TPR MCAT Question. Advice Needed!

[QuentinT88]

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So I'm working through a few TPR diagnostic questions, and this was one of the questions that I got wrong. Not sure if there is something I am missing, or if there is an error in the explanation. But hoping someone would help me get this understanding down. Here is the question.

The sodium-potassium ATPase moves 3 sodium ions out of the cell and two potassium ions into the cell at the cost of one ATP molecule. In the absence of ion leak channels, the ATPase would make the cytoplasm

A) hypertonic to the extracellular environment
B) isotonic to the extracellular environment
C) hyptotonic to the extracellular environment
D) hypersomotic to the extracellular environment

Here is the explanation:
C. Since the ATPase moves more ions out of the cell than in, and since in the absence of leak channels these ions have no way to cross the cell membrane, the cytoplasm would ultimately become hypotonic, meaning it has less particles when compared to the extracellular environment (choice C is correct). Hypertonic (or hyperosmotic) would mean that the cytoplasm had more particles than the extracellular environment (choices A and D are wrong), and isotonic would mean they had the same number of particles (choice B is wrong).

I chose A because I thought that in the absence of K+ leak channels, more K+ ions would be trapped inside the cell, more ions would be in the cytoplasm of the cell, and thus would make it the cytoplasm hyper-tonic compared to the outside environment.

Am I missing something here??

 

[Dr. Trenb]

I did not review Biology yet so I'm a bit rusty on this topic, but from what I remember, there are K+ leakage channels and Na+ leakage channels. Since the question stated "absence of ion leak channels", then I assume both K+ and Na+ channels are shut or not working. Therefore, if you have the ATPase pumping 3 ions out and 2 ions in, more ions would exist outside the cell than inside the cell.

 

[supremus]

Option C:
More non-permeable particles (ions) are going to be found in the extracellular space compared to the cytoplasm due to the uneven exchange rate of 3Na-2K. Also, remember to know the difference between tonicity and osmolarity.

 

[QuentinT88]

Ok thx, I understand what your both saying. But wouldn't there be more ions outside the cell with or without the K+ leak channel? The point of the channel is to allow potassium to leak out of the cytoplasm, which would make the outside environment higher in ion concentration under both situations...

 

[Kariim]

You're assuming that [K+]inside > [Na+]outside, which is generally not true to my knowledge.
Nevertheless, I think the question isn't really testing this. It's just testing the fact that 3 ions are moving out for every 2 that are moving in, regardless of their identity.

 

[Happy_Puppy5000]

So I'm working through a few TPR diagnostic questions, and this was one of the questions that I got wrong. Not sure if there is something I am missing, or if there is an error in the explanation. But hoping someone would help me get this understanding down. Here is the question.

The sodium-potassium ATPase moves 3 sodium ions out of the cell and two potassium ions into the cell at the cost of one ATP molecule. In the absence of ion leak channels, the ATPase would make the cytoplasm

A) hypertonic to the extracellular environment
B) isotonic to the extracellular environment
C) hyptotonic to the extracellular environment
D) hypersomotic to the extracellular environment

Here is the explanation:
C. Since the ATPase moves more ions out of the cell than in, and since in the absence of leak channels these ions have no way to cross the cell membrane, the cytoplasm would ultimately become hypotonic, meaning it has less particles when compared to the extracellular environment (choice C is correct). Hypertonic (or hyperosmotic) would mean that the cytoplasm had more particles than the extracellular environment (choices A and D are wrong), and isotonic would mean they had the same number of particles (choice B is wrong).

I chose A because I thought that in the absence of K+ leak channels, more K+ ions would be trapped inside the cell, more ions would be in the cytoplasm of the cell, and thus would make it the cytoplasm hyper-tonic compared to the outside environment.

Am I missing something here??

Two things:

1. If you realize that both Hypertonic and Hypersomotic are both more or less dealing with the same thing, then you could use the process of elimination.

2. Allows me for a minute to make a really dumb analogy. Here goes:

Let's say everyday 2000 Spring/Winter Breakers head to Cancun Mexico by crossing the southern border (The ATPase.) Meanwhile, 3000 Mexican sneak in via the same ATPase. Now each week some of the US Spring/Winter Breakers head back home via leak channels. While on the other hand, law enforcement deport say 500 of the 3000 Mexican. So that is the status quo. However, lets say the US government shut down because of Congress and college students are unable to get passport or other travel documents and also law enforcement or not doing their usual round up and deporting--the official "Leakage Channels" close. Nevertheless, my Mexican buddies are still figuring a way to get in 3000 per day, while my college friends are hell bend on getting to Senor Frog, but with the government stuff down are unable to get back home.

Therefore, we eventually have more going to one side, but the usually mechanism to "even" things out is not working. Hence, the concentration is greater on the outside than on the inside. In other words, the inside is Hypotonic to the surrounding.

Really stupid explanation, but I hope it helps clear it up a bit, rather than confuse you more.