Captions are on! Click CC at bottom right to turn off. 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.
