MCAT Practice Test | 2026-2027 | Complete
Questions with Answers and Step-by-Step
Explanations to Ace the MCAT Exam
Question 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.
Correct Answer: B. Glutamic acid is isoelectric at a lower pH than is valine.
Rationale: 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).
Question 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.
Correct Answer: D. Phenylalanine will move farther with the mobile phase, because it has a
nonpolar side chain.
,Rationale: 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.
Question 3
Ion flow in neurons can be characterized as an electrical circuit for both the resting neuron
and the active neuron. 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
Correct Answer: D. Thickness of the membrane
Rationale: "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.
Question 4
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.
,Correct 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.
Rationale: 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.
Question 5
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.
Correct Answer: D. RNAs are insusceptible to alkaline hydrolysis.
Rationale: 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).
Question 6
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
Correct Answer: D. 30 cm
Rationale: For rotational equilibrium, the torques on both sides must be equal: (4kg × g × x) =
(1kg × g × (100 - x)). Simplifying: 4x = 100 - x, 5x = 100, x = 20 cm from the 4kg weight. Since the
stick is 100 cm long, the rope must be attached 30 cm from the center (50 cm - 20 cm = 30 cm).
, Question 7
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.
Correct Answer: B. power of each laser pulse.
Rationale: 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).
Question 8
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 1.06 µm, 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.
Correct Answer: D. a smaller energy difference between their normal state and their excited
state.
Rationale: The wavelength of the CO₂ laser is 10 times greater than the 1.06 µm wavelength
laser. Therefore, the frequency and the energy of the CO₂ laser are 10 times lower. Since the
laser light is the energy released by transitions of electrons dropping to a lower energy level,
less emitted energy implies a smaller difference between the energy levels. The mass of an
electron is independent of its atomic or molecular energy state (A and B are wrong).
Question 9
Questions with Answers and Step-by-Step
Explanations to Ace the MCAT Exam
Question 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.
Correct Answer: B. Glutamic acid is isoelectric at a lower pH than is valine.
Rationale: 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).
Question 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.
Correct Answer: D. Phenylalanine will move farther with the mobile phase, because it has a
nonpolar side chain.
,Rationale: 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.
Question 3
Ion flow in neurons can be characterized as an electrical circuit for both the resting neuron
and the active neuron. 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
Correct Answer: D. Thickness of the membrane
Rationale: "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.
Question 4
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.
,Correct 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.
Rationale: 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.
Question 5
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.
Correct Answer: D. RNAs are insusceptible to alkaline hydrolysis.
Rationale: 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).
Question 6
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
Correct Answer: D. 30 cm
Rationale: For rotational equilibrium, the torques on both sides must be equal: (4kg × g × x) =
(1kg × g × (100 - x)). Simplifying: 4x = 100 - x, 5x = 100, x = 20 cm from the 4kg weight. Since the
stick is 100 cm long, the rope must be attached 30 cm from the center (50 cm - 20 cm = 30 cm).
, Question 7
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.
Correct Answer: B. power of each laser pulse.
Rationale: 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).
Question 8
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 1.06 µm, 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.
Correct Answer: D. a smaller energy difference between their normal state and their excited
state.
Rationale: The wavelength of the CO₂ laser is 10 times greater than the 1.06 µm wavelength
laser. Therefore, the frequency and the energy of the CO₂ laser are 10 times lower. Since the
laser light is the energy released by transitions of electrons dropping to a lower energy level,
less emitted energy implies a smaller difference between the energy levels. The mass of an
electron is independent of its atomic or molecular energy state (A and B are wrong).
Question 9
