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Old 03-07-2014, 08:08 AM   #1
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Osmotic pressure

Hmm I'm struggling to understand something with regards to this I'm hoping someone can help.

Basically I want to know where the osmotic pressure is highest between fish in soft low ph water and fish that live in hard high ph waters and why.

Disregarding marine fish altogether. From what I understand water will go to the medium with the highest concentration of a solutes (opposite of diffusion) which means in fresh water fish water is constantly rushing in to there bodies. This means that the need to rid of this water by passing frequent dilute urine as to retain as much solutes as possible. Inevitably the fish will lose some. Luckily they have sodium and chloride cells in the gills that can take up these lost fluids. This uses energy and can stress the fish. To achieve this balance of loss and gain is called osmoregulation.

I also know that water will tend to go towards the side with the highest osmotic pressure. The pressure required to keep this water intake down is known as the osmotic pressure.

What I'm asking is how does the pressure compare in hard and soft freshwater fish. Which of these have the highest osmotic pressure and why?

I hope I'm making sense here?

Thanks
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Old 03-07-2014, 08:43 AM   #2
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With the assumption that soft water and low TDS go hand in hand, then I would say that fish in soft water fish deal with the highest osmotic pressure simply because of the low solute present (greater difference) compared to hard water fish.
However, if the logic in the following link is correct, then soft water can have a low or high TDS. It depends on what was done to make it soft:
http://coffeetime.wikidot.com/total-...lids-tds<br />
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Old 03-07-2014, 10:06 AM   #3
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Originally Posted by Fresh2o View Post
With the assumption that soft water and low TDS go hand in hand, then I would say that fish in soft water fish deal with the highest osmotic pressure simply because of the low solute present (greater difference) compared to hard water fish.
However, if the logic in the following link is correct, then soft water can have a low or high TDS. It depends on what was done to make it soft:
http://coffeetime.wikidot.com/total-...lids-tds<br />

Would this pressure be external exerting on the walls of the fishes skin or internal pressure build up. I'm struggling with which way the pressure builds up and what chemical acts.

If water rushes in to the fish and prefers to head towards the side with greater osmotic pressure. That would mean the osmotic pressure would have to be on fish side of its membrane. This would mean that the solutes in the fish would be diluted and lost through urine.

How does water with lower solutes effect the osmotic pressure in comparison with water with slightly higher solutes.

What exactly is it that causes osmotic pressure to be greater in soft water?
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Old 03-07-2014, 12:17 PM   #4
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the fish will come to equilibrium with the water...the water is slowly changing all the time, so, the fish is slowly changing all the time...a drastic change in water conditions can affect the direction of osmotic flow...too large of a change can damage the fish...i would assume they feel the change as much as we do when we go from one condition to another, say a cool humid room to a hot dusty dry outside...these exchanges in the fish occur in the cells...

direction of flow depends on the change i.e. did you add salt? if so the fish would "lend" some water to the tank until it took in enough salt to come to equilibrium with the new conditions...

think of a tank with a semi-permeable membrane dividing it...put hard water with calcium, salts, etc. in it...on the other side put soft DI/RO water... imagine this as a microscopic view of a fish cell on one side and the tank water on the other....the water on the hard water side will pull some of the soft water through, the soft side will take on salts, and they will exchange until they are the same...
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Old 03-07-2014, 01:03 PM   #5
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Originally Posted by Aldeub View Post
the fish will come to equilibrium with the water...the water is slowly changing all the time, so, the fish is slowly changing all the time...a drastic change in water conditions can affect the direction of osmotic flow...too large of a change can damage the fish...i would assume they feel the change as much as we do when we go from one condition to another, say a cool humid room to a hot dusty dry outside...these exchanges in the fish occur in the cells...

direction of flow depends on the change i.e. did you add salt? if so the fish would "lend" some water to the tank until it took in enough salt to come to equilibrium with the new conditions...

think of a tank with a semi-permeable membrane dividing it...put hard water with calcium, salts, etc. in it...on the other side put soft DI/RO water... imagine this as a microscopic view of a fish cell on one side and the tank water on the other....the water on the hard water side will pull some of the soft water through, the soft side will take on salts, and they will exchange until they are the same...

So are you saying that osmotic pressure is on both sides of the permeable membrane and it is swapping over all the time in an effort to achieve equilibrium?
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Old 03-07-2014, 02:01 PM   #6
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in real world yes...technically the cell will not take in a salt compound but the cell will take in the components to build what is necessary to offset the external pressure or lack of...if using a membrane that is too small that only water molecules can cross then you will see a water rise on the hard water side in the given example...but only enough that the osmosis can push against the head pressure...
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Old 03-07-2014, 02:03 PM   #7
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oh and pressure is both negative and positive...so creating a positive pressure on one side creates a negative on the other...
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Old 03-07-2014, 02:25 PM   #8
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Originally Posted by Aldeub View Post
oh and pressure is both negative and positive...so creating a positive pressure on one side creates a negative on the other...

Ok so in fish that live in soft water the water wants to rush from the water to a place where there is higher solute concentration (in the fish) because the is less solutes in the water this means that the fish has to work harder in order to hold equilibrium?
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Old 03-07-2014, 02:32 PM   #9
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when the fishes environment changes (it does not matter if the tsd is changed that would force a positive or negative osmostic gradient either way) the fish (it's cells) use more energy to regulate themselves to adjust to the change...
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Old 03-07-2014, 02:42 PM   #10
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Originally Posted by Aldeub View Post
when the fishes environment changes (it does not matter if the tsd is changed that would force a positive or negative osmostic gradient either way) the fish (it's cells) use more energy to regulate themselves to adjust to the change...

Ok thanks so much for clearing that up. Big weight off my mind
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