Download CELL PROBLEMS

CELL PROBLEMS
B. Osmosis
13. Red blood cells are normally kept in a solution containing 0.15 M NaCl.
When red blood cells are placed in water, they normally burst. Which of the
following treatments would keep them from bursting?
a. Adding 0.3 M glucose to the water.
b. Soaking them in 0.15 M NaCl before putting them in water.
c. Adding a substance (Hg2+) that blocks aquaporins.
d. Placing the cells and water in a pressure chamber and exerting a
hydrostatic pressure of 7 atmospheres.
14. When red blood cells are placed in a solution of 0.15 M glucose, they
swell and some will burst. Why are they stable in 0.15 M NaCl and not in 0.15
M glucose?
15. When red blood cells are placed in a solution of 0.3 M urea, they burst
quickly; in 0.3 M glycerol, they burst (fairly suddenly) in approximately 15 min;
in 0.3 M glucose they do not burst for hours, if at all.
Explain why they burst in urea and glycerol and not in glucose and why
bursting in glycerol is delayed.
16. Is a plant cell in water always turgid?
17. Assume that two plant cells have identical volumes in water. When the
cells are placed in 0.3 M sucrose (table sugar, consisting of linked glucose
and fructose molecules), cell A shrinks very little and is still turgid; cell B
shrinks more and is flaccid (no turgor pressure).
A. When they were in water, did the two cells have the same turgor
pressure? If not, which had the higher pressure? Did the cells have
the same internal (osmotically effective) concentration of solutes? If
not, which had the higher concentration?
B. When the cells are placed in 0.6 M sucrose, both are plasmolyzed. At
this point, do the cells have the same turgor pressure? If not, which
has the higher pressure? Do the cells have the same internal
(osmotically effective) concentration of solutes?
18. The osmotic force pulling water across a membrane is proportional to the
difference in concentration of solutes across the membrane. A difference of
0.1 M produces a force of 2.4 atmospheres. A certain plant cell in water has
a turgor pressure of 7 atmospheres.
A. What concentration of sucrose would be needed for incipient
plasmolysis? (Assume that the cell wall does not shrink noticeably as
it loses turgor.)
B. What concentration would be needed to plasmolyze the cell to the
point that the volume of the protoplast (contents inside the plasma
membrane) is one-half the volume of the space inside the cell wall.
19. Penicillin kills bacteria by interfering with their cell wall synthesis. When
they grow in penicillin, breaks develop in their cell walls. Suggest reasons
why under certain conditions bacteria might survive in media containing
penicillin.
C. Enzymes
20. Certain proteins called “channels” allow hydrophilic substances (e.g.,
glucose) to cross a cell membrane. Are these proteins enzymes? Explain
why they might be considered to be enzymes (and why not).
21. You purified an enzyme protein (lysozyme) from your tears, but when you
try to assay its activity, it doesn’t work. Suggest reasons why not.