADP+ to NADPH.
What describes a breakdown process in which enzymes degrade complex molecules into simpler ones?
catabolism Explanation: Most of the chemical reactions that cells perform normally occur only at temperatures that are much higher than those inside a cell. Each reaction therefore requires a major boost in chemical reactivity to enable it to proceed rapidly within the cell. This boost is provided by a large set of specialized proteins called enzymes, each of which accelerates, or catalyzes, just one of the many possible reactions that a particular molecule could, in principle, undergo. Rather than being an inconvenience, the necessity for catalysis is a benefit, as it allows the cell to precisely control its metabolism—the sum total of all the chemical reactions it needs to carry out to survive, grow, and reproduce. This control is central to the chemistry of life. Two opposing streams of chemical reactions occur in cells: the catabolic pathways and the anabolic pathways. The catabolic pathways (catabolism) break down foodstuffs into smaller molecules, thereby generating both a useful form of energy for the cell and some of the small molecules that the cell needs as building blocks. The anabolic, or biosynthetic, pathways (anabolism) use the energy harnessed by catabolism to drive the synthesis of the many molecules that form the cell. Together, these two sets of reactions constitute the metabolism of the cell.
In plants, fats and starch are stored in which part of the cell?
chloroplast Explanation: 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. The fats in plants are triacylglycerols, as they are in animals, and they differ only in the types of fatty acids that predominate.In plant cells, fats and starch are both stored in chloroplasts—specialized organelles that carry out photosynthesis. These energy-rich molecules serve as food reservoirs that are mobilized by the cell during periods of darkness. At night, stored starch and fat can be broken down to sugars and fatty acids, which are exported to the cytosol to help support the metabolic needs of the plant.Mitochondria are organelles in which sugars and other food-derived molecules are oxidized to produce ATP.The endoplasmic reticulum in plant cells serves the same function as it does in animal cells—as a site of protein production and folding.Plant cell walls are supportive structures reinforced with cellulose, not starch.
ATP synthase is a large molecular machine that converts the energy in an electrochemical gradient into the bond energy stored in ATP. Which of the following events are required for the synthesis of ATP?
conformational changes of the F1 ATPase. movement of protons down their gradient through ATP synthase. rotation of the rotor in the membrane Explanation: Protons flow down their concentration gradient through the rotor subunit of ATP synthase. The movement of the protons causes the rotor subunit to rotate in the membrane. The rotation causes a shaft embedded in the F1 ATPase subunit to also rotate, leading to conformational changes of the F1 ATPase. Subunits of the F1 ATPase can adopt one of three conformations, empty, able to bind to ADP + Pi, or able to convert ADP + Pi to ATP. ADP and Pi bind to the correct subunit and are converted to ATP as the rotor rotates. ATP is released after synthesis. ATP synthase can also work in the opposite direction if ATP concentration is high and the proton gradient is low to pump protons the reverse direction.
What process generates the largest number of ATP molecules?
electron transport chain Explanation: 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.
During stage I of photosynthesis, excited electrons move in which direction?
from the chlorophyll special pair to an electron carrier to NADP+ Explanation: Building organic molecules from CO2, which takes place during stage 2 of photosynthesis, requires a huge input of energy, in the form of ATP, and a large amount of reducing power, in the form of the activated carrier NADPH. Both of these molecules are generated during stage 1 of photosynthesis.The process begins with the absorption of light energy by chlorophyll. This energy is passed from one chlorophyll molecule to another until it produces an excited electron in a chlorophyll special pair within a photosynthetic reaction center. This electron is passed to an electron carrier in the thylakoid membrane. As excited electrons are passed down the photosynthetic electron-transport chain, a proton pump generates an electrochemical proton gradient. Finally, absorption of another packet of light energy by photosystem I boosts the electron to the high energy level needed to reduce NADP+ to NADPH.
