CHAPTER 14: ENERGY GENERATION IN MITOCHONDRIA AND CHLOROPLASTS

The link between bond-forming reactions and membrane transport processes in the mitochondria is called __________________.

chemiosmotic coupling.

Describe how a standard flashlight battery can convert energy into useful work and explain how this is similar to the energy conversions in the mitochondria.

A battery contains chemicals that generate negatively charged ions at one pole, and it is able to cause the continuous transfer of electrons along a metal wire if that pole is connected to the other end of the battery. The energy released by the electron-transfer process driven by the battery can be harnessed to do useful work, as when it is used to run an electric motor. Likewise, the energy released by the electron transfers that occur between the protein complexes in the electron-transport chain does useful work when it drives the movement of protons to one side of the membrane, since the resulting proton gradient is then used to generate chemicalenergy in the form of ATP.

Based upon what you know about metabolism, explain how electrons are stripped from food molecules and used to drive the electron-transport chain.

Food molecules are ultimately converted into acetyl CoA,. Electrons removed during the

generation of acetyl CoA are added to the cofactors NAD+

and FAD to generate the

reduced cofactors NADH and FADH2, respectively. The two carbon atoms in the acetyl group of acetyl CoA are then fed into the citric acid cycle, where they are oxidized to two molecules of CO2. The electrons removed during this oxidation are also captured by the activated carriers NADH and FADH2. The high-energy electrons in all of these activated carriers, which derived from carbons that were formerly part of food molecules are now transferred to the proteins in the electron-transport chain.

Which of the following is not part of the process known as oxidative phosphorylation?

ATP molecules are produced in the cytosol as glucose is converted into pyruvate.

5 Which of the following statements describes the phosphorylation event that occurs during the process known as oxidative phosphorylation?

A phosphate group is added to ADP

Modern eukaryotes depend on mitochondria to generate most of the cell's ATP. How many molecules of ATP can a single molecule of glucose generate?

30

The citric acid cycle generates NADH and FADH2, which are then used in the process of oxidative phosphorylation to make ATP.

If the citric acid cycle (which does not use oxygen) and oxidative phosphorylation are separate processes, as they are, then why is it that the citric acid cycle stops almost immediately when O2 is removed?

The citric acid cycle stops almost immediately when oxygen is removed because several

steps in the cycle require the oxidized forms of NAD+ and FAD. In the absence of oxygen, these electron carriers can be reduced by the reactions of the citric acid cycle but cannot be reoxidized by the electron-transport chain that participates in oxidative phosphorylation.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

A. The number and location of mitochondria within a cell can change, depending on both the cell type and the amount of energy required.

TRUE

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

B. The inner mitochondrial membrane contains porins, which allow pyruvate to enter for use in the citric acid cycle.

FALSE. The outer mitochondrial membrane contains porins, allowing the passage

of all molecules with a mass of less than 5000 daltons. Although pyruvate must pass through the inner membrane, it does so in a highly regulated manner via specific transporter channels.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

C. The inner mitochondrial membrane is actually a series of discrete, flattened, membrane-enclosed compartments called cristae, similar to what is seen in the Golgi apparatus.

FALSE. Although the cristae do look like individual compartments on the basis of

the images of the inner structure of the mitochondria, the inner membrane is a

single, albeit highly convoluted, membrane.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

D. The intermembrane space of the mitochondria is chemically equivalent to the cytosol with respect to pH and the small molecules present.

TRUE

In which of the four compartments of a mitochondrion is each of the following located?

A. porin

Porin is in the outer membrane.

In which of the four compartments of a mitochondrion is each of the following located?

B. the mitochondrial genome

The mitochondrial genome is in the matrix.

In which of the four compartments of a mitochondrion is each of the following located?

C. citric acid cycle enzymes

The citric acid cycle enzymes are in the matrix

In which of the four compartments of a mitochondrion is each of the following located?

D. proteins of the electron-transport chain

The proteins of the electron-transport chain are in the inner membrane.

In which of the four compartments of a mitochondrion is each of the following located?

E. ATP synthase

ATP synthase is in the inner membrane.

In which of the four compartments of a mitochondrion is each of the following located?

F. membrane transport protein for pyruvate

The transport protein for pyruvate is in the inner membrane.

