Multiple Choice Answers:

21. Cytosolic [Ca²⁺] is normally less than 1 µM. SR (sarcoplasmic reticulum) [Ca²⁺] is normally 300 µM, although under some conditions it increases. Assume the membrane potential for the SR membrane is ΔV = − 0.040 V (SR → cytosol). ΔG_T = R T ln (C₂/C₁) + Z F ΔV

a. Calculate ΔG_T for moving Ca²⁺ into the SR using the Ca²⁺ ATPase, which moves 2 Ca²⁺
      for each cycle.
b. Based upon your answer to part (a), how many Ca²⁺ can be moved into the
     organelle per mole ATP hydrolyzed by the Ca²⁺ ATPase?
c. What happens to both of your answers when SR [Ca²⁺] is doubled? And what effect
      might this change in concentration have on ΔV?

      a. ΔG_T = (8.314 J/mol-K) (298 K) ln ( 300/1 ) + (+4 ) (96480 J/V-mol) (+ 0.040V)
       = (2477.572 J/mol) (5.70) + 15435.8 J/mol) = 14132 J/mol + 15436 J/mol
       = 29568 J/mol = + 29.6 kJ/mol

       I expected you to recognize that moving Ca²⁺ from the cytosol into the SR is
      unfavorable, both in terms of charge and in terms of concentration.

      b. − 30.5 kJ/mol + 29.6 kJ/mol = − 0.9 kJ/mol, so 1 ATP provides enough energy to move
     2 Ca²⁺ only

      c. ΔG_T = R T ln (600/1) + 15436 J/mol = (2477.6 J/mol) (6.40) + 15436 J/mol
        = 31.3 kJ/mol

     This doesn't make a huge difference, but it does mean that it's no longer possible to
     move 2 Ca²⁺ with 1 ATP.

22. a. Leucine rotates out of the channel when 2 ACh bind the receptor.
b and c. Answers could vary. Most people had no trouble with this, and when they did, it was usually the result of answering based on what they had learned in physiology or trying to answer for a G protein receptor rather than the ACh receptor.

23. a. ¹/₂ O₂ + 2 H⁺ + 2 cyt c (Fe²⁺) → H₂O + 2 cyt c (Fe³⁺)

      b. ΔE°′ = 0.816 V − 0.254 V = 0.562 V         Reduction potential is an intrinsic property
                                                                              and does not change with number.

           ΔG°′ = − (2) (96480 J/V-mol) (0.562 V) = − 108444 J/mol = − 108.4 kJ/mol

      c. − 108.4 kJ/mol /reaction ÷ ( 30.5 kJ/mol/ATP) = ~ 3.6 ATP/reaction

24. Glycolysis and gluconeogenesis use many of the same enzymes, but irreversible reactions require different enzymes. Use the table for parts a and b.
Please read the questions carefully and answer only what is asked.
a. Name two enzymes that catalyze irreversible reactions in glycolysis.
b. Name the enzymes that catalyze reactions in gluconeogenesis that reverse the
      glycolysis enzymes you listed in a. If two enzymes are required, you must list both.
c. For each of the reactions you listed, what is the energy gain or loss?

d. Glycolysis produces energy, and gluconeogenesis requires energy.

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