The electron-transport chain pumps protons in which direction?
from the matrix to the intermembrane space Explanation: Each of the respiratory enzyme complexes in the electron-transport chain uses the energy released by the energetically favorable transfer of electrons from NADH to O2 to pump protons across the inner mitochondrial membrane, from the matrix into the intermembrane space. This proton pumping generates an H+ gradient—or pH gradient—across the inner membrane. As a result, the pH in the matrix (around 7.9) is about 0.7 units higher than it is in the intermembrane space (which is 7.2, the same pH as the cytosol). It also produces a voltage gradient—or membrane potential—across the inner membrane. This combined electrochemical gradient represents a form of stored energy.
When food is plentiful, animals can store glucose as what?
glycogen Explanation: 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.
In an animal cell, where are the proteins of the electron-transport chain located?
inner mitochondrial membrane Explanation: In animal cells, oxidative phosphorylation takes place on the inner mitochondrial membrane, which contains both the proteins that form an electron-transport chain and ATP synthase. In this process, high-energy electrons—derived from the oxidation of food molecules—are transferred along the series of electron carriers that form the electron-transport chain. These electron transfers release energy that is used to pump protons across the inner mitochondrial membrane, thus generating an electrochemical proton gradient. These protons subsequently flow back down this electrochemical gradient, into the mitochondrial matrix, through the enzyme ATP synthase. ATP synthase couples the movement of these protons to the production of ATP.
Protons are pumped across the mitochondrial inner membrane to accumulate in the
intermembrane space. Explanation: The electron-transport chain pumps electrons from the mitochondrial matrix into the intermembrane space. This produces a higher concentration of protons in the intermembrane space, which is used to power ATP synthase.
In plant cells, where does the citric acid cycle take place?
mitochondria Explanation: To be able to grow, divide, and carry out day-to-day activities, cells require a constant supply of energy. This energy comes from the chemical-bond energy in food molecules, which thereby serve as fuel for cells. Perhaps the most important fuel molecules are the sugars. While animals obtain sugars and other organic molecules by eating, plants make their own sugars from CO2 by photosynthesis in the chloroplast.Regardless of how these sugars are obtained, the process by which they are broken down to generate energy is very similar in both animals and plants. In both cases, the organism's cells harvest useful energy from the chemical-bond energy locked in sugars as the sugar molecule is broken down and oxidized to carbon dioxide (CO2) and water (H2O). The energy released during these reactions is captured in activated carriers such as ATP and NADH.In all eukaryotic cells, the complete oxidative breakdown of sugars such as glucose begins with glycolysis in the cytosol and continues in the mitochondria with the citric acid cycle and, ultimately, the generation of ATP via oxidative phosphorylation. These processes occur in the same compartments in the cells of both animals and plants.
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?
oxygen Explanation: 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.
In mitochondria, what is the final electron acceptor in the electron-transport chain?
oxygen (O2) Explanation: Like any other chemical reaction, the tendency of oxidation-reduction reactions to proceed spontaneously depends on the free-energy change (ΔG) for the electron transfer, which in turn depends on the relative electron affinities of the participating molecules. Electrons will pass spontaneously from molecules that have a relatively low affinity for some of their electrons, and thus lose them easily, to molecules that have a higher affinity for electrons. With its low electron affinity, NADH is an excellent molecule to donate electrons to the respiratory chain, while O2, with its high affinity for electrons, is well suited to act as the electron "sink" at the end of the pathway.
Which photosystem is depicted in this simplified diagram?
photosystem I Explanation: To generate both ATP and NADPH, plant cells—and free-living photosynthetic organisms such as cyanobacteria—make use of two different photosystems, which operate in series. Although they are similar in structure, these two photosystems do different things with the high-energy electrons that leave their reaction-center chlorophylls. When light is absorbed by photosystem I, electrons are boosted to the very high energy level needed to reduce NADP+. The reaction center of this photosystem passes electrons to a mobile electron carrier called ferredoxin, which brings them to an enzyme that uses them to produce NADPH.
