MCAT Biology : Other Reaction Mechanisms

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Other Reaction Mechanisms

The transformation of compound B to compound C below is known as what type of reaction?

Mcat_1

Possible Answers:

Hydroboration

Hydrogenation

Dehydration

Hydration

Decarboxylation

Correct answer:

Dehydration

Explanation:

The conversion of compound B to compound C results in the elimination of water, which, by definition, is a dehydration reaction. The hydroxyl group on compound B is protonated by the sulfuric acid, generating an leaving group and allowing the formation of the alkene product in compound C.

Hydroboration is the oxidation of an alkene with a borohydride (usually sodium borohydride) reagent, to produce an alkane. Hydration involves the use of a water reactant, usually producing an alcohol product. Hydrogenation is the oxidation of alkene double bonds with the use of a palladium interface to produce an alkane product. Decarboxylation results in product carbon dioxide from a carboxylic acid or carbonic anhydrase reactant.

Example Question #1 : Other Reaction Mechanisms

Which alcohol will react most rapidly via an SN1 mechanism?

Possible Answers:

Correct answer:

Explanation:

Tertiary alcohols react most rapidly via SN1 mechanisms because they form stable tertiary carbocations. Primary and secondary alcohols typically react most rapidly via SN2 mechanisms.

Of the available options,  is the only one that contains a tertiary alcohol.

Example Question #3 : Other Reaction Mechanisms

What type of enzymatic inhibitor binds to an allosteric location on the enzyme with equal affinity for the bound and unbound substrate states?

Possible Answers:

Noncompetitive inhibitor

Suicide inhibitor

Competitive inhibitor

Uncompetitive inhibitor

Correct answer:

Noncompetitive inhibitor

Explanation:

Noncompetitive inhibitors are a specific type of mixed inhibitor that binds to both the free enzyme and the enzyme-substrate complex with equal affinities, resulting in the same binding affinity (Km value) but a decrease in the maximum rate of reaction (Vmax value).

Example Question #21 : Reaction Mechanisms

The enzyme glucose-6-phosphatase plays the most important role in which tissue/organ?

Possible Answers:

Brain

Adipose tissue

Muscle tissue

Liver

Correct answer:

Liver

Explanation:

Glucose-6-phosphatase is responsible for removing the phosphate group from glucose-6-phosphate. The result is free glucose, which can be released into the blood. This process takes place at the end of either glycogenolysis or gluconeogenesis, both processes that are most prominent in the liver due to its large stores of glycogen.

Example Question #5 : Other Reaction Mechanisms

Which of the following statements is false regarding enzyme function?

Possible Answers:

Enzymes increase the activation energy

Enzymes are not used up during the reaction

Enzymes do not affect the thermodynamics of a reaction

All of these are true

Enzymes are a type of catalyst

Correct answer:

Enzymes increase the activation energy

Explanation:

Enzymes function as a biological catalysts by lowering reactions' activation energies. They are not used up in the reaction mechanism, nor do they affect the thermodynamics of the reaction. They only affect the reaction kinetics.

Example Question #6 : Other Reaction Mechanisms

The cellular membrane is a very important structure. The lipid bilayer is both hydrophilic and hydrophobic. The hydrophilic layer faces the extracellular fluid and the cytosol of the cell. The hydrophobic portion of the lipid bilayer stays in between the hydrophobic regions like a sandwich. This bilayer separation allows for communication, protection, and homeostasis. 

One of the most utilized signaling transduction pathways is the G protein-coupled receptor pathway. The hydrophobic and hydrophilic properties of the cellular membrane allows for the peptide and other hydrophilic hormones to bind to the receptor on the cellular surface but to not enter the cell. This regulation allows for activation despite the hormone’s short half-life. On the other hand, hydrophobic hormones must have longer half-lives to allow for these ligands to cross the lipid bilayer, travel through the cell’s cytosol and eventually reach the nucleus. 

Cholesterol allows the lipid bilayer to maintain its fluidity despite the fluctuation in the body’s temperature due to events such as increasing metabolism. Cholesterol binds to the hydrophobic tails of the lipid bilayer. When the temperature is low, the cholesterol molecules prevent the hydrophobic tails from compacting and solidifying. When the temperature is high, the hydrophobic tails will be excited and will move excessively. This excess movement will bring instability to the bilayer. Cholesterol will prevent excessive movement.

Epinephrine binds to its receptor on the surface of the cell. Molecule A does bind to the same receptor but is found to bind a different part of the receptor molecule than does epinephrine, causing the receptor to undergo a confirmation change and no longer fits with its associated ligand. What type of regulation is this? 

Possible Answers:

Partial inhibition

Noncompetitive inhibition

Competitive inhibition

Direct inhibition 

None of these

Correct answer:

Noncompetitive inhibition

Explanation:

According to the question, molecule A acts on a different site to inhibit epinephrine's receptor. This is an example of a noncompetitive inhibitor.  

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