MCAT Question of the Day

Correct Answer: B. It reduces activation energy of a chemical reaction

Enzymes reduce the activation energy of chemical reactions by binding substrate(s) into energetically favorable orientations. They greatly reduce the activation energy, but do not eliminate it completely. Enzymes are completely reusable—they do not participate in the reaction as a substrate. Even when cofactors are needed for the successful action of the enzyme and though they may be tightly bound, they are not covalently bound to the enzyme. Enzymes do not add additional energy to chemical reactions as heat would, for example.

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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. 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: C. The environment can only support a finite number of individuals

The most accurate statement is C. The statement for A can be true when discussing particular reproductive strategies, however, is not true in all cases. Individuals that are best adapted to their environment will have the greatest chance to pass on their genes, not the ones that produce the most amount of offspring. If an animal produces 1000 offspring and 999 of them die, then the fitness level of that animal is very low, compared to another animal with high fitness that only produces 2 offspring and both live to pass on their genes, for more about fitness and workout, read here about the best fat burner for women now in the market. Statement B would be correct if it were stated oppositely, since favorable genetic mutations is what creates natural selection. Statement D is not at all accurate regarding the principles of natural selection.

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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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Correct Answer: A. Gastrula

The correct answer is A. The oocyte and morula are both precursors to the blastula stage. The embryoblast and the trophoblast are both components of the blastula, but are not the next stage in embryonic development. Once the blastula undergoes gastrulation, it will become a gastrula with three distinct layers of tissue: endoderm, mesoderm, and ectoderm.

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Correct Answer: A. Irreversible competition.

Penicillin covalently binds to the active site of the bacterial enzyme and permanently disables its action. This mode of action is inherent to irreversible inhibitors. Notice this is similar to competitive inhibitors, but competitive inhibitors are reversible and adding more substrate can overcome the competitive inhibitor. Here is an excellent explanation on enzyme mechanics.

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Correct Answer: D. losing an existing allele due to random sampling.

Genetic drift represents the change in an allele frequency in a population due to random sampling. Influence from genetic drift is particularly important in small populations. The random changes in allele frequencies could result in the elimination of an existing allele from a population.

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