In my second year of teaching, I was looking to improve an osmosis lab I 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 first steps of the procedure that got to me. To get ready 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 hard-boiled eggs, then. No, no, raw eggs.
But if the shell comes off, well, 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, like a cell. So then you run different scenarios with it for your osmosis lab because it will be a semipermeable membrane, like a cell membrane.
I couldn't visualize this. If the shell comes off, but it's a raw egg, how does it stay together? So I have this area in my house that's 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 cell could never be as large as a single chicken egg, though. Why?
Well, it turns out surface area is a very 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 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, increases, then you will have more need for surface area, as you will have more 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, I'm going to use popular cube models instead of egg-shaped models 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 the smaller model? Six to one ratio! That means the surface area in this small model is six times more than the volume.
Look at this beautiful ratio with so much surface area. But if we look at this bigger cube, and we do some math for this model, that surface area to volume ratio decreases. Sure, the surface area is still larger than the volume in the large model, but it's only two times as large now, not six 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 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 the amazing cell membrane structure itself.
Because the cell membrane structure truly is magnificent, and since every single living thing is made up of one or more cells, which is part of the cell theory, it's a big deal because every single cell has a membrane. 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 be talking 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 what we use to describe the cell membrane.
A mosaic, in case you've ever created one, arranges many small pieces together to make some large piece. You'll see that 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 fluting around, they're not static.
So let's take a look at some of these components in the cell membrane. 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 go in and out of this membrane and which ones can't that you can view, but for now, we're going to 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 the 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 becoming 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 in or on 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're 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 digested 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 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 make them considered a glycoprotein. If the carbohydrate's attached to a phospholipid, you could have what's called a glycolipid. Glycoproteins and glycolipids can identify the cell as belonging to the organism—self and non-self recognition—which is very important when you're 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 system cells to interact with each other and activate. However, it's 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.