Section II: Free Response
Short student-style responses have been provided for each of the questions. These samples indicate an answe r that
would get full credit, so if you're checking your own response, make sure that the actual answers to each part of
the question are similar to your own. The structure surrounding them is less important, although we've modeled
it as a way to help organize your own thoughts and to make sure that you actually respond to the entire question .
Note that the rubrics used for scoring periodically change based on the College Board's analysis of the previous
year's test takers. We've done our best to approximate their structure, based on our institutional knowledge of
how past exams have been scored and on the information released by the test makers.
Our advice is to over-prepare. Find a comfortable structure that works for you, and really make sure that you're
providing all of the details required for each question. Also, continue to check the College Board's website, as
they may release additional information as the test approaches. For some additional help, especially if you're
worried that you're not being objective in scoring your own work, ask a teacher or classmate to help you out.
Good luck!
Question 1 �
a. Describe why a cell membrane is semipermeable. Include examples of molecules that can/cannot pass
through. (2 points)
Cells are semipermeable because certain things can pass through them and some cannot. The membrane
is made of a phospholipid bilayer. This means hydrophobic things like oxygen can pass through and hydrophilic
things like water and ions cannot pass through without assistance, such as a channel.
b. Bacterial cells contain a Na•/K• ATPase pump that helps them maintain an osmotic balance with their
surroundings. In the experimental cells, glycolysis was inhibited to prevent sodium from being pumped
out of the cell. Explain how inhibition of glycolysis likely achieves this. (3 points)
The sodium-potassium pump Is an example of active transport. It pumps sodium ions out of the cell. This
experiment was measuring the amount of sodium in the media and the pump might alter the results. To keep
it tumed off, the cell needs to lack the energy to run the pump. By preventing glycolysis, the cell is kept defi
cient in ATP and the pump will remain off. Glycolysis Is part of cell respiration, and it begins with glucose and
turns it Into PYruvate.
c. Analyze the data and explain how the levels of glucose and sodium inside the cell change throughout the
experiment. (2 points)
The glucose appears to disappear from the media and so It must be entering the cell. At first It enters the
cell quickly and then slows down. The sodium enters the cell quickly, and keeps entering at the quick rate for
longer than the glucose. The media Is almost depleted of sodium so the sodium must be very high Inside the cell.
d. Predict what would happen if the level of sodium inside the cell was not relatively low at the onset of the
experiment. (2 points)
If the sodium was not low, then there would not be a pull for the sodium to enter the cell. Sodium Is entering
due to lt6 concentration gradient. If the sodium was high In the cell, then the sodium would probably remain
In the media.
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