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How canthis agent, in principle, serve as a reducing aid?Such uncoupling agents are no longer prescribedbecause some deaths occurred following their use.Why can the ingestion of uncouplers lead to death?8. Mode of Action of Dicyclohexylcarbodiimide(DCCD) When DCCD is added to a suspension oftightly coupled, actively respiring mitochondria,the rate of electron transfer (measured by O2 consumption) and the rate of ATP production dramatically decrease. If a solution of 2,4-dinitrophenol is596Part III Bioenergetics and Metabolismnow added to the inhibited mitochondrial preparation, O2 consumption returns to normal but ATPproduction remains inhibited.(a) What process in electron transfer or oxidative phosphorylation is affected by DCCD?(b) Why does DCCD affect the O2 consumptionof mitochondria? Explain the effect of 2,4-dinitrophenol on the inhibited mitochondrial preparation.(c) Which of the following inhibitors does DCCDmost resemble in its action: antimycin A, rotenone,or oligomycin?9.
The Malate-a-Ketoglutarate Transport Systemof Mitochondria The inner mitochondrial membrane transport system that promotes the transport of malate and a-ketoglutarate across themembrane (Fig. 18-25) is inhibited by rc-butylmalonate. Suppose n-butylmalonate is added to anaerobic suspension of kidney cells using glucoseexclusively as fuel. Predict the effect of this inhibitor on(a) Glycolysis(b) Oxygen consumption(c) Lactate formation(d) ATP synthesis10. The Pasteur Effect When O2 is added to ananaerobic suspension of cells using glucose at ahigh rate, the rate of glucose consumption declinesdramatically as the added O2 is consumed. In addition, the accumulation of lactate ceases.
This effect, first observed by Louis Pasteur in the 1860s, ischaracteristic of most cells capable of both aerobicand anaerobic utilization of glucose.(a) Why does the accumulation of lactate ceaseafter O2 is added?(b) Why does the presence of O2 decrease therate of glucose consumption?(c) How does the onset of O2 consumption slowdown the rate of glucose consumption? Explain interms of specific enzymes.11. How Many Protons in a Mitochondrion? Electron transfer functions to translocate protons fromthe mitochondrial matrix to the external mediumto establish a pH gradient across the inner membrane, the outside more acidic than the inside.
Thetendency of protons to diffuse from the outside intothe matrix, where [H+] is lower, is the driving forcefor ATP synthesis via the ATP synthase. Duringoxidative phosphorylation by a suspension of mitochondria in a medium of pH 7.4, the internal pH ofthe matrix has been measured as 7.7.(a) Calculate [H+] in the external medium andin the matrix under these conditions.(b) What is the outside: inside ratio of [H+]?Comment on the energy inherent in this concentration. (Hint: See p. 383, Eqn 13-5.)(c) Calculate the number of protons in a respiring liver mitochondrion, assuming its inner matrixcompartment is a sphere of diameter 1.5 /xm.(d) From these data would you think the pHgradient alone is sufficiently great to generateATP?(e) If not, can you suggest how the necessaryenergy for synthesis of ATP arises?12.
Rate of ATP Turnover in Rat Heart Muscle Ratheart muscle operating aerobically fills more than90% of its ATP needs by oxidative phosphorylation.This tissue consumes O2 at the rate of 10 fjunoVmin • g of tissue, with glucose as the fuel source.(a) Calculate the rate at which this tissue consumes glucose and produces ATP.(b) If the steady-state concentration of ATP inrat heart muscle is 5 fimol/g of tissue, calculate thetime required (in seconds) to completely turn overthe cellular pool of ATP.
What does this result indicate about the need for tight regulation of ATP production? (Note: Concentrations are expressed asmicromoles per gram of muscle tissue because thetissue is mostly water.)13. Rate of ATP Breakdown in Flight Muscle ATPproduction in the flight muscles of the fly Luciliasericata results almost exclusively from oxidativephosphorylation. During flight, 187 ml of O 2 /h»gof fly body weight is needed to maintain an ATPconcentration of 7 fimol/g of flight muscle. Assuming that the flight muscles represent 20% of theweight of the fly, calculate the rate at which theflight-muscle ATP pool turns over.
How long wouldthe reservoir of ATP last in the absence of oxidativephosphorylation? Assume that reducing equivalents are transferred by the glycerol-3-phosphateshuttle and that O2 is at 25 °C and 101.3 kPa(1 atm). (Note: Concentrations are expressed inmicromoles per gram of flight muscle.)14.
