Cellular Chapter 14

Approximately how many molecules of ATP can be produced in mitochondria from the complete oxidation of a single glucose molecule?

30

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? A. ATP would not be produced because the conformation of the F1 ATPase head would not be changed. B. The rotor in the membrane would no longer turn. C. ATP synthase would work in reverse, breaking down ATP and pumping protons against their gradient. D. Protons would not cross the membrane using the rotor.

A. ATP would not be produced because the conformation of the F1 ATPase head would not be changed. E: 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

The electron-transport chain in mitochondria accepts high-energy electrons directly from which molecule? A. NADH B. H2O C. pyruvate D. acetyl CoA E. ATP

A. NADH E: 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.

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? A. NADH and the NADH dehydrogenase complex are bound to high-energy electrons while O2 and the cytochrome c oxidase complex are not. B. O2 and the cytochrome c oxidase complex are bound to high-energy electrons while NADH and the NADH dehydrogenase complex are not. C. All three complexes and NADH are bound to high-energy electrons. D. None of the complexes are bound to high-energy electrons.

A. NADH and the NADH dehydrogenase complex are bound to high-energy electrons while O2 and the cytochrome c oxidase complex are not. E: 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.

As the human population grows, it becomes increasingly important to maximize crop yields. As such, scientists search for more efficient ways for plants to convert CO2 into biomass. One approach is to genetically modify plant enzymes involved in photosynthesis to increase their efficiency. Which plant enzyme, directly responsible for carbon fixation, is a focus of research? A. Rubisco B. cytochrome c oxidase C. ATP synthase D. chlorophyll

A. Rubisco E: The enzyme ribulose bisphosphate carboxylase (Rubisco) catalyzes the carbon fixation step of the Calvin cycle, covalently attaching carbon dioxide to ribulose 1,5-bisphosphate (see figure below). Compared to most enzymes, Rubisco is slow, catalyzing only about three molecules of substrate per second. To compensate, plants contain abundant Rubisco enzyme in their leaves. To increase crop yields, scien

The low redox potential of NADH means that it has aChoose one: A. Tendency to give up electrons B. high electron affinity. C. very stable bond.l D. ow free energy.

A. Tendency to give up electrons E: Molecules with a low redox potential are more likely to give up their electrons, and thus have a low affinity for electrons. This means that the electrons are in a "high energy bond" and the bond is easy to break, making NAD+ and FAD good candidates for carrying and delivering electrons to the electron-transport chain, the members of which have higher redox potentials.

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? A.The NADH dehydrogenase, cytochrome b-c1, and cytochrome oxidase complexes all pump protons across the membrane. B.The mitochondria use the proton gradient to synthesize ATP. C.The pH inside the mitochondrial matrix is higher than in the intermembrane space. D.Protons are pumped into the matrix of the mitochondria.

A.The NADH dehydrogenase, cytochrome b-c1, and cytochrome oxidase complexes all pump protons across the membrane. B.The mitochondria use the proton gradient to synthesize ATP. C.The pH inside the mitochondrial matrix is higher than in the intermembrane space. E: 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.

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? A.rotation of the rotor in the membrane B.movement of protons down their gradient through ATP synthase C.binding of ATP to an empty F1 ATPase subunit D.conformational changes of the F1 ATPase

A.rotation of the rotor in the membrane B.movement of protons down their gradient through ATP synthase D.conformational changes of the F1 ATPase E: 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.

When an electron is removed from the reaction center of photosystem II, the missing electron is replaced by an electron from which of the following? A. photosystem I B. H2O C. H+ D. sunlight E. manganese

B. H2O E: Photosystem II includes a water-splitting enzyme that extracts electrons from water, producing O2 as a by-product.

Most of the energy for the synthesis of ATP comes from which molecule? A.NADH produced by glycolysis B. NADH produced by the citric acid cycle C. NADH produced by the conversion of pyruvate to acetyl CoA D. GTP produced by the citric acid cycle E. FADH2 produced by the citric acid cycle

B. NADH produced by the citric acid cycle

In mitochondria, what is the final electron acceptor in the electron-transport chain? A. ADP B. oxygen (O2) C. carbon dioxide (CO2) D. NADH and FADH2 E. water (H2O)

B. oxygen (O2) e: The transfer of electrons to O2 at the end of the electron-transport chain consumes nearly all of the oxygen we breathe.

