Osmotic Pressure (freshwater vs seawater)

[fastnfurious]

My Qbank solution says If an organism goes from Freshwater to Seawater, then the osmotic pressure of the environment Decreases...

I thought it was increases since seawater has more solute and more solute = higher osmotic pressure?? Any one care to explain. If it helps I can paste the question, just let me know!

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[HawkeyePostOp]

Looks like a mistake in the wording.

They probably meant net osmotic pressure instead of environment.

 

[fastnfurious]

The contractile vacuole of Euglena decreases its rate of contraction when the organism is transferred from fresh water to seawater. This is explained by

an increase in the osmotic pressure of the environment.

a decrease in the osmotic pressure of the environment.

the fact that nitrogenous wastes can remain in the cell because of a higher salt concentration outside the cell.

the elimination of excess salts without the loss of water.

the inhibition of the contractile apparatus by salt.
maybe that helps

 

[HawkeyePostOp]

"Of the environment" in this case means "in the new setting", not "from the water"

There is a decrease in the osmotic pressure from the original state. Biological tissue has some osmotic pressuse, fresh water has none. The overall effect is that this gradient creates high osmotic pressure and a tendency for small particle flow into the water and water flow into the organism. This is compensated in Euglena by contractile vacuoles.

If placed in the new salty environment with more osmotic pressure, the difference in osmotic pressure decreases, so the net osmotic pressure (the important one) decreases. Thus the euglena compensates less with the vacuoles.

 

[fastnfurious]

thanks! I'm glad there was someone who was able to explain it because I was thinking it was pertaining to the water's osmotic pressure