Welcome back, everyone! For this week's video, we're going to be discussing the
cell membrane, specifically the fluid mosaic model and what makes it semi
permeable. Okay here we go!
The cell membrane has several major roles.
It protects the cell from its surroundings, maintains an internal environment that
is different from its external environment, and determines which
particles may enter or exit the cell. It has many components that allow it to
perform all of these functions. These include phospholipids, cholesterol,
channel proteins, carrier proteins, glycoproteins, and peripheral proteins.
The membrane they create is both fluid and selectively permeable. Here's a
picture of the cell membrane. Notice how it has two layers. These layers are
comprised of what we call phospholipids. The phosphate head is both polar and
hydrophilic. Attached to it are two nonpolar hydrophobic fatty acid tails.
These tails can be either saturated or unsaturated. The phosphate groups have a
charge, and the tails do not. The nonpolar tails face inwards towards one another,
and the polar phosphate groups are on the outside. This arrangement keeps
molecules from passing through too easily. In between every few
phospholipids is a molecule you've probably heard of; Cholesterol.
Despite what you might see in all those pharmaceutical commercials, cholesterol
is actually really important. It helps the cell membrane maintain an
appropriate level of fluidity by managing the space between phospholipids.
At room temperature, the cell membrane has about the same consistency as
vegetable oil, which is ideal. However, higher temperatures would cause the cell
membrane components to move farther away from each other and the membrane would
disintegrate. Conversely, colder temperatures would
cause the phospholipids to clump together and make the cell membrane too
rigid to work properly. Cholesterol has a very stable structure,
and it helps the phospholipids stay at the optimal distance from one another...
Almost like an overzealous chaperone at a middle school dance.
So, how exactly does it control what molecules can pass in and out?
Certain molecules like oxygen and carbon dioxide can easily squeeze through it without
any special accommodations. This is lucky because we exchange a lot of oxygen
carbon dioxide during cellular respiration. These molecules are small
and uncharged, so they can sneak through using just diffusion. However, other
molecules can't get through without help, and this is where many of our proteins
are going to come into play. Molecules that are large or charged
can't get through the fatty inner layer. Specialized channel proteins allow large
or charged molecules to pass through the membrane. These molecules flow down the
concentration gradient from high concentration to low concentration.
Structures called carrier proteins charge a toll to admit a molecule.
In return for a small amount of ATP, these proteins allow certain molecules to pass through.
They are especially useful because they can often pump items
against their concentration gradients. A great example of this is the
sodium potassium pump, which pumps sodium out of cells, and potassium into cells,
allowing neurons to work. The cell membrane also often includes
glycoproteins, which have a protein component embedded in the cell wall and
then a carbohydrate protrusion that dangles out away from it.
Antibodies involve glycoproteins, but they also play a major role in conception.
Human egg cells are surrounded by layers including the
Corona Radiata and the Zona Pellucida. Sperm cells contain special enzymes in a
compartment called the Acrosome. These enzymes break through the Corona Radiata,
exposing the glycoproteins in the inner Zona Pellucida. The sperm that attaches
itself to these glycoproteins, which begins the process of fusing the two
cell membranes so the genetic material can be combined.
Finally, cell membranes are also studded with peripheral proteins on one side of
the membrane. They can attach and remove themselves from either side of the cell
membrane over and over again. Uou've probably come across a number of
peripheral proteins if you've studied cellular respiration. The compartments of
mitochondria are bound by a phospholipid membrane, which is studded with proteins,
just like the ones earlier in this video. Cytochrome C is an important protein in
the electron transport chain. The final steps of cellular respiration.
Interestingly, enzymes associated with the Cytochrome C protein are one of the
targets of the poison Cyanide. To wrap up, let's consider this idea called the
Fluid Mosaic Model. We've learned that the cell membrane is fluid
thanks to cholesterol, but what about this mosaic idea? A mosaic is a piece of
art, usually on a wall or a floor, that's made up of small pieces of pottery, stone
or tile. The pieces are usually different colors and arranged in a pattern or to
form a picture. For this video we've been looking at the membrane from the side,
but if we flip it around and view it from the top, it does look a lot like a
mosaic. All right, that's it for this week! If you found this video useful, I hope
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Thanks again for watching, everyone!