Reactions - MCAT Biology

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Question

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

Mcat_1

Answer

The conversion of compound B to compound Cresults 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.

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Question

Which alcohol will react most rapidly via an SN1 mechanism?

Answer

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.

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Question

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

Answer

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).

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Question

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

Answer

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.

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Question

Which of the following statements is false regarding enzyme function?

Answer

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.

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Question

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?

Answer

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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Question

Drain cleaners a common household staple, used to open clogged drains in bathtubs and sinks. Human hair is a common culprit that clogs pipes, and hair is made predominately of protein. Drain cleaners are effective at breaking down proteins that have accumulated in plumbing. Drain cleaners can be either acidic or basic, and are also effective at breaking down fats that have accumulated with proteins.

A typical reaction—reaction 1—which would be expected for a drain cleaner on contact with human hair, would be as follows in an aqueous solution:

Pic_1

Another reaction that may occur, reaction 2, would take place as follows in an aqueous solution:

Pic_2

An alcohol reacts with the protein reactant in Reaction 2. Which atom in the protein reactant is likely to be the site of a nucleophilic attack?

Answer

Carbonyl carbons bond with a strongly polar bond to the carbonyl oxygen. This strongly polar bond gives the carbon a partial positive charge, and thus makes it receptive to nucleophilic attack by other oxygens (e.g., alcohols).

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Question

Which of these is a dehydration synthesis reaction?

I. Formation of a disulfide bond between two cysteine residues

II. Carboxylation of a carbohydrate

III. Formation of an amide bond

IV. Splitting of dipeptides into amino acids

V. Addition of hydrogen to an unsaturated fatty acid

Answer

Condensation reactions remove a small molecule—usually water—to bond two reactant molecules. In amide bonding, the carboxylic (–COOH, acid) portion of one amino acid is juxtaposed to the amide (–NH2, basic) end of a second amino acid. A molecule of water is extracted and the amide bond (–CONH) is formed. This is a variant of the classic statement, "Acid plus base yields salt plus water."

Technically, condensation reactions can remove small molecules other than water, but often the terms "condensation" and "dehydration" are used interchangeably in biochemistry (but not in organic chemistry). Incidentally, two cysteine molecules linked by a disulfide bond is renamed cystine (notice spelling alteration).

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Question

Which of the following is not capable of oxidizing a secondary alcohol to a ketone?

Answer

Lithium aluminum hydride is correct because it is a reducing agent, and is therefore not capable of oxidizing secondary alcohols. Instead, LAH could be used to perform the reverse reaction, reducing a ketone to an alcohol. The other answer choices are oxidizing agents.

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Question

In the human body, glycerol and three fatty acids can act together to form triacyl glycerol in a process known as esterification. The reverse of this process is known as __________.

Answer

When an alcohol and carboxylic acid are reacted together they can form an ester, in a process known as esterification. This is what happens with glycerol and fatty acids to form triacyl glycerol. The reverse of this process is an organic chemistry reaction known as saponification, in which an ester and water are reacted to form carboxylic acids and alcohol.

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Question

Which of the following is an example of an anabolic reaction in the body?

Answer

Anabolic reactions are reactions that construct molecules from smaller primary units. Catabolism is just the opposite, where larger molecules are degraded into simpler subunits. In the body larger macromolecules, such as proteins, are frequently created via condensation reactions. In condensation reactions, water is formed as a byproduct of joining two molecules together.

All other three processes describe a scenarios where a larger molecule is broken down into smaller subunits, known as catabolic processes.

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Question

Which of these methods can be used to synthesize a secondary alcohol?

Answer

All of these methods can successfully synthesize secondary alcohols.

Lithium aluminum hydride and sodium borohydride are strong and weaker reducing agents, respectively. Both are able to reduce ketones to secondary alcohols.

Adding water and acid to an alkene (such as propene) results in Markovnikov addition of a hydroxyl group, also creating a secondary alcohol.

Grignard reagents (organometallic halides) add to the carbonyl carbon of an aldehyde, adding an alkane group and forming an alcohol product.

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Question

Triacylglycerols are broken down in the body by lipases. In the presence of water, lipase is used to break down the triacylglycerol into a glycerol molecule and three fatty acids. Based on this information, the breakdown of a triacylglycerol is the opposite of which reaction?

Answer

The molecular structure of a triacylglycerol reveals that the three fatty acid tails are attached to the glycerol molecule, resulting in three ester functional groups. Remember that fatty acids have a carboxylic acid functional group at one end, which is created after the cleavage described in the question. Since the ester bonds are broken and carboxylic acids are created following the hydrolysis of a triglyceride, the opposite of esterification has taken place.

The process described (breakdown of an ester into a carboxylic acid) is also known as saponification.

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Question

Carbonic anhydrase is a very important enzyme that is utilized by the body. The enzyme catalyzes the following reaction:

A class of drugs that inhibits this enzyme is carbonic anhydrase inhibitors (eg. acetazolamide, brinzolamide, dorzolamide). These drugs are commonly prescribed in patients with glaucoma, hypertension, heart failure, high altitude sickness and for the treatment of basic drugs overdose.

