MCAT - Chemical and Physical
Foundations of Biological Systems
Latest Exam set with well detailed and
simplified correct Answers
Cytochrome c oxidase - correct answer-An enzyme responsible for electron transfer in the ETC, so
inhibiting it would slow or stop the ETC.
Dosing of Blast Phase Chronic Myeloid Leukemia (BP-CML) cells with cyanide would result in: - correct
answer-Reduced intercellular ATP and apoptosis or necrosis.
Cyanide binds to cytochrome c oxidase and prevents electron transfer. Inhibiting cytochrome C oxidase
would decrease ATP production. thus, large dose would also cause cell death (apoptosis or necrosis).
Researchers also measure oxygen consumption and intercellular NADH levels in BP-CML cells after
exposure to pyrivinium. What did they most likely observe? - correct answer-An increase in NADH levels
and a decrease in O2 consumption.
According to the study, pyrivinium inhibits intracellular signaling, is effective against cancer cells and
involves heavily in the inhibition of mitochondrial respiration. Because MR consumes oxygen, pyrivinium
should reduce oxygen consumption.
NADH supplies electrons to the ETC, so if the ETC is not functioning properly, the NADH produced by the
citric acid cycle would not be OXIDIZED to NAD+ and H+.
NADH - correct answer-Its oxidized form is (NAD+) and reduced for is (NADH).
Compounds that decouple the proton gradient across the inner mitochondrial membrane would: -
correct answer-Not have a strong effect on cells exposed to pyrivinium.
Decoupling the proton gradient across the inner mitochondrial membrane would be redundant in
pyruvinium-exposed cells, since MR is already functioning poorly.
Which of the following would best explain the observation that pyrivinium is more effective against BP-
CML than colon cancer? - correct answer-Colon cancers rely less on glycolysis for energy production than
BP-CML.
,According to the study, pyrivinium is effective against cancers in glucose-starvation conditions.
Therefore, if colon cancers rely less on glycolysis then we would expect pyrivinium to be less effective
against colon cancers and thus more effective against BP-CML.
Which of the following best describe why hyperventilation would lead to a decreased blood CO2
concentration? - correct answer-The rate of CO2 exhalation exceeds the rate of CO2 production in the
cells.
The main source of CO2 in the human body is cellular production during the Krebs cycle. If an individual
breathes very rapidly, he will exhale CO2 more quickly than his cells produce it.
pH - correct answer-The pH of a buffer solution can be calculated by pH= pKa + log [conjugated base]/
[acid]
Which of the following best describes how a shift to the right in the dissociation curve results in more
oxygen delivered to tissue? - correct answer-(Question #6) The distance between the curves at
100mmHg is less than the distance between the curves at 40mmHg.
When the curve is shifted to the right, the oxygen affinity of hemoglobin at high partial pressure is less
affected than it is at lower pressures.
At a PO2 of 100mmHg, the difference in % saturation between a normal and a right shifted curve is
minimal, but at 40 mmHg, the difference is more profound. As a result, the amount of oxygen that each
red blood cell picks up in the alveoli of the lungs is not significantly decreased, but the amount of oxygen
release in the tissues increases markedly.
Note: in the lungs, hemoglobin picks up O2 from the alveoli, but in the tissue, the affinity for oxygen
must be lowered so that it can diffuse into the tissues.
Relationship between pH and Oxygen Affinity - correct answer-When pH increases, hemoglobin binds
oxygen more readily and vice versa.
In RBC, bicarbonate can be exchanged for chloride in plasma. Why do RBC in venous circulation have a
higher concentration of chloride ions? - correct answer-Answer: Higher concentration of CO2 in venous
erythrocytes produce bicarbonate and drive the exchange of Cl- from the plasma into the cell.
The question stem tell us the RBC can exchange chloride from the plasma with bicarbonate anions.
When more bicarbonate is available, there should thus be a higher concentration of chloride ions in the
cells.
, Know that an increase in CO2 corresponds to an increase in bicarbonate concentration in erythrocytes,
more chloride should take its place.
Most CO2 is not found in the plasma.
Also, know that hyperventilation results in CO2 being expelled from the blood causing the pH to rise. In
response, the buffer needs to release more H+ to lower the pH back to physiological norms.
Which of the following cannot explain a patient who present with a blood pH of 7.0
A. Anaerobic respiration leading to lactic acid buildup
B. Hypoventilation
C. Carbonic anhydrase working only in the reverse direction
D. Underproduction of bicarbonate - correct answer-Answer: C
A blood pH of 7.0 is very low (normally 7.4). Thus, it must be attributed to a mechanism that will
increase hydrogen ion concentration. Therefore, the correct answer will be one that would have an
opposite effect on pH or no effect.
We know that carbonic anhydrase catalyzes the reaction in H20 + CO2 <->H2CO3 <->H+ + HCO3-
Thus, if carbonic anhydrase only worked in the reverse direction, there would be more CO2 and H20 and
less H+ in the blood. Therefore, blood pH would increase. (Not a decrease in pH).
It's not A, because lactic acid is acidic.
It's not B, because hypoventilation result in respiratory acidosis, which would lower the pH.
It's not D, because bicarbonate is a weak base, so decrease bicarbonate production would be expected
to make the blood less basic.
A medical student assisting with the research is interested in applying the results of the experiment to
spinal cord injury in humans. The principal investigator responds that this study does not apply to such
injury, why? - correct answer-(Question #9) the study only examined the behavior of macrophages in
response to uninjured nerve cells and astrocytes.
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