10 Which of the following statements about mitochondrial division is true?

(c) Mitochondrial division is mechanistically similar to prokaryotic cell division.

Which of the following statements describes the mitochondrial outer membrane?

(a) It is permeable to molecules with molecular mass as high as 5000 daltons.

Which of the following statements describes the mitochondrial inner membrane?

(b) It contains transporters for ATP molecules..

Which of the following statements describes the mitochondrial intermembrane space?

(c) It contains proteins that are released during apoptosis.

Which of the following statements describes the mitochondrial matrix?

(d) It contains enzymes required for the oxidation of fatty acids.

15 NADH contains a high-energy bond that, when cleaved, donates a pair of electrons to the electron-transport chain.

What are the immediate products of this bond cleavage?

(b) NAD + H

Mitochondria can use both __________________ and __________________ directly as fuel.

Pyruvate, fatty acids

__________________ produced in the citric acid cycle donates electrons to the electron-transport chain. The citric acid cycle oxidizes __________________ and produces __________________ as a waste product.

NADH, Acetyl groups, carbon dioxide

__________________ acts as the final electron acceptor in the electron-transport chain. The synthesis of ATP in mitochondria is also known as __________________.

Oxygen, oxidative phosphorylation

Electron transport is coupled to ATP synthesis in mitochondria, in chloroplasts, and in the thermophilic bacterium Methanococcus.

Which of the following is likely to affect the coupling of electron transport to ATP synthesis in all of these systems?

(d) an ADP analog that inhibits ATP synthase

Stage 1 of oxidative phosphorylation requires the movement of electrons along the electron-transport chain coupled to the pumping of protons into the intermembrane space.

What is the final result of these electron transfers?

(C) Contrary to what the term "oxidative phosphorylation" may imply, the phosphorylation event does not depend on an oxidative reaction, but rather on the reduction of molecular oxygen, converting it to water.

Osmosis describes the movement of water across a biological membrane and down its concentration gradient.

In chemiosmosis, useful energy is harnessed by the cell from the movement of _______________ across the inner mitochondrial membrane into the matrix _________________ a concentration gradient.

(b) protons, down

20 Which is NOT an act as the following components of the electron-transport chain do proton pump?

(b) cytochrome c

Which component of the electron-transport chain is required to combine the pair of electrons with molecular oxygen?

(d) cytochrome c oxidase

NADH donates electrons to the __________________ of the three respiratory enzyme complexes in the mitochondrial electron-transport chain.

First

__________________ is a small protein that acts as a mobile electron carrier in the respiratory chain.

Cytochrome c

__________________ transfers electrons to oxygen. Electron transfer in the chain occurs in a series of __________________ reactions.

Cytochrome c oxidase, oxidation-reduction

The first mobile electron carrier in the respiratory chain is __________________.

Ubiquinone

In oxidative phosphorylation, ATP production is coupled to the events in the electron-transport chain.

What is accomplished in the final electron-transfer event in the electron- transport chain?

(c) O2 is reduced to H2O

Which of the following statements is true?

(b) The pH in the mitochondrial matrix is higher than the pH in the intermembrane

space.

25 Some bacteria can live both aerobically and anaerobically.

How does the ATP synthase in the plasma membrane of the bacterium help such bacteria to keep functioning in the absence of oxygen?

In the absence of oxygen, the respiratory chain no longer pumps protons, and thus no proton electrochemical gradient is generated across the bacterial membrane. In these conditions, the ATP synthase uses some of the ATP generated by glycolysis in the cytosol to pump protons out of the bacterium, thus forming the proton gradient across the membrane that the bacterium requires for importing vital nutrients by coupled transport.

Which of the following types of ion movement might be expected to require co-transport of protons from the mitochondrial intermembrane space to the matrix, in as much as it could not be driven by the membrane potential across the inner membrane? (Assume that each ion being moved is moving against its concentration gradient.)

(b) import of acetate ions into the matrix from the intermembrane space

The mitochondrial ATP synthase consists of several different protein subunits.

Which subunit binds to ADP + Pi and catalyzes the synthesis of ATP as a result of a conformational change?

(b) F1 ATPase head

Explain how the F0 complex of ATP synthase harnesses the proton-motive force to help synthesize ATP.

