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medical sciences
biochemistry

Questions and Answers of Biochemistry

Mutations in SOD are associated with the neurodegenerative disease amyotrophic lateral sclerosis (ALS). Explain why researchers initially believed that loss of SOD activity would damage neurons.
Calculate (G°( for the oxidation of free FADH2 by O2. What is the maximum number of ATPs that can be synthesized, assuming standard conditions and 100% conservation of energy?
Show that the free energy change for the succinate dehydrogenase reaction catalyzed by Complex II is insufficient to drive ATP synthesis under standard conditions.
Some anaerobic prokaryotes use nitrate (NO3-) as the terminal electron acceptor for energy metabolism. Assuming 100% efficiency, how much ATP could be synthesized by the oxidation of NADH by nitrate?
Some anaerobic prokaryotes reduce elemental sulfur to H2S. Assuming 100% efficiency, how much ATP could be synthesized by the oxidation of acetate by S under standard conditions?
The O2-consumption curve of a dilute, well-buffered suspension of mitochondria containing an excess of ADP and Pi takes the formSketch the curves obtained when (a) Amytal is added at time t = 1
Define the terms light reactions and dark reactions. Do the dark reactions occur in the dark? Explain.
What is the fate of water-derived electrons in chloroplasts treated with DCMU? What simple screening method could be used to identify plants that had been exposed to DCMU?
What do the S states represent in the oxygen-evolving center (OEC)? What S state predominates in dark-adapted chloroplasts?
What chloroplast component generates the majority of the proton gradient used for ATP production? Does it have a mitochondrial counterpart?
Why does the Cu atom of plastocyanin have an unusually high standard reduction potential?
What are the similarities and differences between photosystem I and photosystem II?
Estimate the minimum reduction potential for P680. Estimate the maximum reduction potential for ferredoxin. Explain your answers.
Describe the distribution of LHCs between grana (stacked membranes) and stromal lamellae (unstacked membranes) in chloroplasts under a bright sun and in shady light.
What is the first stable radioactive sugar intermediate seen when 14CO2 is added to algae such as Chlorella? When the supply of 14CO2 is cut off, what compound accumulates? What do these results
Using Figure 19-26, write out the 13 reactions of the Calvin cycle. What is the main difference between Stage I and Stage II reactions? Write a net equation for each stage.
Glycolysis and the Calvin cycle are opposing pathways. Which reactions form a potential futile cycle? How are these reactions controlled in plant chloroplasts?
Draw a cross-section of a chloroplast and indicate the locations of the following proteins and other structural features: Outer membrane..........................Thylakoid lumen Stromal
What is photorespiration? What is the ultimate result of this process?
The concentration of atmospheric CO2 has been increasing for many decades. If this trend continues, how might it affect the relative abundance of C3 and C4 plants?
Calculate the energy of a mole of photons with wavelengths of (a) 400 nm, (b) 500 nm, (c) 600 nm, and (d) 700 nm.
Purple photosynthetic bacteria have different pigments than higher plants. Why is this an advantage for these bacteria?
What distinguishes the chlorophyll in a reaction center from the antennae chlorophyll?
The initial electron transfers in the bacterial photosynthetic reaction center are extremely rapid, but the lifetime of the terminal semiquinone is relatively long. (a) Why is it essential for the
Compare the electron flow in purple photosynthetic bacteria to that in higher plant chloroplasts. What is the origin of the electrons and their eventual fates? How many protons are translocated?
What is the standard reduction potential for the oxidation of H2O (see Table 14-5)? Can this value be obtained from purple bacterial photosynthesis? Compare this to two-center photosynthesis.
The number of O2 molecules released per photon absorbed by a suspension of algae can be measured at different wavelengths. When algae are illuminated by 700 nm light, very little O2 is produced.
Calculate the change in free energy for the transit of two electrons from the quinone pool to cytochrome c2 in bacterial photosynthetic electron transport. Assume that the reduction potential of
Estimate the change in free energy when P960 undergoes photooxidation.
Calculate a and ∆Go' for the light reactions in plants, that is, the four-electron oxidation of H2O by NADP.
Use the solutions of Problems 19-5 and 19-12 to calculate how many moles of photons of d light (λ = 700 nm) are theoretically required to drive the four-electron oxidation of H2O by NADP+ under
Use the solution of Problem 19-12 to calculate how many moles of photons of UV light (λ = 220 nm) would be required to drive the four-electron oxidation of H2O by NADP+ under standard conditions to
Under conditions of very high light intensity, excess absorbed solar energy is dissipated by the action of photoprotective proteins in the thylakoid membrane. Explain why it is advantageous for these
Which of the mechanisms for dissipating light energy shown in Fig. 19-6 would best protect the photosystems from excess light energy?In Figure 19.6
Although the net equation for photosynthesis indicates that the process can be measured in terms of either O2 produced or CO2 fixed, in practice, these measurements are not necessarily equivalent.
