It makes sense to me that water flows to areas of low water concentration, but that’s just intuition. If we look at it chemically, can we say that the water soluble (water attracting) solute particles are attracting in more water particles?
After thinking about my own theory, I don’t even get it.
Let’s say there are more Na and Cl ions outside a cell than inside. Aren’t they already attracting and momentarily sticking to as many water molecules as they possibly can?
That’s not even mentioning something like sugar, which isn’t vey hydrophilic. And of course, a large molecule like sugar has trouble moving through a membrane without special help, so it’s not like releasing two gasses in a room and finding that they mixed evenly by random dispersion.
I know someone out there can explain this in terms that I can explain to high schoolers.
I really don’t get the post, so perhaps I should not be posting, but osmosis means that the concentration of two areas of a water solute separated by a membrane want to become equal. Hence, there is diffusion through the membrane from the stronger solution to the weaker solution, unless there is an active transport system in place to overcome the osmotic effect.
In the case where you have more sodium and chloride ions outside a cell than you do inside, assuming the membrane is permeable to water but not to sodium or chloride, water molecules will constantly be moving back and forth across the membrane at random, but the ones outside the cell will be moving a little slower because of their interactions with the ions. The ones that move out of the cell won’t be replaced by an equal number of ones moving inward because the ones outside are inhibited in their movement, so you get a net flow outward.
You can make osmosis a lot more complicated than that, but I hope that’s what you were looking for.
I don’t think it has anything to do with “interactions,” between the solvent molecules and the solute molecules. Osmosis is simply a matter of probability and energy. Imagine you have pure water on one side (the A side) of a membrane permeable to water but not to a solute, S, and a solution of water and S on the other side (the B side). There are no “interactions” between molecules of S and water molecules except that they occasionally bounce off each other (and the membrane and the walls of the container) as they move around randomly. Now as the molecules on the A side bounce around, they will occasionally pass through pores in the membrane and enter the B side meaning that water will be flowing from A to B. At the same time, the molecules on the B side will be bouncing around with the same energy (both sides are at the same temperature) and will occasionally “hit” the pores in the membrane. However, only the water molecules will pass through. Since the water molecules make up less than 100% of all the molecules on the B side, the amount of water flowing from B to A will be less than from A to B (remember 100% of the molecules on the A side are water molecules). So there will be a net flow of water from A to B. This will last as long as the water molecules (which can pass through the membrane) on the A side hit and pass through the pores of the membrane more frequently than the water molecules on the B side hit and pass through. And it isn’t that the water molecules on the B side are sticking to the S molecules or “interacting” with them in any special way, they are simply not hitting the pores as often because when a molecule hits a pore on the B side it is often an S molecule whereas when a molecule hits a pore on the A side is it is always a water molecule.
Eventually an equillibrium will be reached (and the net flow of water will cease) when water molecules on the B side start hitting and passing through the pores as frequently as water molecules on the A side. This can only happen when the energy on the B side increases so that there there are more molecules hitting pores per unit time on the B side than on the A side. This will happen with the pressure increases on the B side.
Water molecules are electrically polarized. An ion dissolved in water interacts electrostatically with the water molecules to cause the water molecules to form an ordered structure called a hydration sphere around each ion. In the hydration sphere of a negative ion, for example, the partial positive ends of the water molecules are oriented towards the ion, where they are attracted by the electrostatic interaction. The formation of the hydration sphere is what allows ionic solids to dissolve, and it also inhibits the movement of water molecules through membranes. What I posted was a simplified version of this because that’s what the OP wanted.
Thanks. I think I do get it now. It seems that both reasons are good thoughts.
One more thing: water soluble compounds have to be somewhat polar, right? So there’s a polar end to sugars, whereas there’s not to oils, fat, and other lipids?
Oh, and in my search for illustrations of a hydration sphere, I found out that I’m not ingesting enough Clustered Water. I think we’ll have to change the phrase from “snake oil” to “snake water”.
Sugars have several polar OH groups that make them soluble. Lipids tend to be mostly carbon and hydrogen, which have about the same electronegativity, so they don’t form polar bonds.
The posters above did a fine job describing osmosis, but I think the OP was going for something simpler: does the solute pull water toward the solute? Or does the water push itself toward the solute?
The answer is yes. Whichever one you want. It’s just a matter of perspective. Either way, the water still moves.
Unrelated to the OP but illustrative of the same sort of perspective, my high school teacher who taught auto mechanics insisted that a downward moving piston absolutely positively does not “pull” the air-fuel mixture in to the cylinder. He gave all the credit to the atmosphere exerting 14.7lbs psi pressure to “push” the air-fuel mixture in.
No ther’s definitely a polar end to lipids. The hydroxyl group dissociates like it does in any carboxylic acid. The reason it doesn’t dissolve well is the long hyrdocarbon chain attached to the hydroxyl group whic tend to all congregate together.
IMHO the best way to look at osmosis is via thermodynamics. Of course you need to consider the physical characteristics of the semipermeable membrane and the solute/solvent characteristics on both sides of the membrane. But what DRIVES the process is the thermodynamic state of the system.
No. That’s true of fatty acids and membrane lipids, but most of the lipids in your body are in the form of triglycerides, which don’t have any charge on them at all and don’t have a polar end. And the reason they don’t dissolve in water is that the water molecules have a much stronger attraction for each other than they do for the hydrocarbon chains. The hydrocarbon chains are attracted to each other pretty weakly.
Cardinal asked about the polarity of lipids, fats and oils and someone suggested lipids don’t form polar bonds. I addressed this. You’re quite right about fats and oils which may be triglycerides, but for lipids my statement is quite true.
The hydrocarbon tails aggregate because they aren’t as strongly attracted to water, the water aggregates because of hydrogen bonding which doen’t occur as strongly with hydrocarbons. I say the hydrocarbons clumb and the water disperses away, you say the water clumps and the hydrocarbon disperses away.
Sorry Bob Scene, I assumed that the poster of “Reasons for osmosis” wanted to know the general reasons for osmosis, not about the particular case of ions in a polar solvent.