What would happen if the proton gradient were reversed?

In the absence of oxygen, the respiratory chain no longer pumps protons, and thus no proton electrochemical gradient is generated across the bacterial membrane. In these conditions, the ATP synthase uses some of the ATP generated by glycolysis in the cytosol to pump protons out of the bacterium, thus forming the proton gradient across the membrane that the bacterium requires for importing vital nutrients by coupled transport.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

A. The driving force that pulls protons into the matrix is called the proton-motive force, which is a combination of the large force due to the pH gradient and the smaller force that results from the voltage gradient across the inner mitochondrial membrane.

FALSE. Although it is true that both the pH gradient and the voltage gradient are components of the proton-motive force, it is the voltage gradient (also referred to as the membrane potential) that is the greater of the two.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

B. Under anaerobic conditions, the ATP synthase can hydrolyze ATP instead of synthesizing it.

TRUE

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

C. ATP is moved out of the matrix, across the inner mitochondrial membrane, in a co-transporter that also brings ADP into the matrix.

TRUE

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

D. Brown fat cells make less ATP because they have an inefficient ATP synthase.

FALSE. The inner mitochondrial membranes in brown cells contain protein that allows protons to move down their gradient without passing through the ATP synthase. As a result, less ATP is made and most of the energy from the proton gradient is released as heat.

30 Bongkrekic acid is an antibiotic that inhibits the ATP/ADP transport protein in the inner mitochondrial membrane.

Which of the following will allow electron transport to occur in mitochondria treated with bongkrekic acid?

(c) making the inner membrane permeable to protons

The relationship of free-energy change (ΔG) to the concentrations of reactants and products is important because it predicts the direction of spontaneous chemical reactions. In the hydrolysis of ATP to ADP and inorganic phosphate (Pi), the standard free-energy change (ΔG°) is -7.3 kcal/mole. The free-energy change depends on concentrations according to the following equation:

ΔG = ΔG° + 1.42 log10 ([ADP] [Pi]/[ATP])

In a resting muscle, the concentrations of ATP, ADP, and Pi are approximately 0.005 M, 0.001 M, and 0.010 M, respectively.

What is the ΔG for ATP hydrolysis in resting muscle?

(a) -11.1 kcal/mole

The relationship of free-energy change (ΔG) to the concentrations of reactants and products is important because it predicts the direction of spontaneous chemical reactions. In the hydrolysis of ATP to ADP and inorganic phosphate (Pi), the standard free-energy change (ΔG°) is -7.3 kcal/mole. The free-energy change depends on concentrations according to the following equation:

ΔG = ΔG° + 1.42 log10 ([ADP] [Pi]/[ATP])

In a resting muscle, the concentrations of ATP, ADP, and Pi are approximately 0.005 M, 0.001 M, and 0.010 M, respectively.

What is the ΔG for ATP synthesis in resting muscle?

(d) 11.1 kcal/mole

The relationship of free-energy change (ΔG) to the concentrations of reactants and products is important because it predicts the direction of spontaneous chemical reactions. Consider, for example, the hydrolysis of ATP to ADP and inorganic phosphate (Pi). The standard free-energy change (ΔG°) for this reaction is -7.3 kcal/mole. The free-energy change depends on concentrations according to the following equation:

ΔG = ΔG° + 1.42 log10 ([ADP] [Pi]/[ATP])

In a resting muscle, the concentrations of ATP, ADP, and Pi are approximately 0.005 M, 0.001 M, and 0.010 M, respectively.

At [Pi] = 0.010 M, what will be the ratio of [ATP] to [ADP] at

equilibrium?

(c) 7.2 × 10-8

NADH and FADH2 carry high-energy electrons that are used to power the production of ATP in the mitochondria. These cofactors are generated during glycolysis, the citric acid cycle, and the fatty acid oxidation cycle.

Which molecule below can produce the most ATP? Explain your answer.

(c) NADH from the citric acid cycle

35 Human infants have a much larger portion of brown adipose tissue than adult humans. It was found that the mitochondria in brown adipocytes (brown fat cells) have a novel protein in the inner mitochondrial membrane. This protein, called the uncoupling protein (UCP), was found to transport protons from the intermembrane space into the matrix.

