Captions are on! Click CC button at bottom right to turn off. Updates on Twitter (@AmoebaSisters) and Facebook! In my second year of teaching, I was looking
to improve an osmosis lab I had had done the year previously. Osmosis, if you remember from our osmosis
video, involves water traveling through a semi-permeable membrane. Potentially a cell membrane. And I wanted a cool way that students could
model this in different scenarios. And one of my colleagues told me about this
egg lab. I won’t get into the whole lab though- it
was actually one of the very first steps of the procedure that got to me. “To prepare for the lab,” my colleague
told me, “you can soak eggs in vinegar for 24-48 hours and the shell comes off…” “Oh, so I need to make some hardboiled eggs
then…” “No, no, raw eggs.” “But if the shell comes off…” “That’s the whole point, what’s underneath
the shell is going to mimic a cell membrane. It’s kind of modeling how a cell membrane
would function…you know, if the whole egg was actually a cell. So then you can run different scenarios with
it for your osmosis lab, because it will be semi-permeable like a cell membrane.” I couldn’t visualize this …if the shell
comes off…but it’s raw…how does it stay together?! So I have this area in my house that is designated
for things for me to try out. Don’t worry, I always clean up afterwards. I tried this experiment out in advance, just
to be sure. The hard shell is removed, but the membrane
that was always there remains. We often visualize the membrane of a cell
this way, like this membrane around the chicken egg. The cell membrane is semi-permeable, meaning
it lets some materials through but not others. We have an entire video all about cell transport
and how materials can pass through the membrane. A [*body*] cell could never be as large as a single chicken egg though. Why? Well, it turns out surface area is a really
important thing. Remember, that surface area determines the
surface measurements of that cell membrane…and the cell membrane controls what goes in and
out of the cells. That includes food coming in as well as molecules
that are essential for metabolic processes---and then also, waste going out. If volume, which is all this space inside
the cell, increases then you will have more need surface area as you will more of a need
for food to enter, more of a need for waste to be removed, and more metabolic reactions
occurring in this larger volume in the first place. If we do a little bit of math here between
these two models, and I’m going to use popular cube models instead of an egg shape model
because it’s a little faster for me to do surface area and volume calculations. See how, here, there is a big difference in
surface area to volume in this smaller model? 6:1 ratio! That means the surface area in this small
model is 6 times more than the volume! Look at this beautiful ratio with so much
surface area! But if we look at this bigger cube and do
some math for this model…that surface area to volume ratio decreases. Sure, the surface area is still larger than
the volume in this large model, but it’s only 2 times as large now. Not 6 times as large. Cells are way smaller than this small model
here to allow for an exceptionally large surface area to volume ratio. And a major reason why we’re not going to
find a [*body*] cell as big as this chicken egg here. Surface area is important. And while we can model a lot of the processes
of cell transport from this egg membrane and how important the membrane is, I don’t want
to neglect talking about how amazing the cell membrane structure is itself. Because the cell membrane structure---truly---is
magnificent. And since every single living thing is made
up of 1 or more cells - which is part of the cell theory - it’s a big deal because every
single cell has a membrane. So it doesn’t matter whether you’re talking
about bacteria or protists or plants or animals or fungi---even archaea aren’t too cool
to have a membrane. They all have a cell membrane. The structure can vary some, but we’re going
to talk about some major structures of the membrane that you can actually find in most
cells. We should mention that the Fluid Mosaic Model
is often how we describe the cell membrane. A mosaic, in case you’ve ever created one---we
did in some of our art classes over time----arranges many small pieces together to make some larger
piece. You’ll see what that makes sense when describing
the membrane in a minute. The word “fluid” implies movement, and
this is true for the cell membrane, as the components are floating around, they’re
not static. So let’s take a look at some of these components. We’re looking first at a phospholipid bilayer. A phospholipid is a lipid- but an interesting
one. So when you talk about a lipid in general,
many lipids are nonpolar. Think of oil for example. It’s nonpolar. It won’t dissolve in water; water is polar. But a phospholipid is interesting, because
one part of it IS polar---the head----and the other part of it is nonpolar---the tail. It’s amphiphilic! Let’s explain what we mean. We often refer to the polar head of the phospholipid
as hydrophilic, which means that part loves water. Well, you know if a, lipid could love. The nonpolar tails are hydrophobic---they
do not like water. These phospholipids arrange themselves into
a phospholipid bilayer with the nonpolar areas here in between, away from any water. It also allows this area in between to be
separated from the outside and inside---- water can be found on the inside and outside
areas. Also, these phospholipids- they don’t just
stay put. They move around---it’s the fluid mosaic
model after all. This gives the cell membrane flexibility. Phospholipids can even flip-flop around- but
that’s far less common. Remember that this entire phospholipid bilayer
borders the whole cell---it would be a sphere even though we’re just looking at one area
of it. We have an entire video that talks about which
molecules can get through this membrane---and which ones can’t---that you can view, but
for now, we’re going to take a look at some of the other structures more in depth. Cholesterol. You know, cholesterol often gets a bad reputation. And while cholesterol that builds up in arteries
can be a problem, cholesterol in your cell membrane is critical. If temperatures drop, the cholesterol can
actually function kind of like spacers between these phospholipids---keeping them from becoming
too packed. Or vice versa, the cholesterol can actually
function to connect phospholipids to keep them from being too fluid in warm temperatures. Proteins. In protein synthesis, we talk about why it’s
so important for cells to make proteins. Many proteins are found on or in the cell
membrane, and they play major roles. Peripheral proteins, like the name suggests,
tend to be on the peripheral area of the membrane. So while they tend to be on exterior areas
of the membrane, they generally are not going to go through the membrane…that’s for
integral proteins. Integral proteins go through the membrane. Oh, and, peripheral proteins can sit on them. Sometimes. Because of location, these proteins tend to
have different functions. Integral proteins, with their potential to
go through the membrane, are frequently involved in all kinds of transporting methods for all
kinds of materials. Some relevance? Consider the breakfast you ate this morning. Your body digests what you ate for breakfast
to obtain glucose. Once in the bloodstream, those glucose molecules
can’t just squeeze through the phospholipid bilayer to enter all of your cells. The glucose molecules are too big and polar. But your cells need glucose to survive to
make ATP, and they rely on integral proteins to get it. Peripheral proteins tend to be more loosely
attached since they’re generally not stuck in the membrane---they can have an assortment
of functions such as acting as enzymes to speed up reactions or attaching to the cytoskeleton
structures to help with cell shape. Both protein types can have carbohydrates
bound to them---which can then make them considered a glycoprotein. If the carbohydrates attach to the phospholipid,
you have what is called a glycolipid. Glycoproteins and glycolipids can identify
the cell as belonging to the organism---self/non-self recognition---which is very important when
you are fighting pathogens. They can also be involved in many kinds of
cell signaling. In fact, here’s some relevance: a glycoprotein
known as CD4 is found on the surface of some of your immune cells. The CD4 glycoprotein is essential for some
of these immune systems cells to interact with each other and activate. However, it is also exploited by the HIV virus. The HIV virus uses that CD4 glycoprotein as
a way to bind to Helper T cells, which it then can infect. Understanding the components of the cell membrane
and how those components are involved in recognition and cell signaling is critical to understanding
how to fight back against many viral and bacterial diseases. Well that’s it for the Amoeba Sisters, and
we remind you to stay curious!
