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April 24, 2005

Osmo-what?

Osmotic pressure? You want me to talk about osmotic pressure? Well, sure... but why do you ask?

Ha, so ok, it was my idea. But I think it's kind of a neat thing.

Aqueous solutions, like many other chemical systems, tend towards equilibrium. Decoded: if you put a chunk of salt in a pot of water, the salt will slowly dissolve, but will spread the saltiness through the whole pot of water rather than all the saltiness just staying where the chunk of salt was. This process happens by diffusion, which we think we understand intuitively because it happens so often (smells spreading through a room, seasonings spreading in a dish) but really there's a chemical basis to it which can have some pretty nifty effects.

Diffusion really works like the following. Imagine a box. Now put some rubber balls in the box that never stop bouncing - the superest super balls you can find. This box is actually a pretty good approximation of what chemists call an ideal gas. OK, so this box is cool, but it's not useful yet. Let's take all the balls out of the box, and put in a divider so that the box is split in two halves. Let's put holes in this divider that are big enough for the balls to fit through but not so big that the divider isn't there any more :) Now, let's put all the balls back in the box, but we'll put them all on one side of the divider. As the balls bounce around, every once in a while one will approach the divider but will be going at just the right angle so that it goes through the hole and on to the other side. The probability that a ball will make this transition across the divider is a function of a few things: the average ball speed, the size of the hole relative to the size of the ball, and the number of balls on one side.

Now you can imagine what would happen over time. First we start with all the balls on one side. Every once in a while a ball would cross over, but it's far more likely at first that a ball will go from the more-balls side to the less-balls side than the other way around. And eventually there will be the same number of balls on both sides, so the probability of transitioning will be equal and the system is said to be at equilibrium.

Ok, fine, this is all good, but what does it have to do with osmotic pressure? And what is osmotic pressure in the first place? Well, osmosis is the diffusion of water across a semipermeable membrane. It is not, as college students think, that mysterious process where you learn things by sleeping with your textbook under your pillow. In fact, if you put "osmosis is sleeping with your textbook under your pillow" on your chemistry exam then you should have at least been paying enough attention in class to know what osmosis means! Ahem, anyway... osmosis is interesting because there are lots of semipermeable membranes out there. By the way, the definition of a semipermable membrane is a lot like that divider I described above. You could imagine that the divider has holes big enough to fit your superballs through but not baseballs. That's a semipermable membrane, and the most common one that we all deal with on a regular basis is the cell membrane, called the "lipid bilayer" by us biogeeks. All cells have at least one of these membranes, and the cool thing about them is that they let water through, but not big molecules like proteins. This is a very good thing! Otherwise, of course, we would just fall to pieces as all our proteins leaked out.

There is something pretty important, though, about the passage of water across these membranes. See, it's a curious chemical fact that aqueous solutions (stuff dissolved in water) tend towards equilibrium. What that really means is that if you take two different solutions, let's say different amounts of regular table salt dissolved in water, and you put them on either side of a semipermeable membrane which lets water molecules through but not salt ions (this is easy because salt ions are relatively big compared to water molecules) the water will move from the lower salt concentration to the higher salt concentration. It does this because the water wants to be in the same concentration everywhere, and it moves through the membrane in order to bring these concentrations to equilibrium.

But wait, you say, this is silly, if the membrane wasn't there the same thing would happen. Yes, of course, it's the same thing that happens when you put a chunk of salt in a pot of water, the area near the salt gets really salty really quickly, but the water moves in to even things out and slowly the salt diffuses through the whole pot. The point of the membrane is that as the water moves, the salt does not! Which means that you end up having different volumes of solution on either side of the membrane. And any time a volume changes, a pressure is involved... hence osmotic pressure.

Osmotic pressure has several important uses and/or connotations. First, you've all probably heard about reverse osmosis water filters. Well, the osmosis is just what I just described. The reverse part is because the filter works by having a semipermeable membrane where one side has a higher solute concentration (a solute is anything dissolved in a solvent, in this case water) compared to the other; water is added to the the high-solute side and pressurized to overcome the backwards osmotic pressure. This produces clean water on the low-solute side since only water can pass through the membrane.

The other important thing about osmotic pressure is related to biology. As I mentioned above, the most common semipermeable membrane is the cell membrane. It's probably also obvious that cells are chocked full packed of nice solutes like proteins and sugars and salt ions and all sorts of other neat molecules. Well, this is all very useful for the cell but there's one slight problem - if the cell ever interacts with pure water with very little dissolved solute, an osmotic flow happens where the water outside the cell rushes into the cell in order to try to equilibrate the solute concentrations. This creates a pressure inside the cell and the cell literally blows up like a balloon. If the pressure is too great, then the cell bursts! This is why, if you're severely dehydrated, they give you a saline IV which has dissolved salts equal to the dissolved salt in the blood. Otherwise, if you got an IV of pure water, your blood cells would explode and you would get very sick very quickly. This is also why people can die from drinking too much water. We don't drink saline water (for some crazy reason I don't really know why) so if you drink lots and lots and lots of water the salt concentration in the blood drops and you get the same problem of blood cells bursting as well as other cells. This only would happen if you drank like gallons of pure water.

So the moral of this story: When running a marathon, take the Gatorade cups (which have dissolved salts! they like to call them 'electrolytes') rather than the water cups.

P.S. Oh yeah, one more thing. It's way past my bedtime but I think this is the coolest osmosis-related thing. So you know how plants get kinda limp and flabby if you don't water them? Or if they die? Well, here's why - it's osmosis related! Every plant cell has a giant water bubble inside it called a vacuole. This bubble is surrounded by a membrane, yes, a semipermeable membrane like all other biomembranes, which has molecular pumps imbedded in it. So when the cell is alive, those pumps pump salt ions (like potassium and chloride) into the vacuole. As this happens, water molecules follow naturally across the membrane, causing the vacuole to pressurize and expand outwards. This causes the whole cell to pressurize and to stiffen up against the cell walls. Hence, nice firm plants, crisp lettuce, etc.! If you forget to water your plant, though, those pumps stop running, the ions leak out and so does the water - which is a good thing for the plant because it keeps it alive at the cost of a little flabbiness. And if the plant dies, the pumps stop completely and all the ions and water leak out and the plant goes completely limp. So water your plants! They thank you.

Posted by kgutwin at April 24, 2005 10:03 PM

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Comments

Posted by: Rebecca (the girlfriend) at April 25, 2005 06:29 AM

Posted by: Rebecca (the sister) at April 25, 2005 07:19 AM

Posted by: Gramps at April 25, 2005 04:30 PM

Posted by: mom at April 25, 2005 09:20 PM