TPRH SW Bio P47

[circulus vitios]

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Hormones bind to cell-surface receptors, activating the alpha unit of the stimulatory G protein (Gs.) The activation stimulates exchange of GDP bound to Gs-alpha for GTP. GTP bound to the alpha unit is slowly hydrolyzed to GDP, thereby deactivating the cascade.

Adenylate cyclase stimulation takes place after receptors bind hormones because:

A. of the exchange of GTP to GDP bound to Gs-alpha
B. of the exchange of GDP for GTP bound to Gs-alpha

Answer is A because:

when hormone binds to receptor, Gs-alpha releases GDP and binds GTP. As long as it has GTP bound, it stimulates adenylate cyclase activity to produce cAMP (supporting choice A and eliminating choice B.)

Putting it into a sentence with their answer, "stimulation takes place after receptors bind hormones because GTP is exchanged for GDP, deactivating the cascade." That makes absolutely no sense because it's not saying why, it's just saying what would happen next...and if you want to say what happens next, why wouldn't you first mention GDP exchange for GTP which happens before GTP hydrolysis to GDP?

 

[Hemorrage]

Answer is A because:

Putting it into a sentence with their answer, "stimulation takes place after receptors bind hormones because GTP is exchanged for GDP, deactivating the cascade." That makes absolutely no sense because it's not saying why, it's just saying what would happen next...and if you want to say what happens next, why wouldn't you first mention GDP exchange for GTP which happens before GTP hydrolysis to GDP?

I think your reading too into it. The passage clearly states that the receptor has GDP bound to it when it's not interacting with a hormone. Upon binding a hormone it will exchange the GDP (already bound) for GTP. Also there's no given order for hydrolysis and condensation, GTP is utilized by other cell processes too so it's better to think of them as independent events.

 

[circulus vitios]

I think your reading too into it. The passage clearly states that the receptor has GDP bound to it when it's not interacting with a hormone. Upon binding a hormone it will exchange the GDP (already bound) for GTP. Also there's no given order for hydrolysis and condensation, GTP is utilized by other cell processes too so it's better to think of them as independent events.

Exactly. It says GTP to GDP hydrolysis causes a deactivated state. Therefore GDP to GTP exchange would cause the activate state. The question asks for the time when activation takes place. Their reasoning makes absolutely no sense.

 

[sp808]

Your reasoning is correct, the answer choices are just worded really, really badly. Choice A is trying to say "exchange of the GTP in the cytoplasm for the GDP bound to the Gs-alpha" whereas a normal person would write "the Gs-alpha exchanges its bound GDP for cytoplasmic GTP."

This is a really easy question that was made confusing by poor wording - hopefully stuff like this won't be on the real MCAT.

 

[Postictal Raiden]

Your reasoning is correct, the answer choices are just worded really, really badly. Choice A is trying to say "exchange of the GTP in the cytoplasm for the GDP bound to the Gs-alpha" whereas a normal person would write "the Gs-alpha exchanges its bound GDP for cytoplasmic GTP."

This is a really easy question that was made confusing by poor wording - hopefully stuff like this won't be on the real MCAT.

Actually on AAMC 5, there was an ochem question (the very first question of the BS section) that is worded in a similar fashion. The correct answer response states that in one of the steps of synthesis, x compound must be substituted for y to yield the given molecule. However, the original molecule had y compound and the new one must have x compound. I solved this question by process of elimination. Otherwise, I would have thought it was a trap.

 

[circulus vitios]

Your reasoning is correct, the answer choices are just worded really, really badly. Choice A is trying to say "exchange of the GTP in the cytoplasm for the GDP bound to the Gs-alpha" whereas a normal person would write "the Gs-alpha exchanges its bound GDP for cytoplasmic GTP."

This is a really easy question that was made confusing by poor wording - hopefully stuff like this won't be on the real MCAT.

Thank you! Some of the questions are so unbelievably obnoxious and stupid I swear to god I'm on the verge of having a rage blackout.

 

[mcloaf]

This is a really easy question that was made confusing by poor wording - hopefully stuff like this won't be on the real MCAT.

I'm sorry, but this actually made me laugh. You are bound to see stuff like this on the MCAT.