MCAT Question of the Day

Correct Answer: B. The capillary wall and the cell surface membrane

The ISF (interstitial fluid) is the fluid outside the blood vessels while the blood plasma is the fluid within the blood vessels. Thus, the two fluids are separated by the wall of the blood vessels (i.e. the capillary wall). The ICF (intracellular fluid) of the cells bathing in the ISF are separated from their environment by the cell surface membrane, in accordance with answer choice B. You should be aware that human cells, unlike bacteria, do not contain a cell wall.

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Correct Answer: D. RbCl

Ionic character increases with increasing electronegativity of the potential anion and decreasing electronegativity of the potential cation. Electronegativity increases as one moves to the top right-hand corner of the periodic table, and decreases in the opposite direction.

Since the potential anion of all four compounds in question is chloride, we only need to find the compound with the cation located furthest toward the bottom left-hand corner of the periodic table. This compound is RbCl.

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Correct Answer: C. SOCL2, then NaCN

In this reaction, there is a substitution with net retention of configuration at the chiral (stereogenic) center. An SN2 process leads to a single inversion of configuration, whereas, two consecutive SN2 processes will result in the required net retention (double inversion).

In process A, a sulfonateester leaving group is initially formed (with no change in configuration) and then cyanide displaces the leaving group with inversion; i.e., the enantiomer of the required product is formed. In process C, the alcohol is converted by addition of thionyl chloride (SOCl2) to the alkyl chloride with inversion of conversion. Displacement of the chloride by the cyanide anion (with another inversion of configuration) leads to the required product.

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Correct Answer: D. both the temperature and pressure of the gas.

The word “kinetic” means movement or motion. An increase in motion means that there is an increase in kinetic energy which we know is directly related to temperature (CHM 4.1.2). When there is an increase in motion, that means there is an increase in the force of the molecules hitting the walls of the container. Pressure is force per unit area (PHY 6.1.2) so the pressure must increase if the motion increases.

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Correct Answer: B. Initiating uterine contraction prior to birth

The placenta is formed from cells of the chorion of the embryo and the maternal endometrium, following implantation of the fertilized egg into the uterus. The placenta has 3 main functions:

1. the transfer of nutrients from the mother to the fetus and the transfer of wastes from the fetus to the mother (answer choice A.);

2. the production of nutrients for energy (answer choice C.), and

3. the production and secretion of hormones such as hCG, estrogen and progesterone (answer choice D.).

Note that IgG is the only isotype that can pass through the human placenta. An increase in the hormone oxytocin, produced by the posterior pituitary, is associated with the initiation of uterine contractions.

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Correct Answer: D. It would not be oxidized.

Carefully notice that this diol has both alcohol substituents attached to tertiary carbons. Tertiary alcohols are very, very difficult to oxidize and it certainly will not occur under mild oxidizing conditions.

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Correct Answer: C. Initial buoyant force < maximum buoyant force

Tricky! You must recognize that the half immersed block would submerge as long as its density is greater than 1/2 the density of water or a specific gravity of 0.5. Thus this block may: (i) have a specific gravity between 0.5 and 1.0 (which means the block’s density would be less than that of water); or (ii) it could have a density equal to that of water; but (iii) it could NOT have a density greater than that of water (this would make it sink to the bottom but the question says that it remains afloat so (iii) is not possible): nonetheless, it would submerge from a position of being held half immersed [thus answer choice B. is not the best answer because of the preceding points (i) and (ii)]. When the block sinks, it will certainly displace more water than it did initially; therefore, the buoyant force increases to its maximum. {cf. Archimedes Principle}

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Correct Answer: A. the process in which ATP is formed as electrons are transferred from electron carriers to oxygen.

Oxidative phosphorylation is defined by A; B may be true but it is an inadequate description of oxidative phosphorylation; C describes the intermediate step between glycolysis and the Krebs cycle; D describes the Krebs cycle.

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Correct Answer: C. ^aO⁰, ^bC⁰ and ^cN^-1

The normal valency of carbon is 4, oxygen is 2 and nitrogen 3. Adding electrons to make a full octet means adding two lone pairs to both oxygen and nitrogen atoms. For a -1 charge this must therefore be situated on N as it only has two bonding pairs of electrons instead of three.

If you ever get stuck during the exam, just take a moment to consider what you know for sure: (1) when oxygen is bonded twice, it has no formal charge (i.e. water); (2) when carbon is bonded 4 times, it has no formal charge (i.e. methane); (3) when nitrogen is bonded 4 times, it has a positive charge (i.e. ammonium), 3 times and it’s neutral (i.e. ammonia) and so just twice, it must be negatively charged.

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Correct Answer: D. Electric potential

Fields are vectors because they cause forces, and force is a vector. Current is a vector because charge can flow in either direction in a wire. However, electric potential is related to potential energy (PE = qV), and all forms of energy are scalar quantities.

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