happygirl

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Hi,

I was hoping someone could clarify this question for me:

The contractile vacuole of Euglena decreases its rate of contraction when the organism is moved from freshwater to seawater. This can be explained by:

A. an increase in osmotic pressure of the environment
B. a decrease in osmotic pressure of the environment
C. blah blah
D. blah blah

The correct answer is B. I am confused because since the environment is saltier, aka more concentrated, doesn't the environment's osmotic pressure increase, meaning fluid would want to flow into it more than if it was freshwater?

Thanks!!!!!! **oops i guess this should go in the 'study questions' forum...
 

Chemist0157

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I think osmotic pressure is defined as the potential of water leaving the area. Since it's now more concentrated, the environment is now less likely to lose water (so it gains water).
 

SeekerOfTheTree

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Hi HappyGirl,

I am going to take a stab at this...don't take my answer as the best but after reading the question I had thought B because if the vacuole was in a freshwater environment before and it was contracting then when you move it to a salty environment there are more salt particles in the external environment....that's why pressure would tend to flow outwards...the hypo and hyper osmotic stuff comes to mind here.
 

SeekerOfTheTree

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oh and on top of everything a decrease in the osmotic environment of the vacuole occurs because after you put it in an environment with more particles outside then the water will flow out and the pressure will get less in the vacuole because the vacuole will want to have the same concentration inside as the outside environment has.
 
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happygirl

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hi,

thanks for the reply =) but im still confused.

I see that if the seawater is more hypertonic than the vacuole, so fluid should move from the vacuole to the seawater to even out the concentration gradient. But what I don't understand is why the environment (the seawater) has decreased osmotic pressure b/c isn't osmotic pressure the driving force of water in? So if the environment is more concentrated than before (since it's seawater vs. freshwater), shouldn't there be an increased driving force of water into the environment aka increased osmotic pressure of the environment?

Also, the explanation that was given by Kaplan included information about the vacuole. In freshwater, the contraction is to pump water out. In seawater, this is activity is not longer needed since water is not going into the vacuole anymore since it is now hypotonic to the environment. I didnt include this in the original post because I was hoping someone could explain the answer w/o that information because that was information I didnt know while answering the question.


ahhh =(
Thanks, again!!!!
 
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happygirl

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chemist, thanks for the response!

but, I thought osmotic pressure was the force required to resist the movement of water. So, if the environment has become saltier, then water wants to move into it thereby increaseing the force required to resist the water aka increasing osmotic pressure of the environment?

or am i totally getting the reference points confused? water is now less likely to move out of the environment so, from that perspective, the force resisting the movement of water out is decreased??? i can see how that works, but i always thought of osmotic pressure being related to an influx (as opposed to efflux) of water....like hydrostatic pressure is more efflux and osmotic pressure is more influx?

so confused =(
 

Zerconia2921

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Hi,

Accidentally posted this in the mcat discussion thread, but here it is again....
I was hoping someone could clarify this question for me:

The contractile vacuole of Euglena decreases its rate of contraction when the organism is moved from freshwater to seawater. This can be explained by:

A. an increase in osmotic pressure of the environment
B. a decrease in osmotic pressure of the environment
C. blah blah
D. blah blah

The correct answer is B. I am confused because since the environment is saltier, aka more concentrated, doesn't the environment's osmotic pressure increase, meaning fluid would want to flow into it more than if it was freshwater?

Thanks!!!!!!

If it says that the contraction decreased that means there is less pressure for water to go into the cell.

If the contraction increased that would mean the new enviornment was not as concentrated as the inside of the cell. I hope this made sense.

Remember osmosis is the movement of water the moving is the thing that is causing the osmotic pressure. When ever I have trouble with this i always remember two compartments one concentrated the other not. Water will move to the concentrated compartment via semi permeable membrane. The side it moved from will be lowered in volume and the side it moves to will increase up to a point. Once equilb is established no more water will pass due to the osmotic pressure.
 

Vihsadas

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EDIT****See lower post. This post is wrong!!!

