MCAT Question of the Day

Correct Answer: B. An inhibitor binds directly to the active site and prevents the reactant from binding

The correct answer is B. Metal cofactors and ATP are often used to help boost enzyme activity, and are not competitive inhibitors. Non-competitive inhibition(D) occurs when an inhibitor is able to prevent the enzyme from binding with the reactant by binding to the enzyme at a site away from the active site, and change the enzyme’s conformation so it cannot bind to the reactant. Competitive inhibition occurs when the inhibitor competes directly with the reactant at the active site, and this substrate takes the place of the reactant and prevents the reaction from occurring.

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Correct Answer: B. Reflex Arc

A reflex arc is a nervous system process whereby the stimulus causes a motor movement that is coordinated by the spinal cord and not the brain. Sensory neurons are able to capture the environmental stimuli and send the afferent signal to the spinal cord, which is able to send an efferent (motor) signal directly to the effector, a muscle, which then moves. Plasticity refers to the brain’s ability to repair itself following damage. Metabolism is the general term defining the cell’s use and production of energy. Redirection is not an accurate term and is not used in biology. Depolarization is the process of changing the cell membrane’s to become more positive, commonly observed when in the nervous system with the propagation of an action potential. Therefore the correct answer is [B].

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Correct Answer: D. 30N.

Since the object is not moving, its acceleration must be zero, so the net force on it must also be zero. Therefore, the magnitude of the buoyant force upward must equal the magnitude of the weight downward, the specific gravity is extraneous information:

B = w = mg = (3 kg)(10 N/kg) = 30 N

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Correct Answer: C. Vacuole

The correct answer is C. Cells within the Kingdom Plantae have chlorophyll containing cells called CHLOROPLASTS, a rigid cell WALL outside of the plasma membrane, and vacuoles that serve to store carbohydrates. Plant cells do not contain lysosomes, as other eukaryotic cells, instead they have peroxisomes for digestion of foreign material.

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Correct Answer: C. The k value would increase, but only slightly.

The correct answer is C. The Arrhenius equation is given by k = Ae^-E/RT, where A is the pre-exponential factor, R is the gas constant, T is the temperature, and E is the activation energy. Thus, if the temperature were increased by a factor of 4, then the exponent would decrease by a factor of 4, since T is in the denominator. However, since it is e^-x, the actual value won’t change much. For example, exp(-1) = 0.36, but exp(-1/4) = 0.77. Thus, although the value of k will increase, it will not increase by a factor of 4, only slightly.

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Correct Answer: A. PCC/CH2Cl2.

Pyridinium chlorochromate (PCC) is a mild oxidant, which converts primary and secondary alcohols to aldehydes and ketones respectively. KMnO4 and K2Cr2O7 are harsher oxidants, and would further oxidize primary alcohols to the corresponding carboxylic acids. Di-isobutylaluminum hydride (DIBAL) is a reducing agents.

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Correct Answer: C. Stratum corneum

If a layer of epidermis has a low rate of mitosis, this indicates that the cells in this layer are not dividing very frequently. Likewise the presence of keratin, signals that the layer of epidermis must have a waterproofing function. The stratum basale is the most mitotic layer and is the bottom layer of the epidermis. As the cells are produced in the stratum basale, older cells are pushed up to the stratum spinosum, which is also high in mitotic activity. Keratin dominates in the stratum corneum, which is the most superficial layer of the epidermis, characterized by dead skin cells that are no longer undergoing mitosis. Therefore, the correct answer is [C].

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Correct Answer: D. Cl2, FeCl3

This is an electrophilic ring transformation; it proceeds efficiently via electrophilic chlorination (“Cl+”) of the aromatic ring, which usually involves chorine plus a suitable Lewis acid (aluminum chloride).

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Correct Answer: C. 3

Shown here is a Wittig reaction, or Wittig Olefination, which converts aldehydes to alkenes through reaction with a phosphonium ylide.

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Correct Answer: B. 10 m.

All of the kinetic energy will be transferred into potential energy at the maximum height of the cannonball. The trick is to not replace KE with (1/2)mv², the kinetic energy is already given! Therefore:

KE -> PE -> KE = PE -> KE = mgh -> h = KE/mg = 500 J / (5 kg)(10 N/kg) = 10 m.

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