|MadSci Network: Cell Biology|
You've already got the reason, kinetic energy does increase with increasing temperature. But what exactly does that mean? Think of a sheet of paper. Now poke very small holes in it. Think of that as a somewhat permeable (semipermeable) membrane. Now, for convenience and not to make a mess, hold the paper horizontal to the floor, just like you'd have it on a desk. Now have someone sprinkle the paper with those little red or green sugar sprinkles that you put on cookies. Maybe one or 2 will fall through one of the holes, but not many. Now, SLOWLY, start moving the paper back and forth. The sprinkles with start moving, but not very fast. This will cause some of them to "find" a hole and fall through. But it will be relatively slow. Now speed up the movement of the paper, shake it back and forth like you're sieving flour. The sprinkles will move faster and faster (increased kinetic energy from the increase in temperature). The faster they move, the more area they cover in a given unit of time. Therefore, over that period of time they are more likely to find a hole in the paper/membrane and fall through. It's a simple probability idea. The higher the temperature, the more likely any given molecule/sprinkle will find a hole. At some point of course, the molecules that are diffusing through the membrane will be equal on both sides and the system has come to equilibrium. BUT, remember that this doesn't mean that nothing is moving through the membrane, only the the amount moving in one direction is equal to the amount moving in the other direction. Now this example is actually somewhat backward in that we're moving the paper/membrane rather than directly moving the molecules/sprinkles around in a solution, but it's exactly the same idea either way. Further, osmosis doesn't necessarily imply that a molecule is moving through an actual hole and most of time it doesn't since we're usually talking about water. Osmosis is simply a molecule, let's say water, leaving one solution, dissolving in another (the lipid of the membrane), moving around for a while in that solution and then leaving the lipid and redissolving in the solution on one of the other side of the membrane. Once it's in the membrane, it has a roughly equal chance of exiting on either side. Osmosis is simply reflecting the idea that there are more molecules on one side than the other. Therefore more molecules dissolve in the membrane from one side than dissolve from the other. Since they have an equal chance of exiting either side, then numerically, more molecules from the high concentration side will end up on the low concentration side, until equilibrium is reached. There is also an additional component, but it's more dependent on the specific molecule and the composition of the membrane. As temperature increases, a membrane becomes more fluid, more loosely packed and the lipid molecules move around faster (like butter melting in the frying pan). This has the effect of making the membrane more permeable. For some molecules it also will make the molecule more soluble within the membrane. This will also increase the rate of diffusion. In effect, what this component does is alter the properties of the hole in the membrane if we use that type of explanation. Alternatively, in addition to simply increased rate of movement of the molecule, the increased temperature makes it more soluble in the membrane so that once it finally swims around and bumps into the membrane, it has a greater chance of dissolving. Hope that helps.
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