A. What is the impact of UCP on oxidative phosphorylation in the mitochondria of brown fat?

A protein that transports protons into the mitochondrial matrix would diminish the proton gradient. Without the proton gradient, ATP will not be generated. However, the electron-transport chain can still work, as long as oxygen is present. The UCP, therefore, is a biological uncoupler of the oxidative phosphorylation process. The electron-transport chain will run in a futile cycle that does not convert the energy from redox reactions into chemical energy (ATP), but instead releases this energy as heat.

Human infants have a much larger portion of brown adipose tissue than adult humans. It was found that the mitochondria in brown adipocytes (brown fat cells) have a novel protein in the inner mitochondrial membrane. This protein, called the uncoupling protein (UCP), was found to transport protons from the intermembrane space into the matrix.

B. Propose an explanation for the higher proportion of brown fat cells in infants compared to adults.

The thermogenesis resulting from the action of UCP is important for helping infants maintain a constant body temperature. As our body mass increases with to and age, our body temperatures are probably less susceptible fluctuations, therefore adults do not require the same amount of brown fat as infants.

Experimental evidence supporting the chemiosmotic hypothesis was gathered by using artificial vesicles containing a protein that can pump protons in one direction across the vesicle membrane to create a proton gradient.

Which protein was used to generate the gradient in a highly controlled manner?

(d) bacteriorhodopsin

Explain how scientists used artificial vesicles to prove that the generation of ATP by the ATP synthase was not powered by a single high-energy intermediate but rather by a proton gradient. Be sure to describe the two experiments that were negative controls (no ATP generated), the positive control (ATP generated as expected), and a fourth experiment proving that the gradient is the required energy source.

In all the experiments, artificial liposomes were generated and exposed to light, and the surrounding solution was checked for an increase in ATP. In the first experiment, the liposomal membranes contained only bacteriorhodopsin, a bacterial protein that pumps protons and is activated by light. In this negative control, ATP was not expected to be produced, and it was not. In the second experiment, also a negative control, the liposomes contained only ATP synthase. Again, if the chemiosmotic hypothesis was correct, ATP should not have been generated, which was what was observed. In the third experiment, both bacteriorhodopsin and ATP synthase were present in the liposomal membrane. When exposed to light, protons were pumped into the vesicle and ATP was generated. In the fourth experiment, to show that the ATP production was solely a result of the proton gradient, an uncoupling agent was added to the solution containing liposomes with bacteriorhodopsin and ATP synthase. In this case, even though the protons were being pumped into the liposomes, a gradient did not build up; this was because of the presence of the uncoupling agent, which made the membrane permeable to protons. No ATP was generated, proving that it was the proton gradient that was the energy source for ATP synthesis.

The respiratory chain is relatively inaccessible in the experimental manipulation of intact mitochondria. After disrupting mitochondria with ultrasound, however, it is possible to isolate functional submitochondrial particles, which consist of broken cristae that have resealed inside-out into small, closed vesicles. In these vesicles, the components that originally faced the matrix are now exposed to the surrounding medium.

A. How might such an arrangement aid in the study of electron transport synthesis?

This arrangement of components within the vesicles allows the experimental manipulation of the medium surrounding the vesicles, which permits the consequences of different conditions in the mitochondrial matrix to be examined. The medium can be altered by changing pH, adding electron carriers and oxygen, and providing ADP and Pi, for example. The oxidation of electron carriers, the consumption of oxygen, and the production of ATP can be measured in the medium. By changing the composition of the medium, it should be possible, for example, to identify the electron carriers that can donate electrons from the matrix to the transport chain (the side of the membrane that normally faces the matrix is now on the outside), to assess the redox potentials of various components of the transport chain, and to determine the dependence of ATP synthesis on the pH gradient across the membrane and on the ATP/ADP ratio.

The respiratory chain is relatively inaccessible in the experimental manipulation of intact mitochondria. After disrupting mitochondria with ultrasound, however, it is possible to isolate functional submitochondrial particles, which consist of broken cristae that have resealed inside-out into small, closed vesicles. In these vesicles, the components that originally faced the matrix are now exposed to the surrounding medium.

B. Consider an anaerobic preparation of such submitochondrial particles. If a small amount of oxygen is added, do you predict that the preparation will consume oxygen in respiration reactions? Will the medium outside the particles become more acidic or more basic? What, if anything, will change if the flow of protons through ATP synthase is blocked by an inhibitor? Explain your answer.