Which of these is able to boost electrons to the very high energy level needed to make NADPH from NADP+? A. ATP synthase B. photosystem I C. cytochrome b6-f complex D. photosystem II E. H2O

B. photosystem I E: When photosystem II absorbs light energy, its reaction center passes high-energy electrons to plastoquinone. This mobile electron carrier brings them to cytochrome b6-f complex, which uses some of their energy to pump protons across the thylakoid membrane. This proton gradient is tapped by ATP synthase to produce ATP. At the same time, photosystem I captures energy from sunlight and passes its high-energy electrons to ferredoxin. This mobile electron carrier passes them to the enzyme that uses them to reduce NADP+ to NADPH.

In the electron-transport chain, as electrons move along a series of carriers, they release energy that is used to do what? A. split water into protons and oxygen B. pump protons across a membrane C. oxidize food molecules D. hydrolyze ATP E. phosphorylate ADP to form ATP

B. pump protons across a membrane E: 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.

During oxidative phosphorylation, why does a single molecule of NADH result in the production of more ATP molecules than a single molecule of FADH2? A. FADH2 has a lower electron affinity than does NADH. B. FADH2 promotes the pumping of more protons than does NADH. C. FADH2 and NADH feed their electrons to different carriers in the electron-transport chain. D. FADH2 is less likely than NADH to participate in the electron-transport chain. E. NADH donates more electrons to the electron-transport chain than does FADH2.

C. FADH2 and NADH feed their electrons to different carriers in the electron-transport chain.

Carbon fixation occurs in the second stage of photosynthesis, during the light-independent reactions of the Calvin cycle. In the first step of this cycle, the enzyme Rubisco adds CO2 to the energy-rich compound ribulose 1,5-bisphosphate, ultimately producing two molecules of 3-phosphoglycerate. In a culture of green alga that is carrying out photosynthesis in the presence of CO2 in the laboratory, what would happen to the levels of ribulose 1,5-bisphosphate and 3-phosphoglycerate in the minutes after the lights were turned off and the cultures were plunged into darkness? A. Nothing would happen because the Calvin cycle is not light-dependent. B. Both would accumulate. C. Ribulose 1,5-bisphosphate would be depleted, but 3-phosphoglycerate would accumulate. D. Ribulose 1,5-bisphosphate would accumulate, but 3-phosphoglycerate would be depleted. E. Both would be depleted.

C. Ribulose 1,5-bisphosphate would be depleted, but 3-phosphoglycerate would accumulate. E: The carbon-fixation cycle requires an input of ATP and NADPH to form glyceraldehyde 3-phosphate from CO2 and H2O. Both of these activated carriers are products of the first stage of photosynthesis, in which light energy is used to generate ATP and reduce NADP+.

Which metal ion is found in all three respiratory enzyme complexes? A. heme B. manganese C. iron D. copper E. sulfur

C. iron

Stage 1 of photosynthesis is, in large part, equivalent to what process? A. the carbon-fixation cycle B. glycolysis C. oxidative phosphorylation D. the production of acetyl CoA by the pyruvate dehydrogenase complex E. the citric acid cycle

C. oxidative phosphorylation E: In this stage of photosynthesis, the movement of electrons along an electron-transport chain is used to generate a proton gradient that can be used by ATP synthase to produce ATP. The same mechanism occurs during oxidative phosphorylation.

What happens to the ATP produced during stage 1 of photosynthesis? A. It is consumed within the chloroplast to produce NADPH. B. It is exported from the chloroplast and used to produce sucrose. C. It is exported from the chloroplast to fuel the plant's metabolic needs. D. It is consumed within the chloroplast to produce glyceraldehyde 3-phosphate. E. It is consumed within the chloroplast to fuel electron transport.

D. It is consumed within the chloroplast to produce glyceraldehyde 3-phosphate. E: 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.

