|MadSci Network: Cell Biology|
Osmosis works by two principles.
First, the membrane must be permeable to the substance. It does not have to have any "pores", "holes", or "channels" through which a substance can move. Many small molecules can move through the lipids of the membrane without interacting with a transport protein. Water for example can move through a membrane as can things like glycerol and even to a very small extent, ions like Na+ and K+. (Note: A cell/membrane may have specific transport proteins for these same substances. For example, many membranes have "aquaporins" [literally, 'water pores'] through which water is specifically transporter.) Different membranes have different properties. Some membranes of our cells are permeable to water and some are not. Depends on the membrane composition, and it's rather hard sometimes to predict which membranes will be permeable.
There are also "artificial" membranes, made by some company for a particular purpose. These have a different composition. They are not composed of lipid like a cell membrane. They are usually some kind of plastic what has been formed in such a way as to have holes in it of a certain size. Any substance that fits through the hole can pass through the membrane. There are a lot of other things you can do to these artificial membranes to enhance or diminish the ability of certain things to go through them, but basically it's the size of the hole that matters most. This kind of membrane can be used for example to separate salt from sea water and get fresh water. Israel depends on these "desalination" plants for much of its fresh water for example.
Second, now that we've got a membrane, you must also have a concentration gradient. That is, the substance that is diffusing or osmosing through the membrane must be in higher concentration on one side of the membrane than it is on the other. If this is true, then the substance will tend to move from the high concentration side to the low concentration side until the amounts on both sides are equal. This wouldn't be very efficient or useful except in a laboratory. So most osmosis systems artificially keep the concentration on one side of the membrane very low by immediately removing anything that comes through.
Why does some substance tend to go from a high concentration to a low concentration? This is simply a kind of probability, or more succinctly "chance." Suppose you had a membrane with only one hole in it. One one side of the membrane you have 10 molecules of X dissolved in water. On the other side of the membrane you have 100 molecules of X also in water. X will eventually pass through the membrane to the low concentration side until there is an equal concentration of X on either side. (If the volumes on either side were equal, this would be 55 molecules of X on each side in this example.) Why does this happen? Well, suppose we had a camera that could record what happens right at the hole on both sides. If X is dissolved in water in this case, X is not stationary but is always moving randomly through the entire volume it is contained it. Each molecule of X is diffusing or moving around all over the place. Now suppose that one particular molecule of X diffuses into the area of the pore. There is some chance that it will continue to move/diffuse through the pore to the other side. However, if you have 10 molecules of X on one side and 100 on the other side, the number of times that some individual molecules of X would get close to the hole/pore would be 10 times different on the two sides, since there is a 10-fold concentration difference. In other words, the more molecules there are, the greater the chance that one of them will get to the pore. So if molecule X is 10 times as likely to get to the pore on one side versus the other, it is also 10 times more likely to go through the hole. Now we have 11 molecules on one side and 99 on the other. That's still a big concentration difference (or gradient), so the probability is that the next molecule of X to go through the hole will be from the side with 99 molecules. Note that this is a probability. It's entirely possible that the next molecule to go through the hole will be from the side with 11 molecules of X. But over time, since X from the side with the most molecules has the greater chance of finding the pore, it is always more likely that molecules of X, over time, will move from the high concentration side to the low concentration side.
Now suppose this has been going on awhile. Eventually we get 55 molecules of X on either side. The concentrations are equal and there is no concentration gradient. Thus there is no "net" movement of X through the hole. I did NOT say that X is no longer going through the hole. Anytime a molecule of X gets near the hole, it has a chance of going through and may do so. But a molecule from the other side also has a chance. The chance depends ONLY on the concentrations on each side. Over time, each side would always have very close to the same number of molecules.
So osmosis is a process where some substance moves across a membrane that is permeable to the substance. How much of the substance moves is dependent on the concentration of that substance on the two sides of the membrane.
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