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This food is metabolized and the free energyused to synthesize ATP, which is then utilized to dothe body's daily chemical and mechanical work.Assuming that the efficiency of converting foodenergy into ATP is 50%, calculate the weight ofATP utilized by a human adult in a 24 h period.What percentage of the body weight does this represent?(c) Although adults synthesize large amounts ofATP daily, their body weight, structure, and composition do not change significantly during thisperiod.
Explain this apparent contradiction.15. ATP Reserve in Muscle Tissue The ATP concentration in muscle tissue (approximately 70%water) is about 8.0 mM. During strenuous activityeach gram of muscle tissue uses ATP at the rate of300 ^mol/min for contraction.(a) How long would the reserve of ATP last during a 100 meter dash?(b) The phosphocreatine level in muscle is about40.0 mM. How does this help extend the reserve ofmuscle ATP?(c) Given the size of the reserve ATP pool, howcan a person run a marathon?16. Rates of Turnover of y- and ^-Phosphates ofATP If a small amount of ATP labeled with radioactive phosphorus in the terminal position,[y-32P]ATP, is added to a yeast extract, about halfof the 32 P activity is found in Pj within a few minutes, but the concentration of ATP remains unchanged. Explain.
If the same experiment is carried out using ATP labeled with 32 P in the centralposition, [/3-32P]ATP, the 32 P does not appear in Ptwithin the same number of minutes. Why?17. Cleavage of ATP to AMP and PPt during Metabolism The synthesis of the activated form of acetate (acetyl-CoA) is carried out in an ATP-dependent process:Acetate + CoA + ATP>acetyl-CoA + AMP + PP;O/(a) The AG for the hydrolysis of acetyl-CoA toacetate and CoA is -32.2 kJ/mol and that for hydrolysis of ATP to AMP and PP4 is -30.5 kJ/mol.Calculate AG°' for the ATP-dependent synthesis ofacetyl-CoA.(b) Almost all cells contain the enzyme inorganic pyrophosphatase, which catalyzes the hydrolysis of PPj to Pj.
What effect does the presence ofthis enzyme have on the synthesis of acetyl-CoA?Explain.18. Are All Metabolic Reactions at Equilibrium?(a) Phosphoenolpyruvate is one of the two phosphate donors in the synthesis of ATP during glycolysis. In human erythrocytes, the steady-state concentration of ATP is 2.24 mM, that of ADP is0.25 mM, and that of pyruvate is 0.051 mM. Calculate the concentration of phosphoenolpyruvate at25 °C, assuming that the pyruvate kinase reaction(Fig. 13-3) is at equilibrium in the cell.(b) The physiological concentration of phosphoenolpyruvate in human erythrocytes is 0.023 mM.Compare this with the value obtained in (a). Whatis the significance of this difference? Explain.19.
Standard Reduction Potentials The standardreduction potential, Eb, of any redox pair is definedfor the half-cell reaction:Oxidizing agent + n electronsvreducing agentThe E'Q values for the NAD "/NADH and pyruvate/lactate conjugate redox pairs are -0.32 and-0.19 V, respectively.(a) Which conjugate pair has the greater tendency to lose electrons? Explain.(b) Which is the stronger oxidizing agent? Explain.(c) If we begin with 1 M concentrations of eachreactant and product at pH 7, in which directionwill the following reaction proceed?Pyruvate + NADH +=± lactate + NAD~(d) What is the standard free-energy change(AG°') at 25 °C for this reaction?(e) What is the equilibrium constant (K'eq) forthis reaction?20. Energy Span of the Respiratory Chain Electron transfer in the mitochondrial respiratorychain may be represented by the net reaction equationNADHH2O + NAD*Chapter 13 Principles of Bioenergetics(a) Calculate the value of AEQ for the net reaction of mitochondrial electron transfer.(b) Calculate AG°' for this reaction.(c) How many ATP molecules can theoreticallybe generated by this reaction if the standard freeenergy of ATP synthesis is 30.5 kJ/mol?21.
Dependence of Electromotive Force on Concentrations Calculate the electromotive force (involts) registered by an electrode immersed in a solution containing the following mixtures of NAD+and NADH at pH 7.0 and 25 °C, with reference to ahalf-cell of £6 = 0.00 V.(a) 1.0 mM NAD+ and 10 mM NADH(b) 1.0 mM NAD+ and 1.0 mM NADH(c) 10 mM NAD" and 1.0 mM NADH22. Electron Affinity of Compounds List the following substances in order of increasing tendencyto accept electrons: (a) a-ketoglutarate + CO2399(yielding isocitrate), (b) oxaloacetate, (c) O2,(d) NADP+.23. Direction of Oxidation-Reduction ReactionsWhich of the following reactions would be expectedto proceed in the direction shown under standardconditions, assuming that the appropriate enzymes are present to catalyze them?(a) Malate + NAD+>oxaloacetate + NADH + H~(b) Acetoacetate + NADH + H+>/3-hydroxybutyrate + NADr(c) Pyruvate + NADH + H">lactate + NAD+(d) Pyruvate + /3-hydroxybutyrate>lactate + acetoacetate(e) Malate + pyruvate>oxaloacetate + lactate(f) Acetaldehyde + succinate>ethanol + fumarateChapter 18 Oxidative Phosphorylation and Photophosphorylationlyze ATP synthesis from ADP and Pi as protons flow back into the cellthrough proton channels formed by F o .Certain bacterial transport systems bring about uptake of extracellular nutrients (lactose, for example) against a concentration gradient, in symport with protons (see Fig.