In the electron-transport chain in chloroplasts, which molecule serves as the final electron acceptor? A. O2 B. NAD+ C. H2O D. NADP E. +ADP

D. NADP E: This electron acceptor is related to the NAD+ that is required to accept electrons during glycolysis.

Investigators introduce two proteins into the membrane of artificial lipid vesicles: (1) an ATP synthase isolated from the mitochondria of cow heart muscle, and (2) a light-activated proton pump purified from the prokaryote Halobacterium halobium. The proteins are oriented as shown in the diagram. When ADP and Pi are added to the external medium and the vesicle is exposed to light, would this system produce ATP? A. No, because protons are small enough to pass freely in and out of an artificial lipid vesicle. B. No, because no electron-transport chain is present. C. No, because cows and prokaryotes are so distantly related that their proteins cannot be expected to work together. D. No, because ATP synthase is not oriented correctly. E. Yes, because the proton pump will generate a proton gradient that ATP synthase can use to synthesize ATP.

D. No, because ATP synthase is not oriented correctly. E: If the ATP synthase were oriented in the opposite direction, it could take advantage of the proton gradient produced by the pump to generate ATP outside the vesicle. These experiments, performed in the 1970s, demonstrated definitively that a proton gradient could drive the production of ATP.

Some types of bacteria can survive under both aerobic and anaerobic conditions. Regardless of whether oxygen is present, these cells maintain a proton gradient across the plasma membrane to drive ATP synthesis and the import of nutrients. Under aerobic conditions, an H+ gradient across the plasma membrane is produced by the transfer of electrons along the respiratory chain. When oxygen is present, what would be expected to occur in the plasma membrane of these bacteria? A. ATP synthase hydrolyzes ATP, pumping protons into the cell to help maintain the H+ gradient. B. Electrons flow into the cell through ATP synthase, generating ATP. C. Protons flow out of the cell through ATP synthase, generating ATP. D. Protons flow into the bacterial cell through ATP synthase, generating ATP. E. ATP synthase hydrolyzes ATP, pumping protons out of the cell to help maintain the H+ gradient.

D. Protons flow into the bacterial cell through ATP synthase, generating ATP E: In the presence of oxygen, the movement of electrons along the electron-transport chain in the bacterial plasma membrane will generate an H+ gradient. ATP synthase takes advantage of the flow of electrons down this electrochemical gradient to power its production of ATP. But to produce ATP inside the cell, which way would these protons need to flow? The portion of the enzyme complex that catalyzes the phosphorylation of ADP must be situated inside the cell. Protons therefore must flow into the cell, through the membrane-embedded H+ carrier, for ATP synthase to generate ATP for the bacteria to use for its various metabolic needs. ATP synthase is situated the same way in mitochondria, where ATP is generated in the mitochondrial matrix. In this case, protons flow across the inner mitochondrial membrane from the intermembrane space into the matrix. Specialized carrier proteins later export this ATP from the mitochondrial matrix so that it can be made available for use by the rest of the cell.

In photosynthesis, what drives the generation of ATP by ATP synthase? A. the absorption of light by a photosynthetic reaction center B. the phosphorylation of ATP synthase C. the transfer of high-energy electrons to ATP synthase D. the generation of a charge separation in the photosynthetic reaction center E. a proton gradient across the thylakoid membrane

E. a proton gradient across the thylakoid membrane E: The movement of electrons along the photosynthetic electron-transport chain pumps protons across the thylakoid membrane; this proton gradient drives the production of ATP by ATP synthase.

It is energetically favorable for protons to flow in which direction? A. toward the compartment with the most positive charge B. from the mitochondrial matrix to the intermembrane space C. toward the compartment with the lowest pH D. across the outer mitochondrial membrane E. from the intermembrane space to the mitochondrial matrix

E. from the intermembrane space to the mitochondrial matrix E: The movement of electrons along the electron-transport chain releases energy that is used to pump protons from the mitochondrial matrix to the intermembrane space. This action 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). The higher the pH, the lower the concentration of H+. Thus, protons would tend to flow toward the compartment with the higher pH.

What does Photosystem II do?

Photosystem II passes electrons to the mobile electron carrier plastoquinone, which transfers them to the only proton pump involved in the photosynthetic electron-transport chain.