B2.3 Cell Specialisation [IB Biology SL/HL]

this is the video for B 2.3 on Cell specialization and all of this is standard level or Core Curriculum multi cellular 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 um 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 ey 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 and 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 um a group of stem cells right

so that single zygote mitosis mitosis mitosis copy copy copy and I get this bow 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 that 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 um proliferate stem cells um outside of the body which is really cool those very first stem cells that group of cells that results from mitosis um of the zygote or copying of the zygote are what we call toti potent they can become any cell type right and that makes sense because all

of our cells originated from that group of cells so toy I think is being like total totality once that embryo starts to develop okay I'm going to have two rings of cells this tropho blast layer it's not important right now and this Inner Cell mass and that Inner Cell mass will eventually become um the fetus this group of cells is what we call plur potent now plur poent 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 are 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 new eyeball cell or a new brain cell they are what we call multi- poent so multi poent 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 Siz 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 um 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 8 micrometers wide and they kind 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 say they grow from about 10 micrometers to about 30 micrometers when they become activated and then motor neurons these are really cool um these are going to be um 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 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 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 reac 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 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 um kind of like a a descending pattern in My Graph here like this what is the implication there well it tells me that big cells aren't 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 okay so the most efficient cells are going to be these smaller cells that have um much greater or many more opportunities to get materials in and out to service the metabolic reactions going 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

um 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 um 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 in increase the surface area and if I've increased the surface area then I'm going to increase the surface area to volume ratio