7.06_2004_exam1 - 7.06 Spring 2004 Name 7.06 Cell Biology...

7.06 Spring 2004
Name___________________________
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7.06 Cell Biology
EXAM #1
February 24, 2004
This is an open book exam, and you are allowed access to books, a
calculator, and notes but not computers or any other types of electronic
devices. Please write your answers in pen (not pencil) to the questions in the
space allotted; if you use the back of a sheet make it clear which answer is
where.
And be sure to put your name on each page in case they become separated!
Remember that we will Xerox many exams at random.
Good luck!
F=2.3 x 10^4 cal/(V*mol)
R=1.987 cal/(K*mol)
Question 1 (35 pts).
You have recently isolated a yeast that grows in industrial wastewater, and
find that its optimal growth medium contains glucose, glycine, as well as
150 mM KBr (potassium bromide) and 15 mM KCl. Curious as to how such
an organism grows under such strange conditions, you sequence its genome.
You find only one gene sequence (i.e. one encoded protein) homologous to
the mammalian Ca 2+ and K + / Na + pump. Curious as to what ions this pump
pumps, you produce large amounts of it, incorporate it into liposomes, and
find that the protein is an ATP- powered Cl - and Br - pump. For each ATP
molecule hydrolyzed, two Br - ions are pumped outward (from the cytosolic
to the exoplasmic face) and two Cl - ions are pumped inward.
Furthermore, you examine these yeast cells growing in their preferred
medium of 150 mM KBr and 15 mM KCl., and find that their cytosol has a
different ionic composition: 15 mM KBr and 150 mM KCl.
a, 5 pts) To measure binding of radiolabeled Cl - and Br - to the ATP-
powered Cl - and Br - pump, you want to use a detergent to solubilize the

7.06 Spring 2004
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purified protein from your liposomes. Would you use an ionic or nonionic
detergent? Explain your answer.
b, 5 pts ) You find that the potential across the plasma membrane of these
yeast cells is 59 millivolts, with the inside of the cell negative to the outside.
How would you make this measurement?
c, 5 pts ) Would you expect that this potential is generated by open Cl -
channels in the plasma membrane, by open Br - channels in the plasma
membrane, or by the ATP- powered Cl - and Br - pump? Explain your answer
in a sentence or two.
d, 10 pts ) Interested in how these cells take up glucose from the culture
medium, you find in the plasma membrane a chloride- glucose antiporter – it
transports 3 chloride ions in one direction and one glucose in the other.