MCAT Question of the Day

Correct Answer: B. 30^o.

Pressure is force over area. The cross-sectional area of the of the circulatory system is not changing. The direction of the force appears to be changing. But the blood is still flowing orthogonal to the area, creating the same pressure due to the heart. The only additional element is that gravitational force is being applied.

This additional gravitation force opposes the heart force by a factor of sin(theta) times its weight. We are told that the blood acceleration is equal to g. All we need to find now is when the sin is equal to one half. This will yield a pressure of one half of the horizontal value. This occurs at 30^o.

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Correct Answer: A. LiAlH4, followed by HCl

Lithium aluminium hydride is a powerful reducing agent and can reduce all carbonyl containing groups, such as aldehydes, esters, ketones, acids, amides etc.

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Correct Answer: C. S¹⁺ and O^1-

Sulfur (S) is below oxygen in the 6th group in the periodic table. They both have 6 valence electrons and thus can fulfill the octet in the following Lewis structures:

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Correct Answer: D. All of them

The range 3500-3000 cm^-1 is typically associated with stretches between heteroatoms and a hydrogen atom, for example NH and OH bonds. They may appear broad, intense and jagged (COOH), broad and smooth (OH) or less intense and sharper (NH). The preceding sentence is likely more than you would need to know for the MCAT. In general, for MCAT purposes, it is useful to remember approx. 3300 for hydroxyl and approx. 1700 for carbonyl (C=O).

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Correct Answer: C. BF3 and Me3C⁺

A Lewis acid is an electron pair acceptor by virtue of it having an empty orbital. Both BH3 and CH3C⁺ are electron deficient, and therefore are Lewis acids. {Mnemonic: lEwis Acid = Electron Acceptor}

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Correct Answer: A. 4F

Keeping Coulomb’s law in mind, if you reduce the distance by 1/2, then the force increases by the inverse squared of 1/2 which is 4 times. Note the question is only asking about the magnitude of the forces as opposed to their directions. If the direction were asked, then the sign of the charge (i.e. positive or negative) would matter.

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Correct Answer: B. transports a fatty acid.

The diagram (top left corner) shows long chain fatty acids going through the plasma membrane converted to acyl CoA within the mitochondrion (between the mitochondrial double membranes) where it combines with carnitine (catalyzed by CPT I) to produce acylcarnitine, which can cross the inner mitochondrial membrane (IMM). Thus the figure suggests carnitine attaches to a fatty acid thus the fatty acid is a part of the acylcarnitine molecule.

Note that, according to the diagram, carnitine and acylcarnitine are not membrane transporters but rather they are transported through membrane transporters (i.e. “Transporter” and “Translocase”, respectively).

Acylcarnitine’s “job” seems over as it is converted back to carnitine and acyl CoA which, as fatty acids are known to do, undergoes beta-oxidation. In summary, acylcarnitine transported long-chain fatty acids across the IMM.

Background information: A membrane transport protein (= transporter) helps move ions or molecules across the plasma membrane. Transporters are integral (= intrinsic) membrane proteins, thus they span the membrane engaging in carrier-mediated transport (without the need for energy: facilitated diffusion; with the need for energy: active transport).

Most biology MCAT questions do not require discrete knowledge but rather the skill to efficiently interpret graphs, tables and/or diagrams – often without previous specific knowledge. To summarize the diagram provided: (1) Fatty acids -> Acyl CoA which attaches to Carnitine; (2) Acyl CoA + Carnitine -> Acylcarnitine which crosses the IMM through the membrane transporter Translocase then -> Acyl CoA + Carnitine is regenerated; (3) Acyl CoA has now crossed the IMM and then can be oxidized.

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Correct Answer: D. All of the above

All the mentioned forms of compounds in this question show no rotation of plane polarized light. Racemic mixtures showing no rotation of plane polarized light, like a 50-50 mix of D and L compounds, are exact racemic mixtures of two enantiomeric forms of a compound thus cancelling the polarization effect of plane polarized light allowing for optical isomerization. A meso compound is superposable on its mirror image meaning that the molecule is achiral (i.e. optically inactive) though it may contain chiral centers.

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Correct Answer: B. As pKa increases, acidity decreases.

A very low acid dissociation constant Ka means that the acid is weak and thus very little product is made. Since pKa is -logKa, the negative sign means they are always opposites. Thus low Ka means high pKa.

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Correct Answer: B. B

SN2 reactions proceed with inversion of configuration through backside displacement, so molecule A cannot be formed in this case. Molecule C is the product from elimination.

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