Why is it possible for chloroplasts to absorb much more than 8-10 photons per O2 molecule evolved?
Predict the effect of adding myxothiazol, an inhibitor of electron transport in mitochondrial Complex III, to a suspension of chloroplasts exposed to light. Would either ATP or NADPH production be
Why would "knocking out" a gene for a chloroplast fatty acid desaturase (an enzyme involved in synthesizing fatty acids containing three double bonds) increase the rate of photosynthesis at 40°C, a
Calculate the free energy change for moving a proton from the thylakoid lumen to the stroma when ∆pH = 3.4, ∆(, and T = 25°C.
Chloroplast ATP synthase contains 14 c subunits. How many protons must be translocated to the thylakoid lumen to support the synthesis of one ATP?
Chloroplasts are illuminated until the levels of the Calvin cycle intermediates reach a steady state. The light is then turned off. How does the level of RuBP vary after this point?
Calculate the energy cost of the Calvin cycle combined with glycolysis and oxidative phosphorylation, that is, the ratio of the energy spent synthesizing starch from CO2 and photosynthetically
Oil is a major energy-storage molecule in most seeds. During seed development in some plants, oil (triacylglycerols) is synthesized using acetyl-CoA derived from sucrose. The developing seed also
The leaves of some species of desert plants taste sour in the early morning, but, as the day wears on, they become tasteless and then bitter. Explain.
Plants must obtain CO2 but avoid the loss of H2O by evaporation. What would be the effect of increased atmospheric [CO2] on photosynthesis? Would the effect be the same for C3 and C4 plants?
How might the increase in atmospheric [CO2] affect plants' water consumption?
Explain why the chlorophylls in light-harvesting complexes must absorb light of shorter wavelength than the light that would directly excite the special pair of PSI or PSII,
Calculate the energy of one mole of photons of red light (λ = 700 nm).
Describe the functional similarities between the purple bacterial photosynthetic reaction center and PSI.
From a chemical perspective, why is the energy content of fats so much greater than that of carbohydrates or proteins?
Calculate the yield of ATP when one mole of stearic acid is completely oxidized to CO2 and H2O.
In the β oxidation of oleic acid, the presence of the double bond at the ________ position necessitates modification of the β oxidation cycle. The modification occurs after the ______ round of β
Explain why succinyl-CoA arising from the oxidation of odd-chain fatty acids cannot be directly oxidized by the citric acid cycle. How can it be further degraded?
Why does the β oxidation of fatty acids in peroxisomes yield less ATP than the corresponding process in mitochondria?
Infants have high levels of ketone bodies in their blood and abundant 3-ketoacyl-CoA transferase in their tissues (except in liver) prior to weaning. What nutritional advantage does this confer?
Calculate the ATP that is produced when linoleic acid (9,12-octadecadienoic acid; 18:2) is (a) Oxidized to CO2 and H2O (b) Converted to the ketone body acetoacetate in the liver and then oxidized to
Write equations for ketone body synthesis and degradation. What is the net result of combined synthesis and degradation?
Why do individuals who have their gall bladders removed encounter difficulties in digesting large amounts of fats?
A rat liver cytosol preparation is incubated with acetate and all cofactors necessary for the biosynthesis of palmitate. Which carbon atoms of palmitate will be isotopically labeled when the
What are the advantages of the multifunctional dimeric structure of animal fatty acid synthase in the formation of fatty acids?
Which of the following statements is(are) correct? The transport of acetyl-CoA from the mitochondrial matrix to the cytosol for fatty acid biosynthesis: (a) Is necessary because the inner
Explain the role of AMP-dependent protein kinase in regulating fatty acid metabolism.
When tissues are incubated in vitro with 32P-labeled Pi, the incorporation of radioactivity into phospholipids can be readily demonstrated. High concentrations of the drug propranolol dramatically
Fumonisin B is a mycotoxin that inhibits the synthesis of dihydroceramide from sphinganine. When this inhibitor is added to cells in culture that actively synthesize spingoglycolipids, predict how
How do pancreatic lipase and phospholipase A2 differ in the mechanism by which they promote hydrolysis of glycerolipids at interfaces?
Explain how the regulation of HMG-CoA reductase, the principal control site for cholesterol synthesis, can conserve cellular ATP.
Match each term on the left with its description on the right.
Determine the order of the following events involving lipoprotein-mediated transport of ______ dietary triacylglycerols and cholesterol. ______ Triacylglycerols are removed from circulating VLDL by
Why are cholesteryl esters located in the interior of a lipoprotein while cholesterol is located on the exterior?