Respiration reactions will rapidly consume at least some of the added oxygen. During the anaerobic conditions, the electron carriers in the electron-transport chain were reduced; on the addition of oxygen, electrons will be transferred to oxygen, thereby reducing the oxygen and oxidizing the carriers. Concomitantly with the electron flow, protons will be pumped from the medium into the vesicles,

thereby making the medium slightly more basic and the inside of the vesicles acidic. Inhibition of the ATP synthase will not have an immediate effect on oxygen consumption or proton pumping. However, the proton concentration the of the inside the vesicles will quickly become too high to continue activity electron-transport-coupled proton pumping, and thus electron transport and oxygen consumption will cease.

Mitochondrial structure and the reaction products generated inside the matrix are critical for generating stores of energy. Answer the following questions based on what you know about mitochondrial structure and processes.

A. The gradients used to generate ATP are maintained across the inner mitochondrial membrane. Why don't we observe a similar gradient generation across the outer mitochondrial membrane?

The outer mitochondrial membrane contains large, channel-forming proteins called porins, which makes this membrane permeable to small molecules. Protons, other ions, nucleotides, and many other small molecules flow freely across this membrane, making it impossible to establish a gradient of any of these

molecules on either side of this membrane.

Mitochondrial structure and the reaction products generated inside the matrix are critical for generating stores of energy. Answer the following questions based on what you know about mitochondrial structure and processes.

B. The proton-motive force created by the electrochemical proton gradient is the source of free energy utilized in ATP formation. Describe the two components contributing to the total proton-motive force.

One component of the proton-motive force is the concentration gradient of

protons (or pH gradient) across the membrane. This pH gradient makes it energetically favorable for protons to flow back into the matrix. A second component of the proton-motive force is a charge differential across the membrane, referred to as the membrane potential. Because the matrix side of the

membrane has a net negative charge, and protons have a positive charge, this component of the proton-motive force also drives the movement of protons back into the matrix.

40 Which of the following statements about "redox potential" is true?

(b) For molecules that have a strong tendency to pass along their electrons, the standard redox potential is negative.

Which of the following statements is true?

(c) The ΔE0ʹ of a redox pair does not depend on the concentration of each member of the pair.

B. How do these standard redox potentials support our understanding of the stepwise electron transfers that occur in the electron-transport chain?

Each successive member of the electron-transport chain is a better electron acceptor, which permits a unidirectional series of electron transfers until reaching O2, which is the best electron acceptor and the final destination of the electrons, forming water as oxygen is consumed.

C. Why would it not be advantageous for living systems to evolve a mechanism for the direct transfer of electrons from NADH to O2?

If NADH directly donated electrons to O2, a large amount of energy would be released as heat and lost, rather than used as a way for the cell to generate chemical energy in the form of ATP.

Which ratio of NADH to NAD+ in solution will generate the largest positive redox potential?

1:10

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

A. Ubiquinone is associated with the inner mitochondrial membrane as a protein-bound electron carrier molecule.

False. Ubiquinone is an aromatic compound that uses its hydrocarbon associate with the inner mitochondrial membrane.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

B. Ubiquinone can transfer only one electron in each cycle.

False. Ubiquinone can transfer one or two electrons. In the case in which only one electron is transferred, the molecule contains an unpaired electron, which is highly reactive.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

C. The iron-sulfur centers in NADH dehydrogenase are relatively poor electron acceptors

TRUE

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

D. Cytochrome c oxidase binds O2 using an iron-heme group, where four electrons are shuttled one at a time.

TRUE

45 Ubiquinone is one of two mobile electron carriers in the electron-transport chain.

Where does the additional pair of electrons reside in the reduced ubiquinone molecule?

(b) The electrons are added to each of two ketone oxygens on the aromatic ring.

Electron-transfer reactions occur rapidly.

Which of the following statements best describes how the diffusion of ubiquinone is controlled in order to ensure its proximity to the other enzyme complexes?

(b) Ubiquinone is present at high concentrations, minimizing the impact of diffusion

on the electron-transport chain.

