Reading Passage Question
Caffeine, the stimulant in coffee, has been called “the most widely used psychoactive substance on Earth.” Snyder, Daly, and Bruns have recently proposed that caffeine affects behavior by countering the activity in the human brain of a naturally occurring chemical called adenosine. Adenosine normally depresses neuron firing in many areas of the brain. It apparently does this by inhibiting the release of neurotransmitters, chemicals that carry nerve impulses from one neuron to the next.
Like many other agents that affect neuron firing, adenosine must first bind to specific receptors on neuronal membranes. There are at least two classes of these receptors, which have been designated A1 and A2. Snyder et al. propose that caffeine, which is structurally similar to adenosine, is able to bind to both types of receptors, which prevents adenosine from attaching there and allows the neurons to fire more readily than they otherwise would.
For many years, caffeine’s effects have been attributed to its inhibition of the production of phosphodiesterase, an enzyme that breaks down the chemical called cyclic AMP. A number of neurotransmitters exert their effects by first increasing cyclic AMP concentrations in target neurons. Therefore, prolonged periods at the elevated concentrations, as might be brought about by a phosphodiesterase inhibitor, could lead to a greater amount of neuron firing and, consequently, to behavioral stimulation. But Snyder et al. point out that the caffeine concentrations needed to inhibit the production of phosphodiesterase in the brain are much higher than those that produce stimulation. Moreover, other compounds that block phosphodiesterase’s activity are not stimulants.
To buttress their case that caffeine acts instead by preventing adenosine binding, Snyder et al. compared the stimulatory effects of a series of caffeine derivatives with their ability to dislodge adenosine from its receptors in the brains of mice. “In general,” they reported, “the ability of the compounds to compete at the receptors correlates with their ability to stimulate locomotion in the mouse; i.e., the higher their capacity to bind at the receptors, the higher their ability to stimulate locomotion.” Theophylline, a close structural relative of caffeine and the major stimulant in tea, was one of the most effective compounds in both regards.
There were some apparent exceptions to the general correlation observed between adenosine receptor binding and stimulation. One of these was a compound called 3-isobutyl-1-methylxanthine (IBMX), which bound very well but actually depressed mouse locomotion. Snyder et al. suggest that this is not a major stumbling block to their hypothesis. The problem is that the compound has mixed effects in the brain, a not unusual occurrence with psychoactive drugs. Even caffeine, which is generally known only for its stimulatory effects, displays this property, depressing mouse locomotion at very low concentrations and stimulating it at higher ones.
Solution and Explanation
(A) discuss a plan for investigation of a phenomenon that is not yet fully understood
(B) present two explanations of a phenomenon and reconcile the differences between them
(C) summarize two theories and suggest a third theory that overcomes the problems encountered in the first two
(D) describe an alternative hypothesis and provide evidence and arguments that support it
(E) challenge the validity of a theory by exposing the inconsistencies and contradictions in it
Answer: D
Explanation: An idea from recent research about how caffeine impacts behaviour is presented in the first paragraph. An older and well recognised theory regarding how caffeine impacts behaviour is presented in the second paragraph. Offers information that is incompatible with that theory. The third and fourth paragraphs refer back to the more recent idea that was initially presented and include "facts and arguments" in favour of it. As a result, the passage's principal goal is to outline a competing theory and present supporting data and arguments. Choice D suits the answer best.
(A) stimulates behavior in the mouse and in humans, whereas adenosine stimulates behavior in humans only
(B) has mixed effects in the brain, whereas adenosine has only a stimulatory effect
(C) increases cyclic AMP concentrations in target neurons, whereas adenosine decreases such concentrations
(D) permits release of neurotransmitters when it is bound to adenosine receptors, whereas adenosine inhibits such release
(E) inhibits both neuron firing and the production of phosphodiesterase when there is a sufficient concentration in the brain, whereas adenosine inhibits only neuron firing
Answer: D
Explanation: According to the passage, adenosine binds to certain receptors on neuronal membranes to "depress neuron firing." Which in turn prevents neurotransmitter release. Caffeine-related hypothesis from Snyder et al. They suggest that caffeine binds to certain receptors on the membranes of neurons. It allows the neurons to fire more readily than they otherwise would by preventing adenine from binding to those receptors. Snyder et al conclude that caffeine and adenosine vary in that caffeine. When attached to adenosine receptors, it enables neurotransmitter release whereas adenosine inhibits it.
(A) mixed effects in the brain
(B) inhibitory effects on enzymes in the brain
(C) close structural relationships with caffeine
(D) depressive effects on mouse locomotion
(E) the ability to dislodge caffeine from receptors in the brain
Answer: A
Explanation: The passage's final paragraph discusses IBMX's side effects. On the membranes of neurons, IBMX appears to bind to receptors unique to adenosine. But IBMX inhibits rather than increases mouse movement. In contrast to the other caffeine compounds that Snyder et al. tested. According to Snyder et al, IBMX exhibits mixed effects in the brain. A not uncommon phenomenon with psychoactive medicines in response to this experimental finding.
(A) IBMX
(B) caffeine
(C) adenosine
(D) theophylline
(E) phosphodiesterase
Answer: E
Explanation: The enzyme that breaks down the molecule known as cyclic AMP is phosphodiesterase. It is the only substance that has not been linked to a particular type of brain receptor. The levels of coffee that limit the brain's ability to make phosphodiesterase are substantially higher than those that stimulate it, according to Snyder et al. In addition, several substances that inhibit the activity of phosphodiesterase are not stimulants.
(A) The chemical relationship between caffeine and phosphodiesterase
(B) The structural relationship between caffeine and adenosine
(C) The structural similarity between caffeine and neurotransmitters
(D) The ability of caffeine to stimulate behavior
(E) The natural occurrence of caffeine and adenosine in the brain
Answer: B
Explanation: The theory put forth by Snyder et al regarding how caffeine affects behaviour. They assert that caffeine can attach to the brain's A1 and A2 receptors because it is structurally similar to adenosine. Similar to those that adenosine typically binds to. As a result, the passage implies that caffeine's capacity to bind to A1 and A2 receptors may in part be a result of the structural link between caffeine and adenosine.
(A) reveal some of the assumptions underlying their theory
(B) summarize a major finding of their experiments
(C) point out that their experiments were limited to the mouse
(D) indicate that their experiments resulted only in general correlations
(E) refute the objections made by supporters of the older theory
Answer: B
Explanation: The third paragraph provides evidence for Snyder et al’s hypothesis by discussing experiments they conducted on mice. The quotation provides a summary of these experiments' results. Snyder et al. Found some caffeine compounds can connect to specific receptors in mouse brains in a manner similar to adenosine. Furthermore, the derivatives with the best ability to promote locomotion also have the best ability to compete. The binding of adenosine at the receptors. This conclusion is significant because it confirms their theory that caffeine's capacity to compete with adenosine is what causes its simulative effects.
“Caffeine, the stimulant in coffee, has been called “the most widely”- is a GMAT reading comprehension passage with answers. Candidates need a strong knowledge of English GMAT reading comprehension.
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