In patients with hypertension, carbonic anhydrase inhibitors will prevent the reabsorption of sodium chloride in the proximal tubule of the kidney. When sodium is reabsorbed back into the blood, the molecule creates an electrical force. This electrical force then pulls water along with it into the blood. As more water enters the blood, the blood volume increase. By preventing the reabsorption of sodium, water reabsorption is reduced and the blood pressure decreases.

When mountain climbing, the atmospheric pressure is lowered as the altitude increases. As a result of less oxygen into the lungs, ventilation increases. From the equation above, hyperventilation will result in more being expired. Based on Le Chatelier’s principle, the reaction will shift to the left. Since there is more bicarbonate than protons in the body, the blood will become more basic (respiratory alkalosis). To prevent such life threatening result, one would take a carbonic anhydrase inhibitor to prevent the reaction from shifting to the left.

Carbonic anhydrase inhibitors are useful in patients with a drug overdose that is acidic. The lumen of the collecting tubule is nonpolar. Due to the lumen's characteristic, molecules that are also nonpolar and uncharged are able to cross the membrane and re-enter the circulatory system. Since carbonic anhydrase inhibitors alkalize the urine, acidic molecules stay in a charged state.

A patient overdosed on an unknown drug. Two hours after being admitted into the hospital, acetazolamide was administered. A realtime measurement of the drug's concentration in the patient's blood was taken the moment the patient was admitted every 30 minutes. Which of the following best explains the lab results shown?

Answer

From the data, the concentration of the unknown drug in the blood increased shortly after the patient received the carbonic anhydrase inhibitor. The increase in the blood's concentration of the drug meant that the body is reabsorbing the drug. The only way for the drug to be reabsorbed is if it is able to cross the membrane of the renal's lumen. In order to cross the renal's membrane, the drug must be noncharged and nonpolar. Basic molecules in a basic environment will stay uncharged.

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Question

Carbonic anhydrase is a very important enzyme that is utilized by the body. The enzyme catalyzes the following reaction:

A class of drugs that inhibits this enzyme is carbonic anhydrase inhibitors (eg. acetazolamide, brinzolamide, dorzolamide). These drugs are commonly prescribed in patients with glaucoma, hypertension, heart failure, high altitude sickness and for the treatment of basic drugs overdose.

In patients with hypertension, carbonic anhydrase inhibitors will prevent the reabsorption of sodium chloride in the proximal tubule of the kidney. When sodium is reabsorbed back into the blood, the molecule creates an electrical force. This electrical force then pulls water along with it into the blood. As more water enters the blood, the blood volume increase. By preventing the reabsorption of sodium, water reabsorption is reduced and the blood pressure decreases.

When mountain climbing, the atmospheric pressure is lowered as the altitude increases. As a result of less oxygen into the lungs, ventilation increases. From the equation above, hyperventilation will result in more being expired. Based on Le Chatelier’s principle, the reaction will shift to the left. Since there is more bicarbonate than protons in the body, the blood will become more basic (respiratory alkalosis). To prevent such life threatening result, one would take a carbonic anhydrase inhibitor to prevent the reaction from shifting to the left.

Carbonic anhydrase inhibitors are useful in patients with a drug overdose that is acidic. The lumen of the collecting tubule is nonpolar. Due to the lumen's characteristic, molecules that are also nonpolar and uncharged are able to cross the membrane and re-enter the circulatory system. Since carbonic anhydrase inhibitors alkalize the urine, acidic molecules stay in a charged state.

Glaucoma involves the pressure within the eye increasing to a dangerously high level due to improper draining of the vitreous humor. Blindness usually follows due to damage to the nerves. How might administering a carbonic anhydrase inhibitor help in patients with glaucoma?

Answer

From the equation:

We can see that carbonic anhydrase functions in both the forward and backward reactions. In the eye, the equation shifts to the left and more water is produced. Water is one of the main substances in vitreous humor. As the production of water increases, pressure increases as well. By blocking the production of water, the pressure is lowered.

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Question

Carbonic anhydrase is a very important enzyme that is utilized by the body. The enzyme catalyzes the following reaction:

A class of drugs that inhibits this enzyme is carbonic anhydrase inhibitors (eg. acetazolamide, brinzolamide, dorzolamide). These drugs are commonly prescribed in patients with glaucoma, hypertension, heart failure, high altitude sickness and for the treatment of basic drugs overdose.

In patients with hypertension, carbonic anhydrase inhibitors will prevent the reabsorption of sodium chloride in the proximal tubule of the kidney. When sodium is reabsorbed back into the blood, the molecule creates an electrical force. This electrical force then pulls water along with it into the blood. As more water enters the blood, the blood volume increase. By preventing the reabsorption of sodium, water reabsorption is reduced and the blood pressure decreases.

