Here is a box of some air, just a tiny tiny amount.
Room temperature, sea level, 50% humidity.
Pretty normal air, except
the individual molecules (which you can learn more about in a previous video) should be
moving very very fast.
We'll get them moving soon, but first I want to pose a question.
Let's say we introduce a concentrated pocket of carbon dioxide gas here... via pollution
or whatever.
There's a couple other molecules of CO2 here and here, but mostly they're in this area.
My question is: Where do you think these molecules are going to go?
Specifically this molecule.
The idea of diffusion is probably familiar to you, or maybe it's new, but in any case:
Diffusion is commonly defined as: a net movement of particles from a region of higher concentration
to a lower concentration.
So if that made any sense, you might expect these carbon dioxide molecules to move in
this direction.
It's a reasonable guess.
Based on this definition, the question seems straightforward: Simply figure out where the
concentration is higher and where it's lower, and that determines the direction in which
molecules move.
But when I was first introduced to this idea, I couldn't get my head around it.
How do the molecules know how many of their own kind are close by, relative to how many
are farther away?
And it's not like there's more ROOM over there.
There's just as much empty space over here as over there.
And does it really matter that these are a slightly different kind of molecule than these?
So where will this carbon dioxide molecule go?
It was a trick question.
It's going to go over here.
WHY?
Let's run some computer simulations.
To start, let's use just one kind of molecule simplified as spheres in our simulations.
It's worth noting that there's lots of space between particles,
meaning this is a gas, and not a liquid or a solid.
Diffusion happens in liquids too, but we'll stick with a gas for now because it's easier
to see what's happening.
In a gas, all the molecules are constantly moving every which way.
The DIRECTION each molecule is going is totally unpredictable, it's chaos-
Stickman: But you JUST asked us to predict which way-
I know, I know... stick with me.
Get it, stick... stickman...
and the directions change as the particles collide with the walls and with each other.
How FAST a molecule is moving is linked to something called Kinetic Energy.
Kinetic just means movement, so this is the energy of motion.
More kinetic energy means faster motion, and less means slower.
The Kinetic Molecular Theory, is the idea that molecules are always moving, in gases,
liquids and even solids.
That's an important first part of our diffusion puzzle.
Incidentally, the kinetic energy of molecules is also known as heat; it's thermal energy.
And because some particles are moving slower and some moving faster at any given moment,
we say that the average kinetic energy of the particles is the TEMPERATURE of the gas.
If we heat up the container, the particles move faster (on average), and then if they
cool down, they are, by definition moving slower.
Collectively.
On average.
Let's briefly talk about concentration.
No, not how hard you're focusing on this video, concentration of molecules.
Concentration is the amount of a substance in a defined region.
So if we bunched up most of our particles over here, we can see that this volume has
a "high concentration" or number of particles relative to this other volume.
We can put a membrane in the middle to distinguish the two sections,
but in this case it's not a complete barrier.
Our molecule can pass through holes in the membrane, like air passing through a curtain,
though sometimes collisions will happen.
This is the setup you often see when talking about diffusion.
We start with a high concentration here and then it's not terribly surprising when we
see an overall movement kind of in this direction.
This isn't dodgeball though!
The balls aren't being aimed at the other side, but after a bunch of collisions, there's
a good chance each particle will cross eventually.
This is the "net" movement in the definition, a summary of all the particles, if you will.
And another piece of our puzzle.
Eventually, after a period of time, we can say that the system is at EQUILIBRIUM, because
there's about the same concentration of particles on both sides of the membrane, and in fact
everywhere in the container.
There's no more significant migration, but each individual particle is still moving around,
back and forth.
That's IMPORTANT.
It's a DYNAMIC EQUILIBRIUM.
Moving yet balanced.
But let's take another look at this so-called equilibrium.
How would you describe where these molecules might go next, on average?
Hmm... there's no concentration gradient, no high or low region... so nowhere specific,
mostly stay on the same side?
Well, how about now?
Oh hang on, suddenly a concentration difference between here and the surrounding area has
appeared.
Now the particles will go over here and here and here.
They're going to spread out...
But I haven't changed anything about the properties of the particles.
They're identical in every way to the others.
They're just a different color.
In fact we could rewind and color some different particles and play back the exact same simulation.
Look, I'll even overlap them to prove that coloring them hasn't changed any movement.
In this simulation, the red particles, the green particles, AND the blue particles are
diffusing, they're all spreading out, because of their kinetic energy.
If we track one molecule, we can see that it's just going all over the place, without
any concern for what everyone else is doing.
So it's not that a concentration gradient imparts some kind of pushing or pulling FORCE
on the particles.
It's just a natural outcome of particles moving around randomly!
Diffusion doesn't tell molecules where to go.
They just...
GO!
So what does that mean then?
If any given molecule can move anywhere in this container, regardless of what the other
molecules are doing, how do we KNOW that diffusion is going to progress from high to low concentration?
Put another way, why don't molecules sometimes diffuse in the opposite direction from low
to high?
Well they could, but it's just a matter of probability.
Stickman: Oh no... math
Well, I'm tempted to dig into entropy because it's so close to what we're talking about...
but perhaps a more complete discussion can happen in another video, if you're lucky.
I'll try to keep it simple here.
If you had a cup containing half red balls and half blue balls, shook them up, and dumped
them into a tray, how many times do you think you'd have to repeat this process before you'd
expect to get decent mixing in the tray?
Well... one?
Yeah.
And how many times before the balls happened to end up sorted in the tray?
Even partly sorted?
It's just staggeringly unlikely, especially if you're talking about a million billion
trillion molecules.
With our gas simulation, if we have one container that starts mostly segregated, and another
container that starts pretty well mixed...
...we can see that the segregated container shows a trend towards more even mixing, while
the mixed container does NOT show a trend towards unmixing... diffusion doesn't reverse.
*cough* NEITHER DOES ENTROPY *cough*
Our everyday experience confirms that stuff spreads out.
If you spray perfume, a few minutes later you don't smell it just in one place.
No, those perfume molecules collide with air molecules and bounce all over the room.
Stickman: This wall smells nice.
I almost forgot about our air simulation.
This was our starting point, and I asked about this molecule.
Where will it go?
Let's watch.
So...
I told you it was a trick question.
Yes, this molecule did go over here, at least temporarily, but as a whole, the carbon dioxide
spread out, it diffused.
You see, a single molecule can't spread out, it moves randomly.
Diffusion doesn't apply to the individual components.
It only emerges at a larger scale.
This is why diffusion is known as an EMERGENT PROPERTY...
Molecules don't know where to go, nor do they receive directions
The only reason I knew where the CO2 molecule was going to go was because I was able to
preview the computer simulation.
Yes, it's maybe simpler to memorize a definition, but that can lead to misconceptions.
I think forming an intuition for HOW and WHY diffusion actually happens and what it looks
like at the molecular level helps us more easily grasp other molecular biology concepts.
So the definition of diffusion isn't wrong.
For me it just doesn't show the whole picture.
Thank you so much for watching.
Diffusion is one of the central processes that lets our bodies do what they do, and
I hope I've clarified it by visualizing it for you.
If so, I hope you'll subscribe to watch more animated biology videos, and please consider
sharing this video with a science teacher or someone who just enjoys science videos.
I'm Stuart and this is Biocinematics.