Princeton Review MCAT Practice
Test Questions and Answers 100%
Correct
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).
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.
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
Across the membrane of an axon in its rest state:
,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.
C. the potential is lower on the outside of the cell than the inside, and electric field
lines run from the outside to the inside.
D. the potential is lower on the outside of the cell than the inside, and electric field
lines run from the inside to the outside. - 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.
The voltage across the membrane is -70 mV from the exterior to the interior of the
cell; thus, the potential is higher on the exterior of the cell (C and D are wrong).
Since the potential is higher on the exterior than in the interior, the electric field
lines must point from the outside to the inside (choice A). IN PHYSICS, FIELD LINES
GO TOWARD THE MORE NEGATIVE AREA.
Which of the following statements regarding RNA molecules is NOT true?
A. RNAs can act as enzymes to catalyze reactions.
B. Some RNAs have more than four different types of bases.
C. Some RNAs are synthesized in the nucleolus.
D. RNAs are insusceptible to alkaline hydrolysis. - ANSWER - D. RNAs are
insusceptible to alkaline hydrolysis.
RNA molecules have decreased stability compared to DNA in part because of their
susceptibility to alkaline hydrolysis due to the presence of hydroxyl group at 2'-C
position (choice D is not true of RNA and is the correct answer choice). Some RNAs
have enzymatic function (such as in telomerase) and they are termed ribozymes (A
is true). tRNA has unique and modified bases apart from the traditional four bases
A,U,C, and G (such as inosine, B is true). rRNA is synthesized in the nucleolus (C is
true).
Two blocks are suspended from the ends of a massless meter stick; a 4kg weight
from one side and a 1kg weight from the opposite side. How far from the center of
the stick must the rope be attached in order to maintain rotational equilibrium?
, A. 10 cm
B. 20 cm
C. 25 cm
D. 30 cm - ANSWER - D. 30 cm
A Q-switched laser can be used to treat skin blemishes and to remove tattoos. The
Q switch momentarily interrupts the inducing light creating a build-up of energy
within the crystal. This does not increase the overall energy of the laser, but
concentrates it into shorter time periods or pulses. A longer interruption with the Q-
switch most likely would increase the:
A. total amount of work done by the laser.
B. power of each laser pulse.
C. wavelength of the laser light.
D. frequency of the laser light. - ANSWER - B. power of each laser pulse.
Since the overall energy of the laser does not change, neither will the frequency,
wavelength, nor work done by the laser. This leaves choice B: Concentrating the
energy into a shorter time period increases the power of each pulse (since power
equals energy delivered per unit time, by definition).
A CO2 laser used as a laser scalpel produces a beam of laser light with a
wavelength of 10.6 µm. Compared to the excited electrons in a laser with a
wavelength of 1.06um, the excited electrons in the CO2 laser most likely have:
A. greater mass.
B. less mass.
C. a greater energy difference between their normal state and their excited state.
D. a smaller energy difference between their normal state and their excited state. -
ANSWER - D. a smaller energy difference between their normal state and their
excited state.
The wavelength of the CO2 laser is 10 times greater than the 1.06 wavelength laser.
Therefore, the frequency and the energy of the CO2 laser are 10 times lower. Since
