MCAT 2026 EXAM BIOLOGICAL AND BIOCHEMICAL
FOUNDATIONS OF LIVING SYSTEMS ACTUAL EXAM
QUESTIONS AND CORRECT ANSWERS WITH
RATIONALES GRADED A+ LATEST
Question 1 |
Which statement best explains why oxygen consumption increases when compound
| | | | | | | | |
X is added?
| | |
A. Compound X directly donates electrons to oxygen, increasing consumption. | | | | | | | |
B. Compound X increases proton motive force, stimulating ATP synthase and | | | | | | | | |
raising respiration.
| |
C. Compound X dissipates the proton gradient so the electron transport chain | | | | | | | | | |
operates faster to re-establish it.
| | | | |
D. Compound X inhibits the electron transport chain, causing cells to consume | | | | | | | | | |
more oxygen to compensate.
| | | |
Answer: C. |
Rationale: An uncoupler increases proton leak across the inner mitochondrial
| | | | | | | | |
membrane, reducing the proton motive force. The ETC responds by pumping more
| | | | | | | | | | | |
protons (i.e., increasing electron flow and oxygen consumption) to try to re- establish
| | | | | | | | | | | | |
the gradient. Option B is wrong — uncouplers lower the gradient, not increase it. A is
| | | | | | | | | | | | | | | |
incorrect — uncouplers don't donate electrons. D is wrong — inhibition would
| | | | | | | | | | | |
decrease electron flow and oxygen consumption.
| | | | | |
Question 2 |
Given the oxygen consumption numbers, what does the relative increase in oxygen
| | | | | | | | | | |
consumption with pyruvate/malate vs succinate after uncoupler indicate about upstream
| | | | | | | | | |
NADH vs FADH2 electron input?
| | | | |
A. NADH-linked respiration (Complex I entry) can produce a larger maximal | | | | | | | | |
electron flux than Complex II under these conditions.
| | | | | | | |
,B. FADH2 provides more electrons per substrate oxidation than NADH, so
| | | | | | | | |
succinate supports higher rates.
| | | |
C. Complex II is inhibited by uncoupling agents, causing a smaller increase with
| | | | | | | | | | |
succinate.
|
D. Succinate-driven respiration is more efficient and therefore shows less change | | | | | | | | |
with uncoupling.
| |
Answer: A. |
Rationale: Pyruvate/malate generates NADH that feeds electrons into Complex I;
| | | | | | | | |
succinate generates FADH2 feeding into Complex II. The larger jump with
| | | | | | | | | | |
pyruvate/malate (from 100 to 160) vs succinate (70 to 140) suggests the
| | | | | | | | | | | |
NADH/Complex I pathway supports a larger maximal flux under uncoupled
| | | | | | | | | |
conditions. B is false (both donate 2 electrons but enter at different complexes). C has
| | | | | | | | | | | | | | |
no basis here. D confuses efficiency with flux — uncoupling increases flux but
| | | | | | | | | | | | |
decreases coupling efficiency.
| | |
Question 3 |
If the researcher had added oligomycin (an ATP synthase inhibitor) before adding
| | | | | | | | | | |
compound X, what would you expect for oxygen consumption after adding the
| | | | | | | | | | | |
uncoupler?
|
A. Oxygen consumption would remain low after oligomycin and not increase with
| | | | | | | | | |
the uncoupler.
| |
B. Oxygen consumption would still increase after uncoupler despite oligomycin,
| | | | | | | |
because the uncoupler bypasses ATP synthase.
| | | | | |
C. Oxygen consumption would equal zero because both ATP synthase and proton
| | | | | | | | | |
gradient are blocked.
| | |
D. Oxygen consumption would be higher than with uncoupler alone.
| | | | | | | |
Answer: B. |
Rationale: Oligomycin blocks proton flow through ATP synthase, lowering
| | | | | | | |
respiration because pmf isn't used to make ATP and backpressure reduces ETC flux.
| | | | | | | | | | | | |
Adding an uncoupler creates new proton leak pathways independent of ATP
| | | | | | | | | | |
,synthase, dissipating the gradient and driving ETC activity again — so oxygen
| | | | | | | | | | |
consumption rises even with oligomycin present. Thus B is correct.
| | | | | | | | | |
Question 4 |
Which of the following best describes the effect of an increased proton leak on ATP
| | | | | | | | | | | | | |
yield per oxygen consumed (P/O ratio)?
| | | | | |
A. Proton leak increases the P/O ratio. | | | | |
B. Proton leak does not change the P/O ratio.
| | | | | | |
C. Proton leak decreases the P/O ratio. | | | | |
D. Proton leak initially increases then decreases the P/O ratio.
| | | | | | | |
Answer: C. |
Rationale: Proton leak causes protons to re-enter the matrix without passing through
| | | | | | | | | | |
ATP synthase, so fewer ATP molecules are generated per oxygen consumed (lower
| | | | | | | | | | | |
P/O). Therefore ATP yield per oxygen decreases.
