### Re: why does an increase in temperature increase the rate of osmosis

Date: Fri Apr 16 08:40:20 2004
Posted By: Michael Maguire, Professor
Area of science: Cell Biology
ID: 1080069890.Cb
Message:
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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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