10-25). The respiration-linkedtransmembrane proton extrusion provides the driving force for thisuptake. The rotary motion of bacterial flagella, which move cellsthrough their surroundings, is provided by "proton turbines," molecular rotary motors driven not by ATP but directly by the transmembrane electrochemical potential generated by respiration-linked protonpumping (Fig. 18-31). It appears likely that the chemiosmotic mechanism evolved early (before the emergence of eukaryotes).
The protonmotive force can clearly be used to power processes other than ATPsynthesis.Photosynthesis: Harvesting Light EnergyWe now turn to another reaction sequence in which the flow of electrons is coupled to the synthesis of ATP: light-driven phosphorylation.The capture of solar energy by photosynthetic organisms and its conversion into the chemical energy of reduced organic compounds is theultimate source of nearly all biological energy. Photosynthetic and heterotrophic organisms live in a balanced steady state in the biosphere(Fig. 18-32).
Photosynthetic organisms trap solar energy and formATP and NADPH, which they use as energy sources to make carbohydrates and other organic components from CO2 and H2O; simultaneously, they release O2 into the atmosphere. Aerobic heterotrophs (wehumans, for example) use the O2 so formed to degrade the energy-richorganic products of photosynthesis to CO2 and H2O, generating ATPfor their own activities. The CO2 formed by respiration in heterotrophsreturns to the atmosphere, to be used again by photosynthetic organisms.
Solar energy thus provides the driving force for the continuouscycling of atmospheric CO2 and O2 through the biosphere and providesthe reduced substrates (fuels), such as glucose, on which nonphotosynthetic organisms depend.Enormous amounts of energy are stored as products of photosynthesis. Each year at least 1017 kJ of free energy from sunlight is captured and used for biosynthesis by photosynthetic organisms.
This ismore than ten times the fossil-fuel energy used each year by people theworld over. Even fossil fuels (coal, oil, and natural gas) are the products of photosynthesis that took place millions of years ago. Because ofour global dependence upon solar energy, past and present, for bothenergy and food, discovering the mechanism of photosynthesis is a central goal of biochemical research.571FlagellumOuter membraneInner (plasma)membranePeptidoglycanand periplasmicspaceFigure 18-31 Rotation of bacterial flagella byproton-motive force. The shaft and rings at thebase of the flagellum make up a rotary motor thathas been called a "proton turbine." Protons ejectedby electron transfer flow back into the cell throughthe "turbine," causing rotation of the shaft of theflagellum.
This motion differs fundamentally fromthe motion of muscle or of eukaryotic flagella andcilia, for which ATP hydrolysis is the energy source.PhotosyntheticcellsC0 2 H 2 0Figure 18-32 Solar energy is the ultimate sourceof all biological energy. Photosynthetic organismsuse the energy of sunlight to manufacture glucoseand other organic cell products, which heterotrophiccells use as energy and carbon sources.J CarbohydrategHeterotrophiccells572Part III Bioenergetics and MetabolismThe overall equation of photosynthesis describes an oxidationreduction reaction in which H2O donates electrons (as hydrogen) forthe reduction of CO2 to carbohydrate (CH2O):CO 2 + H 2 O09HoOLightreactionsrNADP++ PiNADPHMTCarbon fixationreactionsCarbohydrateCO2Figure 18-33 The light reactions generate energy-rich NADPH and ATP at the expense of solarenergy. These products are used in the carbon fixation reactions, which occur in light or darkness, toreduce CO2 to form trioses and more complex compounds (such as glucose) derived from trioses.Figure 18-34 Schematic diagram (a) and electronmicrograph (b) of a single chloroplast at high magnification.lightO2 + (CH2O)Unlike NADH (the hydrogen donor in oxidative phosphorylation), H2Ois a poor electron donor; its standard reduction potential is 0.82 V,compared with -0.32 V for NADH.
The central difference betweenphotophosphorylation and oxidative phosphorylation is that the latterprocess begins with a good electron donor, whereas the former requiresthe input of energy in the form of light to create a good electron donor.Except for this crucial difference, the two processes are remarkablysimilar. In photophosphorylation, electrons flow through a series ofmembrane-bound carriers including cytochromes, quinones, and ironsulfur proteins, while protons are pumped across a membrane to createan electrochemical potential. This potential is the driving force for ATPsynthesis from ADP and Pi by a membrane-bound ATP synthase complex closely similar to that which functions in oxidative phosphorylation.Photosynthesis encompasses two processes: the light reactions,which occur only when plants are illuminated, and the carbon fixation reactions, or so-called dark reactions, which occur in both lightand darkness (Fig.
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