| | | | | | |
Discrete conceptual questions Question
| | |
| 5
Which amino acid residue is most likely to act as a general base in an enzyme active
| | | | | | | | | | | | | | | |
site (i.e., accept a proton during catalysis) at physiological pH?
| | | | | | | | | |
A. Lysine
B. Aspartate
C. Phenylalanine
D. Tyrosine
Answer: B. |
Rationale: Aspartate has a carboxylate side chain (pKa ~3.9) and is deprotonated at
| | | | | | | | | | | |
physiological pH, allowing it to accept a proton transiently (act as a base).
| | | | | | | | | | | | |
Lysine is protonated at physiologic pH (pKa ~10.5) and usually acts as an acid or
| | | | | | | | | | | | | |
electrostatic residue. Phenylalanine is nonpolar. Tyrosine has a pKa ~10 and is
| | | | | | | | | | | |
mostly uncharged at pH 7.4.
| | | | |
, Question 6 |
A mutation changes a codon from UAU (tyrosine) to UAA (stop). What type of mutation
| | | | | | | | | | | | | |
is this and what is the most likely immediate effect?
| | | | | | | | | |
A. Missense mutation — single amino acid substituted, likely mild effect.
| | | | | | | | |
B. Nonsense mutation — premature termination leading to truncated protein.
| | | | | | | |
C. Silent mutation — no change in amino acid or function.
| | | | | | | | |
D. Frameshift mutation — altered reading frame downstream. | | | | | |
Answer: B. |
Rationale: UAA is a stop codon; changing a tyrosine codon (UAU) to UAA introduces a
| | | | | | | | | | | | | |
premature termination codon — a nonsense mutation — resulting in a truncated protein,
| | | | | | | | | | | | |
likely loss of function or nonsense-mediated decay.
| | | | | | |
Question 7 |
Which bond in DNA is most directly broken by DNase I?
| | | | | | | | | |
A. Phosphodiester backbone between nucleotides | | |
B. Glycosidic bond between base and sugar | | | | |
C. Hydrogen bonds between base pairs | | | |
D. Disulfide bonds in associated proteins | | | |
Answer: A. |
Rationale: DNase I is an endonuclease that cleaves phosphodiester bonds within the
| | | | | | | | | | |
DNA backbone. It does not cleave glycosidic bonds, hydrogen bonds (noncovalent),
| | | | | | | | | | |
or protein disulfide bonds.
| | | |
Question 8 |
During pulse-chase labeling with [35S]-methionine to study a secreted protein,
| | | | | | | | |
radioactivity first appears in the rough ER, then Golgi, then extracellular medium. Which
| | | | | | | | | | | | |
cellular process is this experiment is demonstrating?
| | | | | | |
FOUNDATIONS OF LIVING SYSTEMS ACTUAL EXAM
QUESTIONS AND CORRECT ANSWERS WITH
RATIONALES GRADED A+ LATEST
Question 1 |
Which statement best explains why oxygen consumption increases when compound
| | | | | | | | |
X is added?
| | |
A. Compound X directly donates electrons to oxygen, increasing consumption. | | | | | | | |
B. Compound X increases proton motive force, stimulating ATP synthase and | | | | | | | | |
raising respiration.
| |
C. Compound X dissipates the proton gradient so the electron transport chain | | | | | | | | | |
operates faster to re-establish it.
| | | | |
D. Compound X inhibits the electron transport chain, causing cells to consume | | | | | | | | | |
more oxygen to compensate.
| | | |
Answer: C. |
Rationale: An uncoupler increases proton leak across the inner mitochondrial
| | | | | | | | |
membrane, reducing the proton motive force. The ETC responds by pumping more
| | | | | | | | | | | |
protons (i.e., increasing electron flow and oxygen consumption) to try to re- establish
| | | | | | | | | | | | |
the gradient. Option B is wrong — uncouplers lower the gradient, not increase it. A is
| | | | | | | | | | | | | | | |
incorrect — uncouplers don't donate electrons. D is wrong — inhibition would
| | | | | | | | | | | |
decrease electron flow and oxygen consumption.
| | | | | |
Question 2 |
Given the oxygen consumption numbers, what does the relative increase in oxygen
| | | | | | | | | | |
consumption with pyruvate/malate vs succinate after uncoupler indicate about upstream
| | | | | | | | | |
NADH vs FADH2 electron input?
| | | | |
A. NADH-linked respiration (Complex I entry) can produce a larger maximal | | | | | | | | |
electron flux than Complex II under these conditions.
| | | | | | | |
,B. FADH2 provides more electrons per substrate oxidation than NADH, so
| | | | | | | | |
succinate supports higher rates.
| | | |
C. Complex II is inhibited by uncoupling agents, causing a smaller increase with
| | | | | | | | | | |
succinate.
|
D. Succinate-driven respiration is more efficient and therefore shows less change | | | | | | | | |
with uncoupling.
