Cell Membrane Structure: Fluid Mosaic Model Explained in 5 Minutes

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!