This is the video for B2.3 on cell specialization, and all of this is standard level or core curriculum. Multicellular organisms are made up of lots of different cell types, yet they all originated from a single cell called a zygote. The zygote is the result of the fusion of the male and the female gametes, and then that zygote is going to undergo mitosis many times to make a ball of undifferentiated stem cells.
Now, when those cells start to specialize and become really differentiated, having different structures and functions, that is the process of differentiation. Okay. So all of those cells in that ball of stem cells are going to have different locations and that location determines the type of tissue that it will form.
So some parts of that ball will become the heart and other parts will become your eyeball. Now, why does that happen? All of the cells in your body have identical genomes. They all come from that zygote.
Well, it results from different patterns of genetic expression. So if we think about genes as being turned on and turned off, we're going to need a different pattern of on and off for different cell types. If a liver cell is going to act like a liver cell, it needs to have all of the genes for how to be a liver cell turned off.
And all the genes for how to be other cell types, like a skin cell or a muscle cell, it still has those genes, but they're not expressed. They are turned off. And so that pattern of genetic expression that leads to different structures and functions is the very heart. of this differentiation process. So here's a picture of a group of stem cells, right?
So that single zygote, mitosis, mitosis, mitosis, copy, copy, copy, and I get this ball of undifferentiated cells. Well, what exactly is a stem cell? Two things really cool that it can do.
It can divide endlessly, which means that you can make an infinite number of these stem cells. And it can differentiate into multiple cell types. So it's so cool that this one group of cells can become things so different as a neuron or a cardiac cell or a skin cell or a blood cell.
But becoming multiple cell types is characteristic of a stem cell. After our cells have differentiated, we no longer are just this ball of undifferentiated, unspecialized cells. However, we do contain some locations. where stem cells remain, and those are called stem cell niches. So these are locations in our body, like in our bone marrow or in our liver, that have stem cells, and they provide an environment that allow those stem cells to do both of their really cool jobs, to regenerate and to differentiate.
So changes in that environment might determine... whether or not those stem cells just make more stem cells or whether they start to differentiate. So for example, if I have an injury, then maybe those stem cells will start to differentiate to replenish that tissue.
Okay. And if not, maybe they're doing something else. We can actually recreate the conditions of these stem cell niches in labs to grow and proliferate stem cells outside of the body, which is really cool.
Those very first stem cells, that group of cells that results from mitosis of the zygote or copying of the zygote are what we call totipotent. They can become any cell type, right? And that makes sense because all of our cells originated from that group of cells. So toti, I think, is being like total, totality.
Once that embryo starts to develop, okay, I'm going to have two rings of cells. This is true. trophoblast layer.
It's not important right now. And this inner cell mass, and that inner cell mass will eventually become the fetus. This group of cells is what we call pluripotent. Now, pluripotent means it can develop into many, but not all cell types. So it can still turn into like a lot of different kinds of cells, but not all.
Then once these tissues start to differentiate even further, even the stem cells that we just mentioned, like in your liver or in your bone marrow, can't become any type of cell that they want. So, for example, the stem cells in my bone marrow can make several different types of cells, but they can't make a new. my ball cell or a new brain cell. They are what we call multipotent.
So multipotent can develop into only a few different types of cells. So for example, my bone marrow cells might be able to make some of my blood cells, several types, but they can't do other things. Now, when we say that cells start to differentiate, that means a lot of different things.
They're going to develop different features. Maybe they'll have different proteins, different shapes. but they will also have different sizes and that size is very closely related to its function. Okay so here are some great examples there.
So sperm are long and narrow and so that is definitely going to be related to their function that has a lot to do with locomotion. Eggs on the other hand are huge and rounded and I'm not drawing this to scale the egg is much much much bigger than the sperm. Red blood cells are about eight micrometers wide, and they kind of have this dent in the middle. And again, that's in order to carry oxygen efficiently.
White blood cells, these are really cool. They actually grow. So they grow from about 10 micrometers to about 30 micrometers when they become activated. And then motor neurons, these are really cool.
These are going to be amazing. having a large cell body and then a long skinny axon and look all these different cell types that are different sizes and different shapes and have different features what's so cool is that they all came from undifferentiated stem cells all identical stem cells all of them carry the exact same genome but through different patterns of genetic expression have specialized into having different sizes, shapes, and functions. Let's say I have two cells, a bigger cell and a smaller cell.
Okay, I want to take a look here at their surface area to volume ratio. Because even though we talked about cells having different sizes, cells can't just be huge, okay? There's a restriction on how big they can grow.
So when I talk about surface area, I've drawn that here in blue. Surface area is going to be where things enter and exit the cell, right? So it's cell membrane. The volume or that inside in the cytoplasm, that is where all the metabolic reactions take place.
So this is where all these reactions are taking place. And the blue thing here, that cell membrane is what gets things in and out. You can see here in this picture that the small cell has a much greater volume. ratio of surface area compared to its volume than the big cell.
Okay. And so when I think about the surface area to volume ratio, um, that can be calculated by taking the surface area and dividing it by the volume. So if you're doing this mathematically, you can, um, divide one by the other. You don't have to worry about units here because we're just making a ratio.
Okay, now when I do this, if I do this for several different cells and I throw this up on a graph, if I take a look at increasing cell size and what effect that has on its surface area to volume ratio, I'm going to find that small cells like this one have a much greater ratio of surface area to volume and large cells like this one have... a much smaller ratio of surface area compared to its volume. And so I'm going to get kind of like a descending pattern in my graph here like this.
What is the implication there? Well, it tells me that big cells aren't going to have a lot of surface area, which means they're going to have a much harder time getting things in and out of their cell. in order to utilize all the metabolic pathways in this volume part efficiently. So the most efficient cells are going to be these smaller cells that have much greater or many more opportunities to get materials in and out to service the metabolic reactions going on on the inside.
And not only are they more efficient, but they're also much better at heat distribution. So remember, we don't just need to move materials in and out, but we also need to distribute heat. If I think about like a food that's really hot that I want to eat, and let's say the food is shaped like this, and it's way too hot to eat. This is like a potato or something, okay? And this is going to burn my mouth if I eat it.
Well, one of the ways that I can cool this down is by increasing the surface area to volume ratio. So I might like flatten it out. I still have the same volume of potatoes, okay? Yet now there is more surface area exposed to the cool air. And so this is going to cool down much quicker.
Cells use the same kinds of things, okay? So there are two adaptations that cells can employ in order to have... very large surface area to volume ratios.
Either they can stay small or they can change their shape. So there are very few large cells, but the ones that are large have unique shapes in order to increase that surface area to volume ratio. So let's take a look at this cell here.
There's a lot of volume. It's a pretty big cell, but look at all these folds in the membrane here. The purpose of these folds is to...
increase the surface area. And if I've increased the surface area, then I'm going to increase the surface area to volume ratio.