Osmotic/Hydrostatic Pressure

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Dr Gerrard

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From a Kaplan subject test, don't know why they give 5 choices, but whatever.

3. The contractile vacuole of Euglena decreases its rate of contraction when the organism is transferred from fresh water to sea water. This is explained by
A. an increase in the osmotic pressure of the environment.
B. a decrease in the osmotic pressure of the environment.
C. the nitrogenous wastes can remain in the cell because of a higher salt concentration outside the cell.
D. excess salts are eliminated without the loss of water.
E. salt inhibits the contractile apparatus.

What would you say?

Answer is B.

I thought it was A, since it says "of the environment."

I thought osmotic pressure was the pressure to bring water in, and since there is a higher concentration of solute in the environment, there would be a greater osmotic pressure in the environment.

I think the osmotic pressure inside would remain the same, while the osmotic pressure on the outside would increase and so that is why net movement is outside.

Hydrostatic pressure on the inside increases as well... so is hydrostatic pressure on the inside = osmotic pressure on the outside???

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I thought osmotic pressure was the pressure to bring water in, and since there is a higher concentration of solute in the environment, there would be a greater osmotic pressure in the environment.

Think about it like this, the sea water has a lot of osmotic pressure as you said due to more solute. So when you put a fresh water fish in this environment, water will leave the fish; this as the effect of lowering the concentration of the environment. Similar to hypertrophy, waters leaves the cell to reduce the highly concentrated nature of the environment. Furthermore, if the solute is permeable to the fish, it would enter the fish (high to low gradient) thereby lowering the overall concentration of the environment.

Kinda tricky question that I would definitely get wrong if pressed for time on the mcat.
 
So wouldn't the salt water have a greater osmotic pressure than the fresh water?

According to your first sentence, the answer would be A. Is this correct?
 
See, I think if the answer said "a decrease in the osmotic pressure inside the cell" then it would be correct. But it says "of the environment" instead, which is what gets me.
 
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uhhhhh... well i think i am now officially confused over osmotic pressure..

what i get out of osmotic pressure is that its the tendency of water to flow into an environment due to the solute. the actual chem. term for osmotic pressure is the pressure of water exerted ON the membrane WHEN equilibrium is established. sooo, intially the sea water has a very high osmotic pressure, because it really needs water, when water finally enteres the environment, this pressure subsides at equilibrium..

I THINK?! help would be nice on this topic as it is very important and can appear in both PS and BS


im taking kaplan too, they dont give explanations to those right? I need to get cracking on doing those subject tests because ive only been doing the topical tests..
 
anyone else get this? i've read the other treads regarding the same topic but still dont clearly get it...
 
This question is poorly worded. I would've picked A, B doesn't make sense as per the question itself.
 
I had to google what exactly the contractile vacuoule was. The answer makes sense to me: when the vacuoule is transferred from fresh water to sea water, its water content decreases. It's like putting a permeable bag of pure water in a solution of NaCl...water will leave the bag because it's surrounded by a hypertonic solution.

When water leaves the contractile vacuole, the osmotic pressure inside the vacuole increases (since the concentration of whatever is inside increases as well); at the same time, the osmotic pressure outside (ie. the environment) decreases since it gains water (that leaves the vacuole).

Answer choice A doesn't make sense because you're decreasing the concentration of the sea water, which would mean that its osmolarity is decreasing; it's becoming a less hypertonic solution. If choice A said that the osmotic pressure of the vacuole increases, it would be accurate.

Choice B makes sense because a more diluted solution means that the osmotic pressure decreases.

I usually think of osmotic and hydrostatic pressures in terms of capillary exchange. I hope my explanation made sense; if not, I can explain in terms of capillary exchange as well. Hope this helps.
 
I had to google what exactly the contractile vacuoule was. The answer makes sense to me: when the vacuoule is transferred from fresh water to sea water, its water content decreases. It's like putting a permeable bag of pure water in a solution of NaCl...water will leave the bag because it's surrounded by a hypertonic solution.

When water leaves the contractile vacuole, the osmotic pressure inside the vacuole increases (since the concentration of whatever is inside increases as well); at the same time, the osmotic pressure outside (ie. the environment) decreases since it gains water (that leaves the vacuole).

Answer choice A doesn't make sense because you're decreasing the concentration of the sea water, which would mean that its osmolarity is decreasing; it's becoming a less hypertonic solution. If choice A said that the osmotic pressure of the vacuole increases, it would be accurate.

Choice B makes sense because a more diluted solution means that the osmotic pressure decreases.

I usually think of osmotic and hydrostatic pressures in terms of capillary exchange. I hope my explanation made sense; if not, I can explain in terms of capillary exchange as well. Hope this helps.

do you think my initial reply to this topic was right too? i think we both have the same idea. i just want to make sure i have this concept down
 
do you think my initial reply to this topic was right too? i think we both have the same idea. i just want to make sure i have this concept down
Yea, it seems right. I'm not sure what you mean by the hypertrophy part though. If anything, I'd assume that the water content of a cell would increase in the case of hypertrophy.
 
I had to google what exactly the contractile vacuoule was. The answer makes sense to me: when the vacuoule is transferred from fresh water to sea water, its water content decreases. It's like putting a permeable bag of pure water in a solution of NaCl...water will leave the bag because it's surrounded by a hypertonic solution.

When water leaves the contractile vacuole, the osmotic pressure inside the vacuole increases (since the concentration of whatever is inside increases as well); at the same time, the osmotic pressure outside (ie. the environment) decreases since it gains water (that leaves the vacuole).

Answer choice A doesn't make sense because you're decreasing the concentration of the sea water, which would mean that its osmolarity is decreasing; it's becoming a less hypertonic solution. If choice A said that the osmotic pressure of the vacuole increases, it would be accurate.

Choice B makes sense because a more diluted solution means that the osmotic pressure decreases.

I usually think of osmotic and hydrostatic pressures in terms of capillary exchange. I hope my explanation made sense; if not, I can explain in terms of capillary exchange as well. Hope this helps.

So can we agree that this is a poorly worded problem?

I mean wouldn't the osmotic pressure of the salt water environment be greater than the osmotic pressure of the fresh water environment?

Wouldn't this initial increase in osmotic pressure allow water to flow out of the cell without assistance from the vacuole thing? This is why the vacuole thing stops functioning when it gets put in salt water.
 
So can we agree that this is a poorly worded problem?

I mean wouldn't the osmotic pressure of the salt water environment be greater than the osmotic pressure of the fresh water environment?

Wouldn't this initial increase in osmotic pressure allow water to flow out of the cell without assistance from the vacuole thing? This is why the vacuole thing stops functioning when it gets put in salt water.
The thing is, the question is not asking which compartment has the higher initial osmotic pressure. If the question asked that, then yes, the answer would be the environment.

My understanding of the contractile vacuole is that it pumps water out of the cell. If the vacuole is placed in a hypertonic solution (such as the sea water), water would leave the vacuole in order to equilibrate the osmotic pressures inside and out. So, there would be less water inside the vacuole; this means that the vacuole contracts less frequently. If its primary purpose is to pump water out of the cell and the water content inside has decreased, then it makes sense that it wouldn't contract as often. See what I'm saying? It doesn't have to pump as much water out of the cell, so there is no reason for it to keep contracting at a high frequency. Hope this helps.
 
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