Download CELL PROBLEMS

BIS2A
T.M. Murphy
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CELL PROBLEMS
A. Structure
1. Decide whether microscopy or cell fractionation would be the best way to
answer each of the following questions.
a. What is the nucleus made of?
b. What happens when a plant cell is placed in 0.6 M NaCl?
c. Do all plants’ cells have mitochondria?
d. Are mitochondria and chloroplasts structurally related?
e. Do mitochondria divide like bacteria?
f. How is sugar used for food by an animal cell?
g. Do muscles and cilia move by the same basic mechanism?
2. Assume a cell is a cube 10 µm on a side. What is the volume of the cell in
cubic µm? What is the volume in cubic meters? How many of these cells
would fill a teaspoon (approximately 5 cubic centimeters)?
3. Many textbooks say that the reason cells are small relates to the surfaceto-volume ratio: this ratio is greater for small objects than for large ones, and
a large ratio is needed for the transport of substances in and out of the cell.
What is the surface-to-volume ratio of a sphere 20 µm in diameter? Of a
sphere 200 µm in diameter? Despite this reasoning, many large cells are
known: for instance, the internode cell of an alga, Chara, can be 1 millimeter
(1000 µm) wide and 2 centimeters long. What implicit assumption makes the
reasoning suspect? (Hint: Chara internodes are cylinders that grow in
length.)
4. Again considering surface-to-volume ratio, what organelle, found in plant
cells but not in animal cells, allows the cells of plants to be larger than the
cells of animals?
5. Assume that you are responsible for designing a new space probe to
continue the search for life on Mars. What would you test for, and how would
you design the test, ...
a) if you have to design the one best experiment?
b) if you had an unlimited budget?
6. You have five electron micrographs depicting cells from five different
kingdoms. The labels have dropped off, and you have to figure out which
cells belong to which kingdoms. The only clues you have are the organelles
you can recognize from the pictures.
Two of the micrographs (which for convenience you re-label A and B) show
cells with large central vacuoles; three cells (D, E, and F) do not have such
vacuoles. Among the cells with large central vacuoles, one (A) has
chloroplasts and the other does not. Among the cells without large central
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vacuoles, one (C) has cilia; the others do not. Among the cells without cilia,
one (D) has a cell wall; the other does not.
What are the most likely identities of the five cells?
7. In the diagram below question 8, the best word for A is:
a. organelles b. nucleus c. cell wall d. motility e. microtubules
8. In the diagram below question 8, the best word for B is:
a. plastids b. vacuoles c. motility d. nucleus e. Organelles
9. In the diagram below, the best word for C is:
a. cell walls b. DNA c. photosynthesis d. nucleus e. ribosomes
10. Cytochalasin is a poison that inhibits the polymerization and thus the
function of microfilaments. A scientist observed that cells derived from liver
and placed in a tissue culture medium secreted proteins into the medium;
when treated with cytochalasin, that secretion quickly stopped. Provide a
hypothesis to explain why.
11. A plant cell is said to have over 20 compartments—that is, places to
which newly synthesized proteins may be directed. How many of these can
you name? (Hint: the plasma membrane is one.)
12. The pathway of newly synthesized proteins through the cell can be
followed by a “pulse-chase” experiment. Proteins are synthesized for a short
time with a radioactive isotope (the “pulse”), then they are given the same
compound non-radioactive (the “chase”), the presence of the radioactivity in
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different compartments of the cell is followed by isolating cell organelles and
quantifying their radioactivity. What results would you expect for a) a
lysosomal enzyme? b) a protein that is released from the cell? c) a protein
that is accumulated in the mitochondrial matrix? d) a protein of the plasma
membrane? e) a histone (nuclear protein)?
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.
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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.
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ANSWERS TO CELL PROBLEMS
1. a. fractionation b. light microscopy c. electron microscopy
d. electron microscopy, fractionation e. light, electron microscopy
f. fractionation g. electron microscopy, fractionation
2. 1000 cubic µm; 10-15 cubic meters; 5 x 109.
3. 3/10 µm-1; 3/100 µm-1; think about the surface-to-volume ratio of different
shapes.
4. The vacuole: it confines cytoplasm to the sides of the cell and thus keeps
cytoplasm in close contact with the cell surface.
5. This is an open-ended question. Possible answers depend on the
assumptions you make about the parameters for life. General
possibilities: the presence of complex molecules that are unstable under
the ambient redox conditions; reproduction in the presence of an energy
source (light, food); a heat-destructible catalytic system (but note the
problem with the last test in the Viking missions). More Earth-life-specific
tests could look for RNA, DNA, proteins.
6. A, plant; B, fungus; C, protist; D, bacterium; E, animal
7. cell wall
8. plastids
9. nucleus
10. The secretion of proteins occurs through the function of the
endomembrane trafficking system--ER to Golgi to plasma membrane--with
vesicles moving proteins between components. The vesicle movement is
powered and directed by motor proteins moving along microfilaments.
11. cytosol, plasma membrane, ER membrane, ER lumen, golgi membrane,
golgi lumen, lysosome, other vesicle (peroxisome, lipid body), nuclear
envelope, nucleoplasm, extracellular space (cell wall), tonoplast, vaculolar
solution, mitochondrial outer membrane, mitochondrial inner membrane,
mitochondrial intermembrane space, mitochondrial matrix, chloroplast
outer envelope, chloroplast inner envelope, thylakoid membrane, stroma,
thylakoid lumen
12. a) (rough) ER, ER vesicle, Golgi, lysosome
b) (rough) ER, ER vesicle, Golgi, Golgi vesicle, extracellular compartment
c) polysome, mitochondrial outer membrane, ... matrix
d) (rough) ER, ER vesicle, Golgi, Golgi vesicle, plasma membrane
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e) polysome, nuclear envelope (pore complex), nucleoplasm
13. a and c
14. NaCl dissociates, so the effective osmotic concentration is 0.3 M.
15. Urea and glycerol are smaller molecules than glucose, and they enter the
cell faster. As they enter the cell, water also will enter the cell by osmosis,
and the cell will expand. If the concentration of solute (urea or glycerol)
becomes the same inside and out, then the difference in total
concentration of solutes is as great as if the cells were placed in water.
16. Yes, assuming that its plasma membrane is intact.
17. A. No, cell A had the higher pressure and the higher osmotic
concentration. B. Yes, both cells have the same pressure and the same
osmotic concentration.
18. A. 0.29 M B. 0.58 M
19. The medium has a concentration of solutes higher than the concentration
inside the bacterial cells. The cells are not growing, perhaps because the
medium lacks an essential nutrient.
20. Glucose does not cross a membrane rapidly, because it takes an input of
energy to associate the hydrophilic glucose molecule (dissolve it) in the
hydrophobic lipid phase. That energy is analogous to activation energy of
a chemical reaction. The channel allows the glucose to cross without
acquiring the energy (or as much energy). There is, however, no chemical
reaction during the crossing.
21. Many enzymes need a particular pH and sometimes a specific cofactor
(metal ion, small organic molecule). You may have purified it away from
its cofactor. You may have irreversibly denatured the enzyme during
purification. You may have associated it (possibly covalently) with an
inhibitor.