If the LDL receptor underwent a mutation that increased its affinity for apoB-100, what would be the effect on serum cholesterol levels?
What products would be isolated from urine when dogs are fed (a) phenylheptanoic acid and (b) phenyloctanoic acid? How does this experiment, originally performed by Knoop, shed light on the process
A patient develops an enlarged fatty liver and low blood glucose. These symptoms can be partially overcome only when massive amounts of carnitine are included in the diet. What enzyme defect might be
The removal of fatty acids from triacylglycerols leaves glycerol. Show how the actions of glycerol kinase and glycerol-3-phosphate dehydrogenase on glycerol produce an intermediate of glycolysis.
The complete combustion of palmitate and glucose yields 9781 kJ ∙ mol -1 and kJ ∙ mol -1 of free energy, respectively. Compare these values to the free energy (as ATP) obtained though catabolism
Why are unsaturated fats preferable to saturated fats for an individual whose caloric intake must be limited?
Calculate the ATP yield for the complete oxidation of oleate.
Explain why the degradation of odd-chain fatty acids can boost the activity of the citric acid cycle.
How many ATP can be produced by the complete oxidation of each propionyl-CoA product of fatty acid oxidation? Compare this to the ATP yield for each acetyl-CoA.
Digestion of plant materials generates phytanate (derived from chlorophyll molecules; Fig. 19-2).(a) Explain why phytanate cannot be catabolized by Î² oxidation. (b) Instead, phytanate is
Pristanal can be oxidized to pristanate. Can this compound be further degraded by β oxidation and, if so, what are the reaction products?
Compare the energy cost, in ATP equivalents, of synthesizing stearate from mitochondrial acetyl-CoA to the energy recovered by degrading stearate to acetyl-CoA.
Compare the energy cost, in ATP equivalents, of synthesizing stearate from mitochondrial acetyl-CoA to the energy recovered by degrading stearate to CO2.
Calculate the cost, in ATP equivalents, of converting 12 acetyl- CoA to lignocerate (a C24 fatty acid; Table 9-1) when elongation occurs in the mitochondrion.
Calculate the cost, in ATP equivalents, of converting 12 acetyl- CoA to lignocerate when elongation occurs in the endoplasmic reticulum.
Pharmacological inhibition of acetyl-CoA carboxylase has been proposed as a treatment for obesity. Explain why this enzyme would appear to be a good anti-obesity drug target.
Explain why inhibiting the activity of acetyl-CoA carboxylase might not affect a person's body mass.
An animal is fed palmitate with a 14C-labeled carboxyl group. Under conditions of membrane lipid synthesis, where would the label appear in sphinganine?
Hypercholesterolemic individuals taking statins are sometimes advised to take supplements of coenzyme Q. Explain.
Adipocytes store fat in phospholipid-coated droplets in the cytosol. The protein perilipin is also associated with the surface of the lipid droplet. (a) Describe the likely structure of the perilipin
Biodiesel, a fuel typically derived from plant oils, can be manufactured by treating the oil with a methanol/KOH mixture to produce fatty acid methyl esters. Indicate the structures of the products
Explain why individuals with a hereditary deficiency of carnitine palmitoyl transferase II have muscle weakness.
The first three steps of Î² oxidation (Fig. 20-12) chemically resemble three successive steps of the citric acid cycle. Which steps are these?In Figure 20-12
Indicate whether the following statements are true: (a) Proteins with the sequence Lys-Phe-Glu-Arg-Gln are selectively degraded by proteasomes. (b) Proteins containing sequences rich in Pro, Glu,
Reactions that involve PLP can be classified according to which bond to the alpha carbon is broken. For the amino acid-PLP adduct shown below, indicate what kind of reaction involves cleavage of
Some forms of maple syrup urine disease are amenable to treatment with large doses of dietary thiamine. What metabolic defect is being treated in these cases?
How do sulfonamides selectively inhibit bacterial growth? Why aren't animals also adversely affected by these antibiotics?
S-adenosylmethionine (SAM) is a key methylating agent in several physiological processes, but its regeneration depends on the presence of another methylating agent. How is SAM regenerated?
Calculate the free energy yield for the complete oxidation of the carbon skeletons of (a) aspartate, (b) glutamine, and (c) lysine. Express your answer in ATP equivalents and assume that NADPH is
Rationalize the significance of α-ketoglutarate as an activator of glutamine synthetase?
The four intermediates of the urea cycle are α-amino acids. (a) Which one of these is considered to be an essential amino acid in children? (b) Outline a pathway by which adults can synthesize this

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