Vihsadas said:
So you already see that fluid should flow from inside the vacuole to the outside environment because the solute concentration is greater on the outside. Great!

The question, however, is really testing if you understand WHY this happens, rather than just your knowledge of whether it happens. To tackle this problem, let's think about a few things that you already know from MCAT physics and bio.

Think about what pressure actually is in the context of fluids. Go back to your knowledge of MCAT physics:
Fluid pressure is really the tendency of a fluid to move from one area to another area. In order for this fluid to move, it has to move along a pressure gradient. So, by convention, the direction of fluid flow is from places of higher pressure to places of lower pressure.

Now think about your knowledge of MCAT bio. Osmotic 'pressure' is really the tendency of a fluid to move from one compartment to another because of differences in solute concentration between the two compartments. Because we are talking about the flow of a fluid, this term is denoted as a type of 'pressure'.

So in your example, you already know that the fluid should move from the area of lower solute concentration to the area of higher solute concentration, i.e. from inside the vacuole to the outside. Since we've already established that fluid must flow down a pressure gradient, i.e. form a compartment of higher pressure to a compartment of lower pressure, if the water is flowing from the vacuole toward the external environment, then the vacuole must necessarily be the area of higher osmotic pressure!

Thus the vacuole has the higher osmotic pressure, and the environment has the lower osmotic pressure, and consequently the fluid will flow from the vacuole to the environment.

So while it is counter-intuitive to think of the compartment with more solutes to have less osmotic pressure, this is how science has defined it! I find it easier to think in terms of fluid flow. Just ask yourself, "which way is the fluid flowing?". If you can answer that, you know that the area where the fluid is coming from has higher pressure, and the area where the fluid is going to has lower pressure. Hope this helps!
The confusion in my post here is that osmotic pressure does not define the tendency of water to move, but rather (I think) the tendency of solutes to move. The definition is pi= iMRT, which is a measurement of pressure and it increases as solute concentration increases. So either the test makers just got this one wrong, or there's still something that we're not understanding.
 
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happygirl

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Don't worry! :p

So in your example, you already know that the fluid should move from the area of lower solute concentration to the area of higher solute concentration, i.e. from inside the vacuole to the outside. Since we've already established that fluid must flow down a pressure gradient, i.e. form a compartment of higher pressure to a compartment of lower pressure, if the water is flowing from the vacuole toward the external environment, then the vacuole must necessarily be the area of higher osmotic pressure!

So while it is counter-intuitive to think of the compartment with more solutes to have less osmotic pressure, this is how science has defined it! I find it easier to think in terms of fluid flow. Just ask yourself, "which way is the fluid flowing?". If you can answer that, you know that the area where the fluid is coming from has higher pressure, and the area where the fluid is going to has lower pressure. Hope this helps!

Hi, thanks for the reply! =) I guess I'm still confused because I thought the question was asking more for a comparison of the before and after states of the environment, ie. the osmotic pressure of the freshwater and the osmotic pressure of the seawater. I can follow your logic about how how water is flowing from a high fluid pressure area to a low fluid pressure area (vacuole to seawater), so by this definition I can see how comparing fluid pressure between the vacuole and the seawater, the seawater has lower pressure. But, again, I thought the question was more about the freshwater vs. the seawater. But now that I'm typing this, I guess the same logic can still apply. The freshwater has higher fluid pressure than the seawater, so the pressure of the seawater has decreased when compared to the pressure of the freshwater! Finally!

Ok, so thinking about it that way does make sense, but yeah, that totally goes against my way of thinking about osmotic pressure. So, I guess my main question is (which you did actually already address) the definition of osmotic pressure being the force that resists the flow of water IN is not correct? Because when using that definition, I still don't see how the answer is right because, more concentrated = more water in = more resistance = more osmotic pressure. But what you are saying is to think of osmotic pressure as fluid pressure. The seawater has less fluid pressure than the freshwater.

