MCAT Question of the Day

Correct Answer: C. the activation energy is lowered

A catalyst speeds up a chemical reaction by lowering the activation energy of a reaction. The catalyst is never consumed, nor does it alter the reactants or products. The total enthalpy of the reaction is also not changed from what it would have been without the catalyst. Only the rate at which the reaction occurs and the activation energy is affected. The only answer option that is correct is [C].

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Correct Answer: B. XeF2

Start by drawing the Lewis structure of CO2. We find that the molecular shape is linear. Here is a video to help visualize this process: CO2 Structure

Next, find which of the answer choices is also linear by drawing all of the remaining structures. We find that only XeF2 has the same linear molecular shape, even though it has a different hybridization!
Structure of H2O
Structure of XeF2
Structure of SO2
Structure of SCl2

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Correct Answer: A. the ventricles, spinal canal, and meninges.

The organs of the central nervous system (CNS), the brain and spinal cord, have hollow regions (ventricles and spinal canal, respectively). The CNS also contains the meninges, which consists of three layers of protective covering. The question states that CSF circulates in these areas. Thus A is the correct answer. The spinal nerves and cranial nerves are not part of the CNS, they are part of the peripheral nervous system (with the exception of cranial nerve II, but that’s a little advanced for the MCAT).

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Correct Answer: D. The kinetic energy increases to 9 times its original value.

Kinetic energy is: KE = (1/2)mv². The kinetic energy is directly proportional to the square of the velocity (or speed). If the speed is multiplied by 3 (tripled), the KE must be multiplied by 3² = 9. Sidenote, race car spelled backwards is still race car.

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Correct Answer: D. A and C

During the evolution of living systems, there were two ways the velocity of nerve impulse propagation could be increased. The first way was increasing nerve diameter, while the second way was the formation of the myelin sheath. The latter mechanism is less energetically expensive than the former.

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Correct Answer: A. increased production of HCl by parietal cells acts stimulatory on gastrin secretion

Gastrin stimulates parietal cells to release HCl. If A were true, then there would be a positive feedback loop where gastrin is stimulating more HCl production, and HCl is stimulating more gastrin production, causing us to burn a hole through our stomach! Increased production of HCl by parietal cells leads to somatostatin secretion, which inhibits the secretion of gastrin. Lastly, an elevated gastric pH (i.e. low amount of acid) would stimulate the G cells to release more gastrin in an attempt to stimulate the parietal cells to release more HCl to bring the acidity back to normal.

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Correct Answer: D. Both A and B

Many cellular enzymes exhibit decreasing activity with increasing levels of reaction product. This feedback inhibition help regulate the activity of enzymatic processes. Often this process occurs through non-competitive inhibition. The product of the chemical reaction binds to the enzyme in a location other than the active site, which causes a conformational change in the enzyme and decreases enzymatic activity. Allosteric activation describes a process in which enzymatic activity is increased because of binding of some molecule to a site other than the active site.

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Correct Answer: A. 1, 4

Rotation of plane polarized light is indicative of a chiral molecule. If not, the compounds must be meso or achiral. Compounds 1 and 4 are meso compounds, which, despite having two or more stereocenters, are not chiral (superposable on its mirror image). Compounds 2 and 3 are chiral molecules.

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Correct Answer: C. Delta(Grxn) < 0 at only low temperatures

On the MCAT, numbers can be intimidating, but remember that if the answer choices have no numbers in them, we should think conceptually instead of empirically. Gibbs equation helps us determine if a reaction is spontaneous or non-spontaneous. It is given as:
G = H – TS

In this case, the enthalpy change H is a negative number (exothermic). We know that the entropy change (S) is also negative (the products are more ordered than the reactants) because the reactants are 2 moles of gas and the products are only 1 mole of gas (a synthesis of phosphorus pentachloride). Our equation now be read differently as:
G = (negative number) – temperature * (negative number)

Temperature is in Kelvin and thus always a positive number, but we must consider two possibilities regarding temperature: (1) If this positive temperature is relatively low, the second part of the equation is a small positive number. Thus, if you add a small positive number to a negative number, it will probably still be negative (G<0). (2) If the positive temperature is relatively high, the second part of the equation is a large positive number. If you add a large positive number to a negative number, it will probably become positive (G>0).

In both these cases, ambiguity is important because we don’t know the temperature or S. The only answer that appropriately describes the case we have here is C.

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Correct Answer: A. Cyanide blocks the passage of electrons from cytochrome oxidase a3 to oxygen

Cyanide blocks the passage of electrons from cytochrome oxidase a3 to oxygen since it has a very high binding affinity for the enzyme. The cell cannot use oxygen so even if the person can breathe, the cells cannot use the oxygen that is delivered to tissues. Oligomycin inhibits the electron transport chain by directly affecting ATP synthase. A class of poisons called uncouplers makes the lipid bilayer permeable to hydrogen ions, which eliminates the electrochemical gradient.

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