Transmembrane Movement of Reducing Equivalents Under aerobic conditions, extramitochondrial NADH must be oxidized by the mitochondrialelectron transfer chain. Consider a preparation ofrat hepatocytes containing mitochondria and allthe enzymes of the cytosol. If [4-3H]NADH is introduced, radioactivity appears quickly in the mitochondrial matrix. However, if [7-14C]NADH isintroduced, no radioactivity appears in the matrix.What do these observations tell us about the oxidation of extramitochondrial NADH by the electrontransfer chain?3TT3H HNHo[4- 3 H]NADHChapter 18 Oxidative Phosphorylation and Photophosphorylation15. Photochemical Efficiency of Light at DifferentWavelengths The rate of photosynthesis, measured by O2 production, is higher when a greenplant is illuminated with light of wavelength680 nm than with light of 700 nm. However, illumination by a combination of light of 680 nm and700 nm gives a higher rate of photosynthesis thanlight of either wavelength alone.
Explain.16. Role of H2S in Some Photosynthetic BacteriaIlluminated purple sulfur bacteria carry out photosynthesis in the presence of H2O and 14CO2, butonly if H2S is added and O2 is absent. During thecourse of photosynthesis, measured by formation of[14C]carbohydrate, H2S is converted into elementalsulfur, but no O2 is evolved. What is the role of theconversion of H2S into sulfur? Why is no O2evolved?17.
Boosting the Reducing Power of Photosystem Iby Light Absorption When photosystem I absorbsred light at 700 nm, the standard reduction potential of P700 changes from 0.4 to about -1.2 V.What fraction of the absorbed light is trapped inthe form of reducing power?18. Mode of Action of the Herbicide DCMU Whenchloroplasts are treated with 3-(3,4-dichlorophenyl)-l,l-dimethylurea (DCMU, or Diuron), apotent herbicide, O2 evolution and photophosphorylation cease. Oxygen evolution but not photophosphorylation can be restored by the addition ofan external electron acceptor, or Hill reagent.
Howdoes this herbicide act as a weed killer? Suggest alocation for the inhibitory site of this herbicide inthe scheme shown in Figure 18-44. Explain.19. Bioenergetics of PhotophosphorylationThesteady-state concentrations of ATP, ADP, and Pi inisolated spinach chloroplasts under full illumination at pH 7.0 are 120, 6, and 700 /JLM, respectively.(a) What is the free-energy requirement for thesynthesis of 1 mol of ATP under these conditions?(b) The energy for ATP synthesis is furnished bylight-induced electron transfer in the chloroplasts.What is the minimum voltage drop necessary during the transfer of a pair of electrons to synthesizeATP under these conditions? (You may need torefer to p.
389, Eqn 13-8.)20. Equilibrium Constant for Water-Splitting Reactions The coenzyme NADP+ is the terminalelectron acceptor in chloroplasts, according to thereaction2H2O + 2NADP+> 2NADPH + 2H+ + O2Use the information in Table 18-2 to calculate the597equilibrium constant at 25 °C for this reaction.(The relationship between K'eq and AG°' is discussed on p. 368.) How can the chloroplast overcome this unfavorable equilibrium?21.
Energetics of Phototransduction During photosynthesis, eight photons of light must be absorbed (four by each photosystem) for every O2molecule produced:2H2O + 2NADP+ + 8 photons>2NADPH + 2H + + O2Assuming t h a t these photons have a wavelength of700 nm (red) and t h a t the absorption and utilization of light energy are 100% efficient, calculate thefree-energy change for the process.22.
Electron Transfer to a Hill Reagent Isolatedspinach chloroplasts evolve O 2 when illuminatedin the presence of potassium ferricyanide (the Hillreagent), according to the equation2H2O + 4Fe 3+O24Fe2+where Fe 3+ represents ferricyanide and Fe 2+ , ferrocyanide. Is NADPH produced in this process?Explain.23. How Often Does a Chlorophyll Molecule Absorba Photon ? The amount of chlorophyll a (Mr 892) ina spinach leaf is about 20 ^g/cm2 of leaf. In noonday sunlight (average energy 5.4 J/cm2 • min), theleaf absorbs about 50% of the radiation. How oftendoes a single chlorophyll molecule absorb a photon? If the average lifetime of an excited chlorophyll molecule in vivo is 1 ns, what fraction of chlorophyll molecules are excited at any one time?24.
Effect of Monochromatic Light on ElectronFlow The extent to which an electron carrier isoxidized or reduced during photosynthetic electrontransfer can sometimes be observed directly with aspectrophotometer. When chloroplasts are illuminated with 700 nm light, cytochrome f plastocyanin, and plastoquinone are oxidized. Whenchloroplasts are illuminated with 680 nm light,however, these electron carriers are reduced.
Explain.25. Function of Cyclic PhotophosphorylationWhen the [NADPH]/[NADP+] ratio in chloroplastsis high, photophosphorylation is predominantlycyclic (Fig. 18-44). Is O2 evolved during cyclic photophosphorylation? Explain. Can the chloroplastproduce NADPH this way? What is the main function of cyclic photophosphorylation?.
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