When mountain climbing, the atmospheric pressure is lowered as the altitude increases. As a result of less oxygen into the lungs, ventilation increases. From the equation above, hyperventilation will result in more being expired. Based on Le Chatelier’s principle, the reaction will shift to the left. Since there is more bicarbonate than protons in the body, the blood will become more basic (respiratory alkalosis). To prevent such life threatening result, one would take a carbonic anhydrase inhibitor to prevent the reaction from shifting to the left.

Carbonic anhydrase inhibitors are useful in patients with a drug overdose that is acidic. The lumen of the collecting tubule is nonpolar. Due to the lumen's characteristic, molecules that are also nonpolar and uncharged are able to cross the membrane and re-enter the circulatory system. Since carbonic anhydrase inhibitors alkalize the urine, acidic molecules stay in a charged state.

When a patient is not ventilating efficiently, the kidneys try to compensate by producing more bicarbonate to buffer the decreasing pH of the blood due to the buildup of . Which of the following will increase the amount of in the blood?

I. Increase level in the blood

II. Increase level in the blood

III. Increase metabolism

Answer

To answer this question, we must understand Le Chatelier’s principle. The reaction favorites the product when there is more of the reactants. The opposite is also true in that when there is a buildup of the products, the reaction will reverse and favorite the reactants. Increasing the metabolism will produce more , which will shift the equation to the right.

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Question

Carbonic anhydrase is a very important enzyme that is utilized by the body. The enzyme catalyzes the following reaction:

A class of drugs that inhibits this enzyme is carbonic anhydrase inhibitors (eg. acetazolamide, brinzolamide, dorzolamide). These drugs are commonly prescribed in patients with glaucoma, hypertension, heart failure, high altitude sickness and for the treatment of basic drugs overdose.

In patients with hypertension, carbonic anhydrase inhibitors will prevent the reabsorption of sodium chloride in the proximal tubule of the kidney. When sodium is reabsorbed back into the blood, the molecule creates an electrical force. This electrical force then pulls water along with it into the blood. As more water enters the blood, the blood volume increase. By preventing the reabsorption of sodium, water reabsorption is reduced and the blood pressure decreases.

When mountain climbing, the atmospheric pressure is lowered as the altitude increases. As a result of less oxygen into the lungs, ventilation increases. From the equation above, hyperventilation will result in more being expired. Based on Le Chatelier’s principle, the reaction will shift to the left. Since there is more bicarbonate than protons in the body, the blood will become more basic (respiratory alkalosis). To prevent such life threatening result, one would take a carbonic anhydrase inhibitor to prevent the reaction from shifting to the left.

Carbonic anhydrase inhibitors are useful in patients with a drug overdose that is acidic. The lumen of the collecting tubule is nonpolar. Due to the lumen's characteristic, molecules that are also nonpolar and uncharged are able to cross the membrane and re-enter the circulatory system. Since carbonic anhydrase inhibitors alkalize the urine, acidic molecules stay in a charged state.

Based on the passage, which of the following statements, if true, will contradict the effectiveness of carbonic anhydrase inhibitors as a treatment?

Answer

Carbonic anhydrase inhibitors are used to alkalinize the urine. When the urine is alkalinized, acidic molecules will lose its proton and go into a charged state. Charged molecules are unable to cross the membrane of the lumen of the collecting tubule. Without the ability to cross the membrane, the molecule is therefore unable to be reabsorbed. Therefore, if the statement "Acidic molecules will not release its proton in the basic environment of the lumen of the collecting tubule" was true, then alkalinizing the urine will have no effect.

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Question

Which of the following compounds can be oxidized to form a ketone?

Answer

An alcohol can be oxidized and form a ketone only if the alcohol is a secondary alcohol. Tertiary alcohols cannot be oxidized and primary alcohols are oxidized to form aldehydes and carboxylic acids.

3-pentanol is a secondary alcohol, while 2-methyl-2-butanol is a tertiary alcohol. Methanol and propyl alcohol are both primary alcohols.

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Question

Compound B is dissolved in methylene chloride, and then treated with trifluoroacetic acid. Over the next thirty minutes, gas evolution was observed from the reaction mixture. What gas was being given off?

Mcat__4

Answer

Treatment of a carboxylic acid with acid results in decarboxylation, and the evolution of , especially if the resulting compound contains a benzylic or allylic carbon, as is the case here.

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Question

The combustion of pentane with oxygen gas is an exothermic reaction that produces carbon dioxide and water as products. Is this a spontaneous reaction?

Answer

A balanced equation of the combustion of pentane indicates that one mole of pentane reacts with eight moles of oxygen gas to produce five moles of carbon dioxide and six moles of water.

Because there are nine moles of reactant and eleven moles of product, entropy increases in this reaction. Exothermic reactions that increase entropy are favorable at all temperatures, as seen in the Gibb's free energy equation.

In our scenario, H is negative and S is positive.

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