Caffeine, the stimulant in coffee, has been called "the most widely used psychoactive substance on Earth." Snyder, Daly, and Bruns...
GMAT Reading Comprehension : (RC) Questions
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.
The primary purpose of the passage is to
1. Passage Analysis:
Progressive Passage Analysis
| Text from Passage | Analysis |
|---|---|
| Caffeine, the stimulant in coffee, has been called "the most widely used psychoactive substance on Earth." | What it says: Coffee's caffeine is an extremely common drug that affects the brain. What it does: Opens with a hook to establish the importance and relevance of the topic. Source/Type: General fact (attributed to unnamed sources) Connection to Previous Sentences: This is our starting point - no previous information to connect to. Visualization: Imagine billions of people worldwide drinking coffee daily, making caffeine more widespread than any other mind-altering substance. Reading Strategy Insight: This opener tells us the passage will be about something universally relevant - caffeine. Feel confident that this affects you personally! |
| 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. | What it says: Three researchers think caffeine works by blocking a brain chemical called adenosine. What it does: Introduces the main theory the passage will explore. Source/Type: Researchers' hypothesis/claim Connection to Previous Sentences: This builds on sentence 1 by moving from "caffeine is important" to "here's a specific theory about HOW caffeine works." Visualization: Think of adenosine as a "brake pedal" for the brain, and caffeine as something that blocks that brake pedal. What We Know So Far: Caffeine is widespread, and researchers think it works by blocking adenosine What We Don't Know Yet: What adenosine actually does, how the blocking works Reading Strategy Insight: The word "countering" suggests opposition - caffeine vs. adenosine will be our main relationship. |
2. Passage Summary:
Author's Purpose:
To explain how scientists figured out the real way caffeine works in the brain by comparing a new theory with an old one and showing why the new theory is better.
Summary of Passage Structure:
In this passage, the author walks us through a scientific investigation that settles a debate about how caffeine affects our brains:
- First, the author introduces caffeine as an important substance and presents a new theory that it works by blocking a brain chemical called adenosine that normally slows down brain activity.
- Next, the author explains the old theory that scientists used to believe - that caffeine worked by affecting a different chemical system involving an enzyme called phosphodiesterase.
- Then, the author shows why the old theory has serious problems by presenting evidence that caffeine works at doses too low to affect that enzyme, and other chemicals that do affect the enzyme don't act like stimulants.
- Finally, the author presents experimental evidence supporting the new adenosine theory, showing that chemicals better at blocking adenosine are also better stimulants, and explains why some exceptions to this pattern don't undermine the theory.
Main Point:
Caffeine most likely works by blocking adenosine receptors in the brain rather than by affecting the phosphodiesterase enzyme system, and the scientific evidence strongly supports this adenosine-blocking explanation over the older competing theory.
Why It's Wrong:
- The passage doesn't discuss a "plan for investigation"—it presents completed research with actual results and conclusions
- The phenomenon (how caffeine works) is being explained, not left as "not yet fully understood"
- The passage actively argues for one theory over another rather than just planning future research
Why It's Wrong:
- The passage doesn't "reconcile" the two theories—it shows why one is wrong and the other is right
- The adenosine theory and phosphodiesterase theory are presented as competing, not complementary
- The evidence clearly favors one theory over the other rather than finding middle ground
Why It's Right:
- The passage clearly presents two theories: the adenosine receptor theory (new) and the phosphodiesterase theory (traditional)
- It systematically shows problems with the phosphodiesterase theory (wrong concentrations, other compounds don't work)
- It provides positive evidence supporting the adenosine theory (correlation experiments)
- It addresses potential problems with the preferred theory (explaining exceptions)
Why It's Wrong:
- While the passage does describe the adenosine theory as an "alternative," it doesn't just describe and support it
- The passage also presents the traditional phosphodiesterase theory and shows why it's inadequate
- This choice misses the comparative aspect that's central to the passage's structure
Why It's Wrong:
- The passage doesn't "challenge" a theory by "exposing inconsistencies"—it systematically compares two theories
- The tone is analytical and constructive rather than purely critical
- The passage builds up the adenosine theory as much as it critiques the phosphodiesterase theory