Which of the following reactions has a sufficiently large free-energy change to enable it to be one of ATP used, in principle, to provide the energy needed to synthesize a molecule from ADP and Pi under standard conditions? Recall that:

ΔG° = -n (0.023) ΔE0ʹ, andΔE0ʹ = E0ʹ (acceptor) - E0ʹ (donor).

(a) the reduction of a molecule of pyruvate by NADH

Cytochrome c oxidase is an enzyme complex that uses metal ions to help coordinate the transfer of four electrons to O2.

Which metal atoms are found in the active site of this complex?

(b) one iron atom and one copper atom

Consider a redox reaction between molecules A and B. Molecule A has a redox potential of -100 mV and molecule B has a redox potential of +100 mV. For the transfer of electrons from A to B, is the ΔG° positive or negative or zero? Under what conditions will the reverse reaction, transfer of electrons from B to A, occur?

The ΔG° is negative. The sign of ΔG° is the opposite of that of ΔE0ʹ = E0ʹ (acceptor) - E0ʹ (donor). The acceptance of electrons by B from A has a ΔE0ʹ = 100 - (-100) = 200. The reverse reaction, the donation of electrons from B to A, has a positive ΔG° and is therefore unfavorable under standard conditions. Remember that, by definition, the concentrations of A and its redox pair Aʹ are equal under standard conditions; similarly, the concentration of B is equal to the concentration of its redox pair Bʹ. B will be able to donate electrons to A only when [B] > [Bʹ] and/or [A] < [Aʹ] to such an extent that the ΔG for electron transfer becomes negative.

50 An electron bound to a molecule with low affinity for electrons is a [high/low]-energy electron.

high

Transfer of an electron from a molecule with low affinity to one with higher affinity has a [positive/negative] ΔG° and is thus [favorable/unfavorable] under standard conditions.

negative; favorable

If the reduced form of a redox pair is a strong electron donor with a [high/low] affinity for electrons, it is easily oxidized; the oxidized member of such a redox pair is a [weak/strong] electron acceptor.

low; weak

Which of the following statements is true?

(d) Mitochondrial electron carriers with the highest redox potential generally contain copper ions and/or heme groups.

Which of the following is not an electron carrier that participates in the electron-transport chain?

(c) rhodopsin

Which of the following statements about cytochrome c is true?

(c) The pair of electrons accepted by cytochrome care added to the porphyrin ring of the bound heme group.

Photosynthesis is a process that takes place in chloroplasts and uses light energy to generate high-energy electrons, which are passed along an electron-transport chain.

Where are the proteins of the electron-transport chain located in chloroplasts?

(d) thylakoid membrane

55 In stage 1 of photosynthesis, a proton gradient is generated and ATP is synthesized.

Where do protons become concentrated in the chloroplast?

(a) thylakoid space

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

A. Carbon fixation can be described as a process by which gaseous carbon-containing molecules are captured and incorporated into biological hydrocarbon

molecules.

TRUE

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

B. The electron-transport proteins, utilized in stage 1 of photosynthesis, reside in the inner membrane of the chloroplast.

FALSE. The electron-transport system in chloroplasts resides in the thylakoid

membrane.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

C. Similar to oxidative phosphorylation, the electrons passed along the chloroplast electron-transport chain are ultimately passed on to a molecule of O2, to produceH2O.

FALSE. The recipient of electrons in the chloroplast electron-transport chain is the NADP+cofactor, which becomes reduced to NADPH.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

D. Stage 2 of photosynthesis involves a cycle of reactions that do not directly depend on energy derived from sunlight.

TRUE

The ATP synthase found in chloroplasts is structurally similar to the ATP synthase in mitochondria.

Given that ATP is being synthesized in the stroma, where will the F0 portion of the ATP synthase be located?

(d) thylakoid membrane

Stage 2 of photosynthesis, sometimes referred to as the dark reactions, involves the reduction of CO2 to produce organic compounds such as sucrose.

What cofactor is the electron donor for carbon fixation?

(d) NADPH

In the electron-transport chain in chloroplasts, ________-energy electrons are taken from __________.

(b) low; H2O.

60 The photosystems in chloroplasts contain hundreds of chlorophyll molecules, most of which are part of _______________.

(b) the antenna complex.

