MadSci Network: Cell Biology

Re: what is the difference between diffusion and osmosis?

Date: Thu Feb 24 23:05:49 2000
Posted By: Erin Cram, Grad student, Molecular and Cellular Biology, University of CA, Berkeley
Area of science: Cell Biology
ID: 951443200.Cb

Simply put:
Osmosis is the movement of water across a barrier.  Diffusion is the movement of 
other molecules, not necessarily across a barrier.

Here is a more detailed answer, from one of our archived answers:

Diffusion. If we place a red molecule (just one) into the very middle of a glass 
of water, what do you think will happen? Right, it will move somewhere else. The 
red molecule is being bumped about on either side, just like I am bumped around 
in a shopping mall at Christmas time. Try to stand in one place in the middle of 
a crowded store. It won't be long before somebody says "excuse me, could you 
please move out of my way." The red molecule is constantly being pushed from all 
sides by other water molecules that are impatient to get somewhere. This pushing 
is random, and it is uneven. If the pushing were even then the molecule would 
not move, it would just get squished. If the pushing was not random, then we 
could predict where the red molecule would end up. As it is, the red molecule 
does move, and we can not predict where it will go. Now imagine one-thousand red 
molecules all placed together in the center of the water. These molecules will 
all be pushed in different directions and so they will move. It just happens 
that we placed them very close together so if they move in different directions, 
they are bound to spread out. This spreading movement of the red molecules 
through the water is called diffusion. It is a very hard thing to understand. 
Individually, the red molecules just move about, but collectively we can see
them spread apart. There is no force pulling them apart, it is just more likely 
that they will spread out. There are so many ways that the molecules can spread 
because we don't mark individual molecules. Try it yourself with a bunch of 
paper scraps. Rip one piece of paper into about up about thirty pieces. Now 
throw them up into the air and watch them hit the floor. If you do this again, 
you will see that the pattern is about the same. However, if you had put numbers 
on them and recorded the position of each individual piece of paper then you 
will see very different results every time. We have no way to mark the red 
molecules, so what we see is only the general pattern. Also,
the number of red molecules in one drop of foor colouring is much more than a 
billion! Imagine how random your room would look if you had used more than a 
billion scraps of paper instead of thirty. I hope you have understood diffusion.
Osmosis. Osmosis is the process of water molecules passing through a very thin 
solid membrane.

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