That just seems so weird. Then if the question were to be asked, "does the seawater or the vacuole have greater osmotic pressure?" how would you answer that? Would you say that the vacuole has greater osmotic pressure because it has a greater fluid pressure? But now that I'm thinking about it, that seems more like hydrostatic, not osmotic pressure though.

omg, it's no use.....haha, it's just me. theres no hope =( but, thanks for all the replies. i def. appreciate it!!!!!!!!!
 

nlax30

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I think it's just a poorly worded question, that or it's always possible they listed the wrong answer.

Was this in a review book or something?

You're correct in your thinking though. The salt water will have a higher osmotic pressure than the freshwater because of the extra solutes in the saltwater (more hypertonic). Osmotic pressure is proportional to Molarity of the solution.

So if the question is really looking for the difference in osmotic pressures between the freshwater environment and the saltwater environment, I would have to go with "A", that the osmotic pressure of the environment would have increased (the saltwater).

Run the question by a teacher or tutor, but don't tell them what the book says the answer is. You know the "mechanism" so that's the important part.

No textbook or review is going to be 100% accurate, even med school texts. Every once in a while we'll come across a review question in a book that just does not make sense and usually the answer key was just marked wrong.

Did the book offer any sort of explanation?
 
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happygirl

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The explanation is as follows:
"The contractile vacole of Euglena removes excess water from the organism after it comes through the cell membrane osmotically. In fresh water, the osmotic pressure drives water inside the organsim due to a higher solute concentration with the Euglena. The organism must use energy (ATP) to force water back out. When this organism is transferred to salt water, the solute concentration is greater outside the cell, thus the osmotic pressure is smaller. This causes water to leave the Euglena. The contractile vacuole no longer has to remove water."

So, it's saying the decrease in osmotic pressure of the seawater is slowing the pumping of water out of the cell....but, to me, it sounds like it should be the decrease in hydrostatic pressure, not osmotic pressure.

Yeah, I asked my instructor (with the answer already circled), but her explanation didn't really help me understand it any better, because I fundamentally still don't see how higher solute concentration can be a decrease in osmotic pressure. I can see how less water will move out and thereby lowering fluid pressure, but not osmotic pressure. Maybe I'll try reasking now that I've talked it out with you guys =)
 

nlax30

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I'm with you on this. The hydrostatic pressure is decreased, but I would describe the osmotic pressure of the saltwater as being higher than that of the freshwater.

When the organism is in freshwater the conc. of solute is higher in the organism, so the osmotic pressure in the organism is greater than the surrounding water, and that helps drive water in.

Then you place the sucker in saltwater, now, compared to before, the osmotic pressure IN the organism is less than that of the surrounding saltwater and this helps driver water out.

You have the right understanding, just an odd question.
 
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happygirl

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thanks! and thanks to everyone for their input! =)
 

BloodySurgeon

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It's just worded oddly like other questions ive came across. The lower osmotic pressure its referring to is actually the vacuole not the salt water. Therefore you are correct even though you are marking the wrong answer... I fall victim to these answers all the time, but I have only seen one mistake on the aamc test so you can be reassured it wont be like this on the real test.

FYI the aamc mistake is that they said acetal instead of ketal (i don't remember which aamc it was)

The salt water has higher osmotic pressure to both distilled water and the vacuole (if no ATP is being used to force H2O out).
 

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Okay, I think you guys were right. I confused osmotic potential with the fluid pressure. I can't resolve this problem, but I do think that you guys are right in thinking that osmotic pressure increases with solute concentration. I just looked at the formula and it is clearly:

pi = iMRT, which has units: [number]*[Mol/L]*[L*Atm/(mol*K)]*[T]

which is a measurement of pressure. So in higher molarity, osmotic pressure is defined as increasing. The only way that I can resolve this is if osmotic pressure was defined as the tendency of solute particles to move, and NOT the tendency of water to move like I said before.

That being said, either the test maker made a mistake here, or there is not a consensus on whether osmotic pressure describes the tendency of the solutes to move or the tendency of water to move. From wikipedia, it would seem that the test maker just messed up on this one.

