Osmosis question

[bioislife]

When water diffuses through a semipermeable membrane it is due to the difference in water concentration or potential between membrane sides. What is keeping the water from diffusing out of the membrane at places where air would be on the other side of the membrane? Can you explain it in "middle school" terms?

[blcr11]

Nothing. (Well, not completely nothing. There is a tendency for gels to try and hold on to strongly bound water of hydration.The cytoplasm will give up tightly bound water very reluctantly, but it will eventually have to if the cell stays exposed to the atmosphere.) The water moves through the membrane and evaporates, becoming water vapor rather than liquid water. Eventually, if it’s a cell, the cell dessicates, although the membrane itself may dehydrate and change it’s physical properties before the cell completely dries up.

[mith]

As blcr11 said, you end up getting evaporation. The reason why you don't see evaporation happen as fast as say salt/water diffusion is because water has to become a gas in order to mix with air(imagine large liquid water droplets floating in air, now that's quite trippy). Now we know that you have to heat water to get water vapor/steam otherwise only a tiny bit of moisture is dissolved. Given the energy available at room temperature that's highly unlikely(but still happens).

[bioislife]

Thank you both. You have made it much easier to understand. Makes perfect sense.