Caffeine, the stimulant in coffee, has been called...

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. 11 ln general," they reported, 11the ability of the compounds to compete at the receptors correlates with their ability to stimulate locomotion i  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 adenosinereceptor 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 ofthe passage is to ...
(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

A. Mursyidatul · Accepted Answer

Halo Mawar, kakak bantu jawab ya.

Jawaban yang benar adalah ''(D) describe an alternative hypothesis and provide evidence and arguments that support it''.

Soal menanyakan tujuan utama dari teks ini.

Opsi jawaban:
(A) mendiskusikan rencana penyelidikan suatu fenomena yang belum sepenuhnya dipahami.
(B) menyajikan dua penjelasan tentang suatu fenomena dan mendamaikan perbedaan di antara mereka.
(C) merangkum dua teori dan menyarankan teori ketiga yang mengatasi masalah yang dihadapi dalam dua teori pertama.
(D) menggambarkan hipotesis alternatif dan memberikan bukti dan argumen yang mendukungnya.
(E) menantang validitas suatu teori dengan mengekspos inkonsistensi dan kontradiksi di dalamnya.

Paragraf pertama memperkenalkan hipotesis terbaru tentang bagaimana kafein mempengaruhi perilaku yaitu ''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.'' (Snyder, Daly, dan Bruns baru-baru ini mengemukakan bahwa kafein mempengaruhi perilaku dengan melawan aktivitas di otak manusia dari bahan kimia alami yang disebut adenosin.)

Paragraf kedua melihat pada hipotesis sebelumnya yang diterima secara luas dan kemudian menyajikan keberatan yang dibuat oleh para ilmuwan yang mengemukakan hipotesis yang lebih baru yaitu ''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.'' (Snyder dkk. mengemukakan bahwa kafein, yang secara struktural mirip dengan adenosin, mampu mengikat kedua jenis reseptor, yang mencegah adenosin menempel di sana dan memungkinkan neuron untuk menembak lebih mudah daripada yang seharusnya.)

Paragraf ketiga dan keempat memberikan bukti untuk mendukung hipotesis baru itu yaitu ''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.'' (Tapi Snyder dkk. menunjukkan bahwa konsentrasi kafein yang dibutuhkan untuk menghambat produksi fosfodiesterase di otak jauh lebih tinggi daripada yang menghasilkan stimulasi. Selain itu, senyawa lain yang menghambat aktivitas fosfodiesterase bukanlah stimulan.)

Dapat disimpulkan bahwa sebagian besar bagian ini dikhususkan untuk hipotesis terbaru, jelas tujuan utamanya adalah untuk menyajikan hipotesis itu kepada pembaca.

Jadi, jawaban yang benar adalah ''(D) describe an alternative hypothesis and provide evidence and arguments that support it''.

Semoga membantu ya.

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