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The giant, single-cell alga Nitella lives in fresh water and is readily
cultured in the laboratory. For this reason and also because of
its large size and favorable optical properties, this alga has been a
favorite object of study by cell biologists and cell physiologists.
Consider the following experiments and answer all of the following questions
concerning the Nitella plasma membrane. Remember! What useful information
is provided by the question, what must you remember from text and reading,
and what questions are (and are not) being asked?
Don't forget to answer question D, which
is found at the bottom of the page after the sample answers and comments
for questions A, B, and C.
A. (5 pts) Why doesn't this alga swell osmotically and burst in its natural
environment? How could you test the validity of your explanation?
B. (5 pts) Using the Fick equation, how would you determine the permeability
coefficient of the Nitella plasma membrane to water?
C. (5 pts) If you place Nitella in a solution of 10% glycerol and
observe it under a phase contrast microscope, you would note first its
shrinkage followed by a gradual return to its normal shape.
What is the simplest accurate explanation of these results?
Having answered the questions yourself, now consider the following answers
others
provided.
A. Why doesn't this alga swell osmotically and burst in its natural
environment? How could you test the validity of your explanation?
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Answer
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Comment
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| Example 1. Most
likely, this organism has a number of "pumps" to take in
solutes against their gradients, keeping the cell from hemolysing.
There are a number of ways to test this hypothesis. One way would
be to introduce an inhibitor that binds to the "pump" sites
and restricts pumping of solutes (e.g., ouabain). |
Interesting answer and logical
test, but wrong! Pumping in solute would make osmotic gradient steeper,
causing more water to diffuse inwards by osmosis. |
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| Example 2. Osmosis
occurs only on the tail of another ion which is diffusing, as in binding
to a polar molecule or ion (ex. Na+). If these concentrations
are kept at a certain level in the cell, the water will not be admitted.
To test this, change the concentration of the water carrier in or
out of the cell and monitor the rate of osmosis. |
Principle is partially
correct, but very unlikely that solute concentration in cytoplasm
is equal to solute in fresh water. It is unclear what test is showing. |
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Example 3. Due to high concentrations of intercellular ions
and other substances the flow of H2O according to the
activity gradient into the cell by osmosis, Nitella must have a
system for regulating H2O flow. Diffusion is determined
by polarity and molecular size. Although H2O is polar
and therefore not readily admissible it is so small it usually passes
through the membrane uninhibited. Nitella may have developed a membrane
which is relatively impermeable to H2O. This solutions
would prevent excess H2O from entering the cell.
Another possible explanation is that Nitella has developed a pump
system to remove excess H2O from its interior. The probable
explanation contains both these elements.
One way to test this hypothesis is through an examination of Nitella's
permeability according to the Fick equation. A method to test the
possibility of a system for removing excess H2O could
be developed by labeling H2O within the cell by means of a dye and
observing to see if this H2O is removed from the cell to the surrounding
medium.
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Good opening sentence but answer is very wordy.
Much of second and third sentences is irrelevant.
Relatively impermeable membrane would only affect rate of H2O
entry, but cell would eventually swell and burst.
There are no known H2O pumps! Creative, but must
first rule out simpler explanations.
Excellent test of weak (unlikely) hypothesis.
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B. Using the Fick equation, how would you determine the permeability
coefficient of the Nitella plasma membrane to water?
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Answer
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Comments
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| Example
1. Taking the Fick equation, I would test the rate of diffusion
across the Nitella plasma membrane and compare it to the other substances
of known permeability coefficients. By creating a solution where only
water was moved across the membrane the rate of diffusion could be
calculated comparing the rate of diffusion to other substances of
known permeability coefficients the permeability coefficient of Nitella
could be obtained. |
Where's the Fick equation? Comparison of the permeability coefficient
of H2O with that of other substances irrelevant here.
How is the rate of diffusion to measured?
Weak answer!
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| Example
2. To determine the permeability, you would want to hemolyze cells.
It would be calculated by dividing the change in the H2O concentration
by the time it took. Then plug in for [H2Oo]
and [H2Oi]--this will give you K. It's really
a pointless calculation because everything happens so fast that the
permeability coefficient is huge and meaningless. |
Where's the Fick equation?
Nitella won't "hemolyze" - see answer to Question
A.
How is the concentration of H2O measured? What does
it mean to talk about the concentration of a solvent?
Don’t “fight” the question! The student here
doesn’t know enough to draw this conclusion.
