AP® Biology 2021 Scoring Guidelines
(a)
Describe the characteristics of the plasma membrane that prevent simple diffusion of
Na
+
and
K
+
across the membrane.
1 point
Accept one of the following:
• The interior of the plasma membrane is hydrophobic/nonpolar.
• The phospholipid tails are hydrophobic/nonpolar.
• The exterior of the pla
sma membrane is hydrophilic/polar.
• The phospholipid heads are hydrophilic/polar.
Explain why ATP is required for the activity of the
+ +
Na /K ATPase
.
1 point
• The
+ +
Na /K ATPase
pumps ions against their concentration gradients. This requires
an input of (metabolic) energy.
Total for part (a) 2 points
(b) Identify a dependent variable in the experiment represented in Figure
1. 1 point
• The number of cells
Justify the use of normal human kidney NHK cells as a control in the experiments. 1 point
Accept one of the following:
• It allows the scientists to determine the effect of PKD on the cells’ responses to (various
concentrations of) ouabain.
• It allows the scientists to compare the responses of PKD cells and nor
mal cells (to
ouabain).
Justify the use of a range of ouabain concentrations in the experiment represented in
Figure 1.
1 point
Accept one of the following:
• The scientists need to determine whether different concentrations have different effects
on the
cell numbers.
• The scientists did not know at which concentrat
ion of ouabain there would be an effect.
Total for part (b) 3 points
(c)
Based on the data shown in Figure 2, describe the rel
ationship between the concentration of
ouabain and the
+ +
Na /K ATPase
activity both in normal human kidney (NHK) cells AND
in PKD cells.
1 point
Accept one of the following:
• Increasing concent
rations of ouabain result in decreasing ATPase activity (in both types
of cells).
• There is an inverse relationship/negative correlation between t
he concentration of
ouabain and the ATPase activity (in both types of cells).
The scientists de
termined that
+ +
Na /K ATPase
activity in PKD cells treated with
1 pM
ouabain is 150 units of ATP
hydrolyzed/sec
. Calculate the expected
+ +
Na /K ATPase
activity (units/sec) in PKD cells treated with
10
6
pM
ouabain.
1 point
• 45 (Accept between 40 and 50)
Total for part (c) 2 points
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