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: 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: B. Polyphyletic and heterotrophic.

Fungi are a classified as a group based upon common biological traits acquired through convergent evolution, without a common ancestor, and are therefore considered polyphyletic. Unlike plants, fungi cannot make their own food supply. Instead, they obtain nutrition by parasitism, saprobism or symbiosis.

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Correct Answer: B. The shark and the dolphin

Convergent evolution is the process where organisms from different ancestral linage, grow to look alike and share similar physical characteristics because they live in the same or very similar environments. Whereas divergent evolution is defined as the evolutionary process whereby organisms from the same ancestral lineage differ in appearance. This difference would result because of the different habitats of the organisms. Only the shark and the fish are from different animal lineages and look very similar because they both live in aquatic environments. The other animal pairings all originate from a similar ancestral lineage and have grown to look very different over time because of their terrestrial habitats.

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Correct Answer: B. They all contain codons.

There are 3 types of RNA: (1) ribosomal RNA (rRNA); (2) messenger RNA (mRNA) and (3) transfer RNA (tRNA). RNA is a single-stranded polymer made up of four bases, namely uracil, adenine, guanine and cytosine. Only mRNA can contain a codon, a genetic code consisting of three base pairs that can be translated by the cellular machinery into an amino acid during protein synthesis. RNA elongates elongates from the 5? to 3? direction using a DNA strand with the opposite orientation as a template for transcription.

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Correct Answer: D. Hypothalamus

The direction of release is as follows: hypothalamus -> (TRH) -> pituitary -> (TSH) -> thyroid.

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Correct Answer: D. Bladder

The parasympathetic fibers travel mainly through the vagus nerves and the anterior roots of the sacral segments of the spinal cord. They are not as extensively distributed in the body as the sympathetic fibers and their ganglia (= collection of nerve cell bodies outside the CNS, eliminating answer choice B.) are located near or within the visceral organs. From the answer choices provided, the bladder is the only organ innervated by the parasympathetic system. Recall that the parasympathetic system is important in the ‘vegetative responses’ of the body.

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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: 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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