Oversecretion of gastric HCl can be treated by severing the vagus nerve in a procedure called a vagotomy which reduces parasympathetic activity. Which of the following effects is LEAST likely to be caused by a vagotomy?
Answer(s): A
The parasympathetic nervous system is involved in "rest-and-digest" functions which include lowering the heart rate and promoting digestion. The question stem states that a vagotomy will reduce parasympathetic stimulation. This would tend to increase heart rate and reduce gastrointestinal activity. The question asks for the LEAST likely effect of a vagotomy. A vagotomy would not decrease heart rate.Choices B and C are incorrect because decreased parasympathetic activity would decrease gastric motility and HCl production.Choice D is incorrect because decreased parasympathetic activity would increase heart rate, and thus increase blood pressure.
Tetrodotoxin, the extremely potent poison produced by the puffer (fugu) fish, binds tightly to Na+ channels and blocks the flow of Na+ ions but does not affect K+ or Cl- channels. Tetrodotoxin directly blocks which phase of action potential propagation?
An action potential begins with the opening of voltage gated sodium channels that allow sodium to flow into the axon, depolarizing the cell. This is followed by opening of potassium channels that allow potassium to flow out of the cell, repolarizing it. Blocking the sodium channels would block depolarization.Choice B is incorrect because repolarization is achieved primarily by opening potassium channels.Choice C is incorrect because hyperpolarization is also due primarily to the flux of potassium ions out of the axon.Choice D is incorrect because saltatory conduction refers to conduction between nodes of Ranvier in myelinated fibers.
Female athletes may fail to menstruate (amenorrhea) because extremely low body fat reduces the release of GnRH from the hypothalamus, affecting the secretion of LH and FSH. Which of the following is most likely to be absent in a female long-distance runner?
Answer(s): C
The question stem states that female athletes may not menstruate and discusses a decrease in LH and FSH secretion. It can be inferred that ovulation does not occur. The corpus luteum is formed, following ovulation, by the cells of the follicle. Without ovulation, there will be no formation of the corpus luteum.Choices A, B, and C are incorrect because lack of any of these structures could not be caused by excercise.The anterior pituitary and hypothalamus are structures in the brain which serve endocrine roles. The ovary is the female gonad, located in the abdomen, which is responsible for egg production and release.
Sugars are carbohydrates, that is, molecules usually with the empirical formula C(H2O), and structural formulas made up of polyhydroxy aldehydes or ketones. Because of their polyfunctional nature, sugars can undergo a wide variety of transformations upon treatment with acids, bases, or heat, and upon reaction with other simple reagents and enzymes. While many sugars occur in nature and are thus readily available, the synthesis and modification of simple sugars is a necessary step in studies of enzymatic processes.Higher sugars can be synthesized from the simple carbohydrate D-glyceraldehyde with the following procedure:D-glyceraldehyde (Compound A) is reacted with HCN to produce a cyanohydrin (Compound B). Compound B is then treated with hydrogen gas and a modified palladium catalyst (similar to the Lindlar reagent) to give Compound C. Compound C is hydrolyzed to give the higher sugars in Mixture D. This reaction is summarized in Figure 1. Mixture D contains two compounds, which can be separated by crystallization. Two doublets near 9.5 (, ppm) are observed in the 1H NMR spectrum of mixture D, with each doublet corresponding to one of the two products present in the mixture. IR spectroscopy shows broad absorptions for both products around 3300 cm1.The hydroxyl groups of carbohydrates can also participate in reactions. For example, D-glyceraldehyde can react with chloromethane under basic conditions to yield a completely methylated product. This SN2 reaction is shown in Figure 2. Figure 2 Methylation of D-glyceraldehydeThe reaction shown in Figure 2 most likely proceeds via the formation of a(n):
Answer(s): B
The passage states that an SN2 reaction occurs. This requires a good nucleophile. Alkoxides (RO-) are strong bases and thus good nucleophiles. Sodium hydroxide (NaOH) is used in this reaction to remove the hydrogen from the hydroxyl group to form the alkoxide. The SN2 reaction is shown belowChoice A is incorrect because a hydroxyl group would be converted to water, a good leaving group, under acidic conditions. The reaction in Figure 2 occurs under basic conditions. Choice C is incorrect because carbocation intermediates occur in SN1 and E1 reactions, but not in SN2. Choice D is incorrect because tetrahedral intermediates are formed during addition reactions to a carbon/oxygen double bond. The reaction in Figure 2 does not involve attack on the carbonyl group.
Sugars are carbohydrates, that is, molecules usually with the empirical formula C(H2O), and structural formulas made up of polyhydroxy aldehydes or ketones. Because of their polyfunctional nature, sugars can undergo a wide variety of transformations upon treatment with acids, bases, or heat, and upon reaction with other simple reagents and enzymes. While many sugars occur in nature and are thus readily available, the synthesis and modification of simple sugars is a necessary step in studies of enzymatic processes.Higher sugars can be synthesized from the simple carbohydrate D-glyceraldehyde with the following procedure:D-glyceraldehyde (Compound A) is reacted with HCN to produce a cyanohydrin (Compound B). Compound B is then treated with hydrogen gas and a modified palladium catalyst (similar to the Lindlar reagent) to give Compound C. Compound C is hydrolyzed to give the higher sugars in Mixture D. This reaction is summarized in Figure 1. Mixture D contains two compounds, which can be separated by crystallization. Two doublets near 9.5 (, ppm) are observed in the 1H NMR spectrum of mixture D, with each doublet corresponding to one of the two products present in the mixture. IR spectroscopy shows broad absorptions for both products around 3300 cm-1. The hydroxyl groups of carbohydrates can also participate in reactions. For example, D-glyceraldehyde can react with chloromethane under basic conditions to yield a completely methylated product. This SN2 reaction is shown in Figure 2.Figure 2 Methylation of D-glyceraldehydeThe modified palladium catalyst is used instead of platinum to:
The cyanohydrin (compound B) is reduced to an imine (compound C). If platinum were used, reduction would proceed all the way to the amine as shown below. This must be avoided in order to produce the carbonyl group on the extended sugar.Choice B is incorrect because a catalyst will not alter the thermodynamic favorability of a reaction. A catalyst will only increase the rate of reaction by lowering the activation energy.Choice C is incorrect because a Lewis acid is an electron pair acceptor. The palladium catalyst acts as an activator of hydrogen. Platinum would function in the same way.Choice D is incorrect because compound B does not even have a carbonyl group (carbon double bonded to oxygen).
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