Which photosystem is depicted in this simplified diagram?
photosystem II Explanation: To generate both ATP and NADPH, plant cells—and free-living photosynthetic organisms such as cyanobacteria—make use of two different photosystems, which operate in series. Although they are similar in structure, these two photosystems do different things with the high-energy electrons that leave their reaction-center chlorophylls.When the first photosystem (which, paradoxically, is called photosystem II for historical reasons) absorbs light energy, its reaction center passes electrons to a mobile electron carrier called plastoquinone, which is part of the photosynthetic electron-transport chain. This carrier transfers the high-energy electrons to a proton pump, which—like the proton pumps in the mitochondrial inner membrane—uses the movement of electrons to generate an electrochemical proton gradient. The electrochemical proton gradient then drives the production of ATP by an ATP synthase located in the thylakoid membrane.
In the electron-transport chain, as electrons move along a series of carriers, they release energy that is used to do what?
pump protons across a membrane Explanation: In oxidative phosphorylation, which occurs in mitochondria, an electron-transport system receives high-energy electrons derived from the oxidation of food. The movement of these electrons along the electron-transport chain releases energy that is used to pump protons across the membrane. The flow of electrons back down this electrochemical proton (H+) gradient can then be harnessed to generate ATP.
The movement of electrons through the electron-transport chain in mitochondria does which of the following?
pumps protons out of the mitochondrial matrix Explanation: The stepwise movement of these electrons through the components of the electron-transport chain releases energy that can then be used to pump protons across the inner mitochondrial membrane. This action represents stage 1 of chemiosmotic coupling. The electrons are ultimately passed to oxygen (the final electron acceptor) and the proton gradient is used to drive the synthesis of ATP. This chemiosmotic mechanism for ATP synthesis is called oxidative phosphorylation because it involves both the consumption of O2 and the addition of a phosphate group to ADP to form ATP.
In cells that cannot carry out fermentation, which products derived from glycolysis will accumulate under anaerobic conditions?
pyruvate and NADH Explanation: In cells that can carry out fermentation, the pyruvate generated by glycolysis is converted into products that can be excreted from the cell: lactate in muscle cells, for example, or ethanol and CO2 in the yeast cells used in brewing and breadmaking. In the process, NADH gives up its electrons and is converted back to the NAD+ required to maintain the reactions of glycolysis.Without fermentation, NADH produced during glycolysis would accumulate. Because oxygen is the ultimate acceptor of electrons donated by NADH, in the absence of oxygen, NADH would be forced to keep hold of its electrons. As a result, the NAD+ needed for glycolysis to continue would no longer be regenerated.The enzyme complex that converts pyruvate into acetyl CoA—and the citric acid cycle that completes the oxidation of these carbons—also requires a source of NAD+. Without fermentation reactions to regenerate this NAD+, cells lacking oxygen would be unable to further oxidize pyruvate. Thus, this intermediate, too, would accumulate.
How is pyruvate imported into the mitochondrial matrix for use in the citric acid cycle?
sodium gradient-driven antiport Explanation: Pyruvate is transported across the mitochondrial inner membrane using the proton gradient. The transport protein is a symporter, moving protons in the same direction as the pyruvate molecule.
When nutrients are plentiful, plants can store glucose as what?
starch Explanation: 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 what?
the electron transport chain Explanation: 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.
What is gluconeogenesis?
the synthesis of glucose from pyruvate Explanation: Gluconeogenesis is the reversal of glycolysis—the synthesis of glucose from pyruvate. This process allows glucose to be remade if it becomes scarce.
Once excited by sunlight, chlorophylls in the antenna complex do which of the following?
transfer the energy to the chlorophyll special pair Explanation: In an antenna complex, hundreds of chlorophyll molecules are arranged so that the light energy captured by one chlorophyll molecule can be transferred to a neighboring chlorophyll molecule in the network. In this way, energy jumps randomly from one chlorophyll molecule to the next—either within the same antenna or in an adjacent antenna. When this energy is passed to the chlorophyll special pair in the reaction center, which holds its electrons at a slightly lower energy than do the other chlorophyll, it becomes effectively trapped there. The trapped energy excites an electron within the special pair, and this high-energy electron is passed to a set of electron carriers that ferry it to the electron-transport chain. This electron transfer converts the light energy that entered the special pair into the chemical energy of a transferable electron.