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Example 3. We
can measure in
laboratory the rate of diffusion of particles and we also can control
the solute concentrations inside and outside of the cell. Therefore
we can determine the coefficient of permeability. We can measure in
laboratory the rate of diffusion of particles and we also can control
the solute concentrations inside and outside of the cell. Therefore
we can determine the coefficient of permeability. |
Good start at an
answer! Variables need to be related, with constants, to rate. State
the Fick equation! |
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| All three answers suffer from the same weakness. If
the questions requires the use of an equation to dermine a solution,
the equation must be used. |
C. If you place Nitella in a solution of 10% glycerol and observe
it under a phase contrast microscope, you would note first its shrinkage
followed by a gradual return to its normal shape. What is the simplest
accurate explanation of these results?
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Answer
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Comment
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| Example 1. Nitella,
in its natural environment exists in a situation in which the outside
environment contains much less solute than its interior. Therefore
if it is immediately immersed in a 10% glycerol solution, it will
atrophy due to loss of H2O moving out of the cell according
to the activity gradient--the higher concentration of solute outside
this cell. Gradually the cell would adjust to the situation by regulation
of its H2O uptake and loss and regain the concentration
of solute/H2O which is necessary for its survival. This
ability to regulate the flow of H2O allows Nitella to exist
in solutions of various concentrations. |
Good answer so far! although “atrophy” is the wrong
verb.
Vague--how does water regulation wor and how would it affect outcome?
Last sentence interesting but irrelevant.
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Example 2. Phase contrast gives good contrast observation
so that substances are dark enough to see instead of the faintness
you might get under bright field. However, what is lost is the little
details (physically) that you would have been able to note under
bright field. That may account for the shrinkage at first of the
cell and vacuole. Perhaps it was losing detail in favor of a sharper,
more focused albeit smaller image.
The other explanation is that glycerol is an alcohol with a polar
-OH end (from the electronegative oxygen). It is more difficult
for polar molecules to pass through membranes so their rate of passage
is considerably slower. So perhaps this "gradual return"
could be attributed to the time it may take for the glycerol to
move into Nitella.
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First part of answer suggests initial change is an optical artefact.
Accept the details provided at face value: Don’t side-step
the question.
Second part of answer changes tack and is essentially correct, but
doesn't address initial shrinking (due to H2O loss) in
an explicit manner.
(Focus changes, resulting in an incomplete answer!)
How does crenation result from a hyperosmotic environment? What
is a "dynamic equilibrium"? Answer is much too general.
It may reflect complete understanding, but answer lacks sufficient
detail.
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D. Like other cells Nitella exhibits a membrane, or "resting",
potential whereby its cytoplasm is negative with respect to its environment.
The actual value of this potential (at 18o C) is - 138 mV.
How is this potential likely to be generated and how could you test your
hypothesis? (Your reasoning must be explicit and detailed and all relevant
calculations must be shown.) The intracellular and extracellular concentrations
of the major inorganic ions, as well as log values for their concentration
ratios, are indicated below.
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Concentration in
mM |
Logs of Concentration
Ratios |
| Location |
Na+ |
K+ |
Cl- |
Nai/Nao
= 1.15
Nao/Nai = -1.15 |
| cytoplasm |
14.0 |
119.0 |
65.0 |
Ko/Ki
= -3.08
Ki/Ko = 3.08 |
| stream/culture |
1.0 |
0.1 |
1.3 |
Cli/Clo
= 1.70
Clo/Cli = -1.70 |
Having answered the questions yourself, now consider the following answers
others
provided.
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Answers
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Comments
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| Example 1. This resting potential
is generated because ions are moving in and out of the cell. It is
negative because ions like Na+ and K+ are leaving the making the cytoplasm
negative The log of Na outside/Na inside is negative so that is why
the resting potential is negative. The hypothesis could be tested
by changing the concentrations outside and inside and determining
the change difference in the membrane potential |
Generally true answer, but lacking calculations,
it's much too vague.
Test is correct, but what exactly would results show? |
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Example 2. The resting potential is determined by the electro
chemical gradient that is maintained by the cell. It takes into
account the changed ions of Na+, K+, Cl-. The electro chemical gradient
is determined by the Nerst equation. v = . In determining the resting
potential the permeability of each ion must also be considered and
factored into the equation.
To test the hypothesis to see if it is these ions that make up
the resting potential, one could place the cell in a test solution
that is higher in concentration of the ions than the cell. A voltage
meter may be inserted into the cell and the solution, to see if
the resting potential is changed.
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Accurate answer but no calculations shown; therefore, answer
is incomplete.
Good test, but . . .
. . . how specifically would voltage change under condition described?
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