Photons from sunlight that are in the ______________ wavelength range are preferentially absorbed by chlorophyll molecules to raise the energy levels of electrons in the __________ ring.

red; porphyrin

The __________ reflected are lower in energy, which is indicated in the ________, green wavelengths detected by the human eye.

photons; longer

If you shine light on chloroplasts and measure the rate of photosynthesis as a function of light intensity, you get a curve that reaches a plateau at a fixed rate of photosynthesis, x, as shown in Figure Q14-62.

Which of the following conditions will increase the value of x?

(b) increasing the number of reaction centers

If you add a compound to illuminated chloroplasts that inhibit the NADP+reductase, NADPH generation ceases, as expected. However, ferredoxin does not accumulate in the reduced form because it is

able to donate its electrons not only to NADP++(via NADP reductase) but also back to the cytochrome b6-f complex. Thus, in the presence of the compound, a "cyclic" form of photosynthesis occurs in which electrons flow in a circle from ferredoxin, to the cytochrome b6-f complex, to plastocyanin, to photosystem I, to ferredoxin.

What will happen if you now also inhibit photosystem II?

(c) Plastoquinone will accumulate in the oxidized form.

The enzyme ribulose bisphosphate carboxylase (Rubisco) normally adds carbon dioxide to ribulose 1,5-bisphosphate. However, it will also catalyze a competing reaction in which O2 is added to ribulose 1,5-bisphosphate to form 3-phosphoglycerate and phosphoglycolate. Assume that phosphoglycolate is a compound that cannot be used in any further reactions.

If O2 and CO2 have the same affinity for Rubisco, which of the following is the lowest ratio of CO2 to O2 at which a net synthesis of sugar can occur?

(c)3:1

65 Which of the following statements is not true about the possible fates of glyceraldehyde 3-phosphate?

(d) It can be transported into the thylakoid space for use as a secondary electron acceptor downstream of the electron-transport chain.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

A. The dark reactions of photosynthesis occur only in the absence of light.

FALSE. The dark reactions are those involved in carbon fixation and are named as such because they do not require light

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

B. Much of the glyceraldehyde 3-phosphate made in the chloroplast ends up producing the molecules needed by the mitochondria to produce ATP.

TRUE

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

C. Ribulose 1,5-bisphosphate is similar to oxaloacetate in the Krebs cycle in that it is both regenerated at the end of their respective cycles.

TRUE

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

D. Each round of the Calvin cycle uses five molecules of CO2 to produce one molecule of glyceraldehyde 3-phosphate and one of pyruvate.

FALSE. Three molecules of CO2 are required for each round of the Calvin cycle, and the product is one molecule of glyceraldehyde 3-phosphate and the recyclingof the ribulose 1,5-bisphosphate molecule.

In 1925, David Keilin used a simple spectroscope to observe the characteristic absorption bands of the cytochromes that participate in the electron-transport chain in mitochondria. A spectroscope passes a very bright light through the sample of interest and then through a prism to display the spectrum from red to blue. If molecules in the sample absorb light of particular wavelengths, dark bands will interrupt the colors of the rainbow. His key discovery was that the absorption bands disappeared when oxygen was introduced and then reappeared when the samples became anoxic. Subsequent findings demonstrated that different cytochromes absorb light of different frequencies. When light of a characteristic wavelength shines on a mitochondrial sample, the amount of light absorbed isproportional to the amount of a particular cytochrome present in its reduced form. Thus, spectrophotometric methods can be used to measure how t

This result allows you to order the electron carriers in the respiratory chain because when oxygen is added, the last carrier in the chain will be oxidized first. This is because oxygen is the final sink for the electrons that flow through the chain, and it participates directly in a redox reaction with the last electron carrier. The wave of oxidation will then proceed backward through the chain toward the first electron carrier in the chain; this is because the oxidation of each carrier will convert it to a form that can accept electrons from the "upstream" carrier in the chain, thereby oxidizing each upstream carrier sequentially. The order of cytochromes in the respiratory chain is the reverse of the order in which they are oxidized (that is, the order in which the reduced form is lost).

Listed from first to last, the cytochromes in the chain are b, c1, c, and (a1 + a3).

In the carbon-fixation process in chloroplasts, carbon dioxide is initially added to the sugar __________________.

ribulose 1,5-bisphosphate.