| |
Answer: A. |
Rationale: Pyruvate/malate generates NADH that feeds electrons into Complex I;
| | | | | | | | |
succinate generates FADH2 feeding into Complex II. The larger jump with
| | | | | | | | | | |
pyruvate/malate (from 100 to 160) vs succinate (70 to 140) suggests the
| | | | | | | | | | | |
NADH/Complex I pathway supports a larger maximal flux under uncoupled
| | | | | | | | | |
conditions. B is false (both donate 2 electrons but enter at different complexes). C has
| | | | | | | | | | | | | | |
no basis here. D confuses efficiency with flux — uncoupling increases flux but
| | | | | | | | | | | | |
decreases coupling efficiency.
| | |
Question 3 |
If the researcher had added oligomycin (an ATP synthase inhibitor) before adding
| | | | | | | | | | |
compound X, what would you expect for oxygen consumption after adding the
| | | | | | | | | | | |
uncoupler?
|
A. Oxygen consumption would remain low after oligomycin and not increase with
| | | | | | | | | |
the uncoupler.
| |
B. Oxygen consumption would still increase after uncoupler despite oligomycin,
| | | | | | | |
because the uncoupler bypasses ATP synthase.
| | | | | |
C. Oxygen consumption would equal zero because both ATP synthase and proton
| | | | | | | | | |
gradient are blocked.
| | |
D. Oxygen consumption would be higher than with uncoupler alone.
| | | | | | | |
Answer: B. |
Rationale: Oligomycin blocks proton flow through ATP synthase, lowering
| | | | | | | |
respiration because pmf isn't used to make ATP and backpressure reduces ETC flux.
| | | | | | | | | | | | |
Adding an uncoupler creates new proton leak pathways independent of ATP
| | | | | | | | | | |
,synthase, dissipating the gradient and driving ETC activity again — so oxygen
| | | | | | | | | | |
consumption rises even with oligomycin present. Thus B is correct.
| | | | | | | | | |
Question 4 |
Which of the following best describes the effect of an increased proton leak on ATP
| | | | | | | | | | | | | |
yield per oxygen consumed (P/O ratio)?
| | | | | |
A. Proton leak increases the P/O ratio. | | | | |
B. Proton leak does not change the P/O ratio.
| | | | | | |
C. Proton leak decreases the P/O ratio. | | | | |
D. Proton leak initially increases then decreases the P/O ratio.
| | | | | | | |
Answer: C. |
Rationale: Proton leak causes protons to re-enter the matrix without passing through
| | | | | | | | | | |
ATP synthase, so fewer ATP molecules are generated per oxygen consumed (lower
| | | | | | | | | | | |
P/O). Therefore ATP yield per oxygen decreases.
| | | | | | |
Discrete conceptual questions Question
| | |
| 5
Which amino acid residue is most likely to act as a general base in an enzyme active
| | | | | | | | | | | | | | | |
site (i.e., accept a proton during catalysis) at physiological pH?
| | | | | | | | | |
A. Lysine
B. Aspartate
C. Phenylalanine
D. Tyrosine
Answer: B. |
Rationale: Aspartate has a carboxylate side chain (pKa ~3.9) and is deprotonated at
| | | | | | | | | | | |
physiological pH, allowing it to accept a proton transiently (act as a base).
| | | | | | | | | | | | |
Lysine is protonated at physiologic pH (pKa ~10.5) and usually acts as an acid or
| | | | | | | | | | | | | |
electrostatic residue. Phenylalanine is nonpolar. Tyrosine has a pKa ~10 and is
| | | | | | | | | | | |
mostly uncharged at pH 7.4.
| | | | |
, Question 6 |
A mutation changes a codon from UAU (tyrosine) to UAA (stop). What type of mutation
| | | | | | | | | | | | | |
is this and what is the most likely immediate effect?
| | | | | | | | | |
A. Missense mutation — single amino acid substituted, likely mild effect.
| | | | | | | | |
B. Nonsense mutation — premature termination leading to truncated protein.
| | | | | | | |
C. Silent mutation — no change in amino acid or function.
| | | | | | | | |
D. Frameshift mutation — altered reading frame downstream. | | | | | |
Answer: B. |
Rationale: UAA is a stop codon; changing a tyrosine codon (UAU) to UAA introduces a
| | | | | | | | | | | | | |
premature termination codon — a nonsense mutation — resulting in a truncated protein,
| | | | | | | | | | | | |
likely loss of function or nonsense-mediated decay.
| | | | | | |
Question 7 |
Which bond in DNA is most directly broken by DNase I?
| | | | | | | | | |
A. Phosphodiester backbone between nucleotides | | |
B. Glycosidic bond between base and sugar | | | | |
C. Hydrogen bonds between base pairs | | | |
D. Disulfide bonds in associated proteins | | | |
Answer: A. |
Rationale: DNase I is an endonuclease that cleaves phosphodiester bonds within the
| | | | | | | | | | |
DNA backbone. It does not cleave glycosidic bonds, hydrogen bonds (noncovalent),
| | | | | | | | | | |
or protein disulfide bonds.
| | | |
Question 8 |
During pulse-chase labeling with [35S]-methionine to study a secreted protein,
| | | | | | | | |
radioactivity first appears in the rough ER, then Golgi, then extracellular medium. Which
| | | | | | | | | | | | |
cellular process is this experiment is demonstrating?
| | | | | | |
