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Old 04-29-2012
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Question Osmolality of Zero!

A 49-year-old male in end-stage renal failure is able to perform peritoneal dialysis at home. The osmolality of the solution chosen for peritoneal dialysis will determine the rate of ultrafiltration. Which of the following statements best characterizes a molecule whose osmolality is zero?

It will not permeate the membrane
It can only cross the membrane through the lipid bilayer
It causes water to flow across the membrane
It is as diffusible through the membrane as water
It is transported across the membrane by a carrier
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diffuses as water?
==

btw! what is the difference between osmolarity and osmolality?

same thing?
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Originally Posted by DocSikorski View Post
diffuses as water?
==

btw! what is the difference between osmolarity and osmolality?

same thing?
osmolarity = no of moles of solute/L of solution
osmolality = no of moles of solute/Kg of solvent
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Default my answer

It is as diffusible through the membrane as water

Goljan says in his audios that osmolalilty and osmolarity is the same thing. It's not 100% true but for med calculations I think his statement is pretty safe
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I think such a molecule would cause water movement across the membrane...


Lets take the example of urea- suppose we add 200 mOsm/L urea solution to one compartment and 300 mOsm/L Nacl to another compartment separated by a membrane..then the solution with urea is like pure water since urea diffuses rapidly across the membrane and its contribution to osmolarity is ZERO( similar to the substance in the above question).
Hence in this case, water moves from urea containing sol to Nacl one...this is my explanation

What's the answer btw????????????
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Default hmmm

Quote:
Originally Posted by anomali View Post
I think such a molecule would cause water movement across the membrane...


Lets take the example of urea- suppose we add 200 mOsm/L urea solution to one compartment and 300 mOsm/L Nacl to another compartment separated by a membrane..then the solution with urea is like pure water since urea diffuses rapidly across the membrane and its contribution to osmolarity is ZERO( similar to the substance in the above question).
Hence in this case, water moves from urea containing sol to Nacl one...this is my explanation

What's the answer btw????????????
En example with urea is good but I would think about it this way:

Only effective osmolality can cause water movement. e.g. increase in Na in one of the compartments. Effective osmolality is created only by nonpenetrating solutes (those that would cause this water to go from compartment with lower Osm to the compartment with higher Osm). Urea is a penetrating substance. It will diffuse through the membrane and in the result the concentration in both compartments will be equal. So I would say that urea gives zero osmolality, it goes through the membrane as easily as water so it does NOT cause water movement through the membrane.
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Originally Posted by Casandra View Post
En example with urea is good but I would think about it this way:

Only effective osmolality can cause water movement. e.g. increase in Na in one of the compartments. Effective osmolality is created only by nonpenetrating solutes (those that would cause this water to go from compartment with lower Osm to the compartment with higher Osm). Urea is a penetrating substance. It will diffuse through the membrane and in the result the concentration in both compartments will be equal. So I would say that urea gives zero osmolality, it goes through the membrane as easily as water so it does NOT cause water movement through the membrane.
second that .
motion moved
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Quote:
Originally Posted by Casandra View Post
En example with urea is good but I would think about it this way:

Only effective osmolality can cause water movement. e.g. increase in Na in one of the compartments. Effective osmolality is created only by nonpenetrating solutes (those that would cause this water to go from compartment with lower Osm to the compartment with higher Osm). Urea is a penetrating substance. It will diffuse through the membrane and in the result the concentration in both compartments will be equal. So I would say that urea gives zero osmolality, it goes through the membrane as easily as water so it does NOT cause water movement through the membrane.
Thanks for the effort........really appreciate that.....
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