On another note, it seems that I missed this question before as well and wrote it down in my MCAT notes...just shows how easily you forget. :) I'm not quite sure what I eventually did about this. I think that I stuck to the wikipedia/pi = iMRT definition and it didnt' steer me wrong.

Sorry for the confusion
 

physics junkie

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Per usual I haven't read what other people have posted before posting this. All these posts and no diagrams!

Osmotic pressure is kind of a misnomer. A look at this diagram gives you a physical interpretation of what osmotic pressure means.



Let's do a thought experiment. First imagine that both are pure solutions with no solutes and the membrane isn't permeable meaning water can't travel between it. The water height on the left is still higher in this hypothetical situation. The additional pressure on the left side on the impermeable membrane as compared to the right side would then be given by pressure=density*gravity*height. This is equivalent in magnitude to the osmotic pressure. Now we have one way of thinking about what this pressure is.

Now let's return to the actual situation where the membrane is permeable and there is solute on the left side. Because there is solute on the left side it's going to attract more water and thus the water level will be higher on the left side. The solute side can be thought of as having a "sucking force" that pulls water into the solute side. This sucking force is called osmotic pressure and is equal to pressure=density*gravity*height. Alternatively, you could imagine, that if you had two pure solutions with a semi-permeable membrane the side has the lower water level has an additional amount of pressure(let's say by a pump that is pushing air into the tube) that is equal to pressure=density*gravity*height. 'Height' in this equation refers to the difference in height of the water level of the left and right hand side. This could also be thought of as "osmotic pressure" and is more intuitive for some people because pressure is always thought of as being positive. Indeed, the people who coined the term osmotic pressure thought of it this way instead of as a sucking force because it allows them to say the pressure is positive. To physicists pressure is always positive since it is considered a "pushing force" and if you ever use the term 'negative pressure' around a physicist they will not appreciate it. :laugh: Instead they will tell you that you describe situations like that using another system of units, mmHg, because negative pressure has no physical interpretation. You can look up how they define mmHg online but it's not really worth your time to do so.

Now, you might be asking yourself if the equation for osmotic pressure is pi = iMRT then what the hell am I talking about saying it is equal to density*gravity*height? Well, the first equation, pi = iMRT, is an equation you could use to predict the difference of water height. My definition, pressure=density*gravity*height, is how you could figure out what the osmotic pressure is from doing the experiment in the diagram and making a measurement of the height. So one equation has predictive powers and is stated in terms of the number of molecules of solute particles and the other has relevance for observational measurements. The two in fact measure the same thing so iMRT=density*pressure*height.

Up to this point all I'm trying to do is clear up what osmotic pressure is by building up your physical intuition. Hopefully I've been clear. If I haven't been please let me know and I'll be glad to try to simplify it further.


In regards to your question, I found this online of how Euglena contractile vacuoles work.

Toward the posterior of the cell is a star-like structure: the contractile vacuole. This organelle helps the cell remove excess water, and without it the euglena could take in some much water due to osmosis that the cell would explode.
So it's contraction rate would decrease because less water wants to flow in because the water outside is saltier and thus retains a good grip of the water that solvates the salt. That's why it's rate of contraction decreases--because it has to pump out less water. I guess if you considered the environment to be the vacuole of the cell then the osmotic pressure decreases. I think this is kind of a stupid question and I wouldn't waste my time wondering whether Kaplan's answer is correct or not. The important thing is that you understand osmotic pressure.
 

physics junkie

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I haven't read ur post physics junkie, but I just wanted to ask... How do you always have a diagram lying around to explain your answers?
One or two I've drawn by hand with a wacom tablet if I couldn't find an image on GIS(Google Image Search) :D I usually stick to answering general physics questions and for the most part those questions are the same across all undergrad physics classes so it's easy to find images with GIS.

Vihsadas(how do you pronounce that, btw?), that diagram was definitely an mspaint. Must've been a talented professor. My mspaint skills are on par(for other forums I frequent) but damn, hat one must've been a pain to make.