sfk
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Solutions A and B are seperated by a semipermeable membrane. Solution A contains 1mM sucrose and 1 mM urea. Solution B contains 1mM sucrose . Which of the following is correct?
a) Solution A has higher effective osmotic pressure than solution B
b) Solution A has lower effective osmotic pressure than solution B
c) Solutions A and B are isosmotic
d) Solution A is hyperosmotic with respect to solution B, and the solutions are isotonic.
e) Solution A is hyposmotic with respect to solution B, and the solutions are isotonic.
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| Jackofknives
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D
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| sfk
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plz explain
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| sfk
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no one? 
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| new_n_lost
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I think this is a Qbank Question and if memory serves me correct the Explanation given had something to do with Osmolarity of Urea. I cant where i have seen it but i have. If i find the explanation i will post it here.
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| sfk
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its got something to do with the fact that urea can permeate the membrane, but not sucrose. So would that make the 2 solutions isotonic or isosmotic?
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| Jackofknives
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osmosis is an account of osmotic particles, whereas tonicity is an account of EFFECTIVE osmosis.
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| mikky
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c... coz urea diffue across as it is a penetrating substance
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| Jackofknives
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mikky, i think what sfk meants was that "Jack, you got the right answer, could you please explain how you derived to that answer?"
Its a question testing definitions here.
Osmolarity is a measure of the amount of osmotically active particles in a solution.
Solution A is 2 mOsm/L, for B its 1 mOsm/L. so A is hyperosmotic compared to B.
tonicity is osmosis times reflection coefficient. bc urea has a reflection coefficient of nearly zero, it is not an effective solute, therefore A and B are isotonic.
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| sfk
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yes jok has the right answer. and a good explanation too. i was i bit confused about osmolarity and tonicity, now i get it. tonicity is effective osmolarity, urea contributes to osmolarity but not to tonicity.
Thanks jok. sometimes i just cant understand whats written in the book. and now its so clear, that i'm wondering what was it that i didnt understand. 
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| SmokyWaters
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its Q book question :P
C
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| amritt786
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hey guys its C , dont u remeber why DR goljan excluded osmolarity of urea
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| sfk
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no guys the answer is d. check brs physio.
Smokywaters what q book r u talking about?
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| step122
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the answer is c as miky said . with his explanation
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| i_m_legend
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d) Solution A is hyperosmotic with respect to solution B, and the solutions are isotonic
Solution A contains both sucrose and urea at conc. of 1mM,whereas B contains only sucrose at 1mM.Calculated osmolarity of A is 2mosm/L,& calculated B is 1mosm/L.Therefor sol A which has higher osmolarity,is hyperosmotic with respect to sol B.Actually,sol A and B have the same effective osmotic pressure (isotonic) because the only effective solute is sucrose,which has same conc in both the solutions.
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| luckiest
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Well think of its this way,
you got two kind of employee in each company.
Company who has more employee that actually WORK in ONE PLACE will be more matters to company.(TONIC)
Solution A:  
Solution B:
Urea: Lousy employee just walk back and forth two company doen't matter for TONIC
Sucrose: Work addict, stay in one place .. matter=TONIC
Company A : more people(hyper OSM)
But same amount of people who actually work.(isotonic)
I know it's non sense metaphor but just so we can learn togeter...
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| shohreh
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C...because urea osmolarity is zero..so to compatment have the same osmolarity...i think we can not predict tonicity with this information....
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| luckiest
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Osmolarity = Number of particle X concentration
The fact that Urea has reflective coefficient= 0 (It will go throiugh any membrane
will effect Tonicity not Osmotic.
Effecitve osmotive pressure= concentrationX number of particleX temperature X REFELCTION COEFFITIENT
This is where urea make 0 in the formula te istonicity(I guess the environment are the same)
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