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Have you ever heard of the ich?
The disease, that is, that can afflict aquarium fish?
I will never forget the first time my fish caught Ich.
While it’s called ich, it stands for this.
So, ich, it is.
It’s actually caused by a protist---a parasitic one.
In our ecology video, we talk about parasites and how parasites benefit while causing harm
to their host.
And in my case, it was all of my beloved fish.
Well, okay technically it affected Gertrude first, but then it spread, because it is a
contagious disease in fish.
Ich typically causes these white dots.
It gets on their fins.
On their bodies.
On their gills.
All over.
It can even be lethal if not treated.
So it’s bad news, and being a guppy enthusiast, I was really worried about all my guppies.
This is not a video about parasites; this is a video about diffusion.
So you are probably wondering how is she going to connect diffusion to this parasitic protist
disease?
Well it’s because of the treatment.
You see, there is a commonly purchased treatment for the disease of Ich.
One common treatment option contains an antiparasitic called methylene blue.
Its name is not misleading; it is quite blue.
The directions on the back of the medication outlined how many drops to add per gallons
of water.
And I watched and tried to…encourage…my fish to swim towards the medicine.
But they were panicked---maybe because a portion of their water was changing colors but also
probably because of the kid trying to get their attention to swim towards the medicine.
But you know what I didn’t know then?
I didn’t need them to swim over there.
The blue drops would spread on their own, in a process known as diffusion.
Diffusion is when the net movement of a substance travels down its concentration gradient.
That means it moves from a high concentration to a low concentration.
When the methylene blue drops were added into the water, they didn’t just sit there.
The molecules traveled in the water through diffusion: the high concentration of molecules---which
is where they were dropped---spread to areas of low concentration in the water.
Pretty soon, the water had a uniformly blue appearance as the molecules eventually are
evenly dispersed.
This was very important for treating my fish, because the treatment reached all of them.
And they soon got better.
Diffusion doesn’t just happen in water.
For example, it occurs if you spray an air freshener in the air.
The net movement of molecules travel from an area of high concentration of the air freshener
to a low concentration of the air freshener.
Pretty soon, it’s likely others would smell the air freshener even if they were quite
a distance away.
Now, I want to mention two important points.
First, we say “net” movement because that’s the overall movement.
But it doesn’t mean that the molecules can’t move around the other direction, and it doesn’t
mean that the molecules eventually stop moving either.
The molecules are continuously moving--- even when equilibrium is reached.
It’s just the overall movement, the net movement, is from a high concentration to
low concentration until equilibrium is reached.
Second, diffusion is a passive transport.
That means an input of energy is not needed; we say that passive transport does not require
added energy.
When we talk about diffusion of molecules in cells--- for example the diffusion of oxygen
molecules into a cell--- it is classified as passive transport.
A concentration gradient is itself a form of potential energy.
Passive transport is different from other processes that we detail in our cell transport
video like active transport where an input of energy is required.
And speaking of our cell transport video, we outline that there is also a type of diffusion
called facilitated diffusion.
Facilitated diffusion is when molecules still have a net movement of a high concentration
to a low concentration, but they may be too large or have other characteristics that prevent
them from directly traveling across the selective cell membrane.
Therefore, they have to go through a protein channel.
It’s still diffusion, because it’s still passive transport and it’s still traveling
down its concentration gradient.
It’s just that, in that case, it generally has to go through a protein to get inside.
Now, there are factors that can affect the rate of diffusion.
We’ll mention just a few of them when considering these small molecules here diffusing in simple
diffusion.
Distance.
The greater the distance that needs to be traveled, the slower the diffusion rate.
For example, you could observe a difference in a 5 gallon tank versus a 55 gallon one.
Temperature!
Ok, really quick, would you think a higher temperature or a lower temperature would increase
the diffusion rate?
[Assuming everything else is a constant for the point of the comparison] Generally a higher
temperature.
A higher temperature generally would mean there is more movement of the molecules so
the diffusion rate is increased---this has to do with energy so check out the link in
the description for more about that.
Characteristics of the solvent.
For example, is the solvent very dense?
That could slow the molecules down and decrease the diffusion rate.
Characteristics of the molecules, or whatever substance is traveling through diffusion since
technically not everything that travels in diffusion is defined as a molecule.
What is the mass of the substance traveling?
Generally, a substance with a greater mass will have a lower diffusion rate when compared
to a substance with less mass.
Characteristics of the barrier, if diffusion is crossing a type of barrier.
To explain that, let’s consider diffusion across this cell where the barrier would then
be the cell membrane.
Small, nonpolar substances pass through a cell membrane easier than something large
or polar.
That will affect the rate of diffusion.
The surface area and thickness of the cell membrane also will alter the diffusion rate.
For example, a large surface area and thin membrane would generally allow a faster diffusion
rate than a smaller surface area and thick membrane.
By the way, there are more factors that can affect the diffusion rate than just those
five we listed.
In fact, you may already have thought of another factor when seeing our simple diffusion image.
Generally, if I were to increase the concentration of this substance here, that would cause a
larger difference between these concentrations, and that would also increase the rate of diffusion.
Before we end, you might wonder, “Why should I care about this diffusion thing?”
Ok, other than the aquarium fish that I love, realize that diffusion is critical for all
of life.
Cells are the living unit that builds up all of life, and materials that cells need to
survive have to get in and waste molecules of the cell have to get out.
Now while diffusion is certainly not the only kind of cell transport, diffusion is responsible
for many critical items.
One example?
As you sit here right now, diffusion is responsible for oxygen leaving the alveoli of the lungs
to your blood.
Likewise, the waste gas carbon dioxide is able to exit your blood to the alveoli.
A critical process made possible by diffusion.
Well, that’s it for the Amoeba Sisters, and we remind you to stay curious.