PRINCETON REVIEW MCAT PRACTICE TEST
QUESTIONS
1. Sickle-cell anemia (HbS) results from a substitution to valine from glutamic
acid at position 6 of the β chain of hemoglobin. Which of the following best
explains why the isoelectric point of HbS is higher than that of HbA?
A. The side chain of glutamic acid is less acidic than that of valine.
B. Glutamic acid is isoelectric at a lower pH than is valine.
C. Valine is isoelectric at a lower pH than is glutamic acid.
D. Glutamic acid has a net charge of 0 at its isoelectric point.
Answer B. Glutamic acid is isoelectric at a lower pH than is valine.
The isoelectric point is the pH at which an amphoteric molecule has a net electric charge of zero. Glutamic acid is more
acidic, not less acidic than valine (A is wrong). Glutamate would require more acidic pH to neutralize its charge (C is
wrong). D is a true, but irrelevant statement. The best answer is B. The negative charge of glutamic acid would require
a more acidic pH to neutralize, making the isoelectric point of HbA (with glutamic acid) lower than that of HbS (with
,valine).
2. A mixture of aspartate and phenylalanine is separated into its component
molecules by thin layer chromatography on a silica plate eluted with benzene.
Which of the following best explains why the separation occurs?
A. Aspartate will move farther with the mobile phase, because it has a polar
side chain.
B. Aspartate will move farther with the mobile phase, because it has a nonpolar
side chain.
C. Phenylalanine will move farther with the mobile phase, because it has a polar
side chain.
D. Phenylalanine will move farther with the mobile phase, because it has a
nonpolar side chain.
Answer D. Phenylalanine will move farther with the mobile phase, because it has a nonpolar side chain.
The side chain on aspartate is -CH2COO-, which is very polar, while the side chain on phenylalanine is -CH2Ph, which is
,nonpolar. Since "like dissolves like," and the mobile phase is nonpolar, the molecule which is less polar (phenylalanine,
in this case) will move farther with the mobile phase.
3. SEE OTHERSIDE FIRST
D. Thickness of the membrane Correct Answer
"Charge stored per unit voltage," Q / V, is the definition of capacitance, C. The
equation for the capacitance of a parallel-plate capacitor is C = κε0A / d, where
κ is the dielectric constant, ε0 is a universal constant (the permittivity of free
space), A is the area of each plate, and d is the distance between the plates. Of
the choices given, only D, decreasing the thickness of the membrane (that is,
decreasing d), would increase the capacitance, C.
Answer Ion flow in neurons can be characterized as an electrical circuit for both the resting neuron (Figure
1) and the active neuron (Figure 2). The membrane is a capacitor, slow leakage channels are a 25 MΩ resistor, and the
Na+/K+ pump is a voltage generator. In a resting axon, there is no net transfer of charge across the axon membrane.
, Figure 2 includes the additional Na+ influx (a 4 kΩ resistor) of an action potential. Other ion fluxes are ignored.
According to the figures, decreasing which of the following would create the greatest increase in charge stored per
unit voltage on an axon membrane in its rest state?
A. Leakage channel resistance
B. Na+ channel resistance
C. Area of the membrane surfaces
D. Thickness of the membrane
4. Across the membrane of an axon in its rest state
Answer
A. the potential is higher on the outside of the cell than the inside, and electric
field lines run from the outside to the inside.
B. the potential is higher on the outside of the cell than the inside, and electric
field lines run from the inside to the outside.
QUESTIONS
1. Sickle-cell anemia (HbS) results from a substitution to valine from glutamic
acid at position 6 of the β chain of hemoglobin. Which of the following best
explains why the isoelectric point of HbS is higher than that of HbA?
A. The side chain of glutamic acid is less acidic than that of valine.
B. Glutamic acid is isoelectric at a lower pH than is valine.
C. Valine is isoelectric at a lower pH than is glutamic acid.
D. Glutamic acid has a net charge of 0 at its isoelectric point.
Answer B. Glutamic acid is isoelectric at a lower pH than is valine.
The isoelectric point is the pH at which an amphoteric molecule has a net electric charge of zero. Glutamic acid is more
acidic, not less acidic than valine (A is wrong). Glutamate would require more acidic pH to neutralize its charge (C is
wrong). D is a true, but irrelevant statement. The best answer is B. The negative charge of glutamic acid would require
a more acidic pH to neutralize, making the isoelectric point of HbA (with glutamic acid) lower than that of HbS (with
,valine).
2. A mixture of aspartate and phenylalanine is separated into its component
molecules by thin layer chromatography on a silica plate eluted with benzene.
Which of the following best explains why the separation occurs?
A. Aspartate will move farther with the mobile phase, because it has a polar
side chain.
B. Aspartate will move farther with the mobile phase, because it has a nonpolar
side chain.
C. Phenylalanine will move farther with the mobile phase, because it has a polar
side chain.
D. Phenylalanine will move farther with the mobile phase, because it has a
nonpolar side chain.
Answer D. Phenylalanine will move farther with the mobile phase, because it has a nonpolar side chain.
The side chain on aspartate is -CH2COO-, which is very polar, while the side chain on phenylalanine is -CH2Ph, which is
,nonpolar. Since "like dissolves like," and the mobile phase is nonpolar, the molecule which is less polar (phenylalanine,
in this case) will move farther with the mobile phase.
3. SEE OTHERSIDE FIRST
D. Thickness of the membrane Correct Answer
"Charge stored per unit voltage," Q / V, is the definition of capacitance, C. The
equation for the capacitance of a parallel-plate capacitor is C = κε0A / d, where
κ is the dielectric constant, ε0 is a universal constant (the permittivity of free
space), A is the area of each plate, and d is the distance between the plates. Of
the choices given, only D, decreasing the thickness of the membrane (that is,
decreasing d), would increase the capacitance, C.
Answer Ion flow in neurons can be characterized as an electrical circuit for both the resting neuron (Figure
1) and the active neuron (Figure 2). The membrane is a capacitor, slow leakage channels are a 25 MΩ resistor, and the
Na+/K+ pump is a voltage generator. In a resting axon, there is no net transfer of charge across the axon membrane.
, Figure 2 includes the additional Na+ influx (a 4 kΩ resistor) of an action potential. Other ion fluxes are ignored.
According to the figures, decreasing which of the following would create the greatest increase in charge stored per
unit voltage on an axon membrane in its rest state?
A. Leakage channel resistance
B. Na+ channel resistance
C. Area of the membrane surfaces
D. Thickness of the membrane
4. Across the membrane of an axon in its rest state
Answer
A. the potential is higher on the outside of the cell than the inside, and electric
field lines run from the outside to the inside.
B. the potential is higher on the outside of the cell than the inside, and electric
field lines run from the inside to the outside.
