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Welcome to MCAT Basics, your ultimate guide to the essential topics you need to master for the MCAT. Brought to you by the physicians at Med School Coach. Every week, Sam Smith breaks down high-yield MCAT topics, ensuring you're primed for success on test day. Join SAM as we explore the most crucial subjects outlined by the AAMC, pulled from official practice materials and third-party resources. Get ready to elevate your MCAT game with topics tailored to maximize your score potential.
Hello, I'm Sam Smith. This podcast is going to cover cell and tissue types. The first thing I'm gonna go into is the difference between prokaryotic and eukaryotic cells. And then within each of these different sets of cells, I'm gonna bring up even more cells. So I'll talk about what's the difference between animal cells, plant cells, gram positive and gram negative bacteria, just some kind of things like that. Then I'll talk a little bit about erythrocytes.
And then once I get past the different types of cells, I'm going to get into the different types of tissue. And there's four main types of tissue that is muscle tissue, connective tissue, epithelial tissue, and nervous tissue. And this material is going to show up on two out of the four MCAT sections, which is going to be the biobiochem section, and then also potentially in the physics chemistry section.
The main purpose of this podcast is to really have you understand the difference between these really common cell types or these different tissue types. And I have seen this come up on the MCAT, definitely a few times. So this stuff is important and I hope this podcast helps.
All right, the first thing I want to talk about here are different cell types. And this might seem like kind of like a boutique topic or like really specific and you might be thinking, yeah, do I really need to know this? Should I be listening to this podcast right now? The answer is yes. So how I came up with these different topics is I just went through a lot of the different practice material put out by the AMC. So I went through their practice tests, their question packs and kind of picked out these different topics.
and then, you know, made a big list, ranked, and whatever. So this is important. This does come up, I think, 10 or 15 different times throughout their practice tests. So with that said, let's get into the different cell types. So first of all, what is a cell? A cell is the basic building block of life. I'm sure everyone who's listening to this knows that.
And, you know, there's a ton of different types of cells in the body. I went through Wikipedia and was kind of just looking at a long list of all these cells. And, you know, there are cells called cartwheel cells that live in the brain. No idea what they do or why they're called cartwheel cells, but kind of cool. You have cells in the ear, for instance, that secrete ear wax, and that's kind of their main function. You have cells in the lungs, in the heart, and they're all different. So, a lot of different types of cells.
And some organisms in life are made of a single cell. So bacteria and archaea are both single-celled organisms. Archaea are extremophiles, and they tend to live in these weird places where, you know, very hot, very cold, they're really high pressure or really low pressure. And they're often called extremophiles because they like to live in these extreme conditions. I'll get into all this a little bit more. Amoebas are also a type of organism that are single-celled.
And these are the kind of organisms that you hear, you know, these crazy horror stories of them eating people's brains. And TV shows like Monsters Inside Me. So, organisms can be made of a single cell, or they could be made of multiple cells like us humans.
And the first difference I want to get into in terms of cell type are the difference between eukaryotic cells and prokaryotic cells. So prokaryotes are broken down into archaea and bacteria, and these are both single-cell organisms. And eukaryotic cells are broken down into human or animal cells, and then fungi, plant, and protozoa.
And I don't think I really understood what exactly protozoa were before studying for the MCAT. You know, it's a word that you've heard before, but maybe you don't really know what it means. Well, these are just single-celled eukaryotic organisms. So things like a meebas. And so in general, eukaryotic cells are larger than prokaryotic cells.
and have a membrane-bound nucleus, membrane-bound organelles, and rod-shaped chromosomes. And then prokaryotic cells, on the other hand, are single-celled organisms that don't have a membrane-bound nucleus and a lack membrane-bound organelles. And so that's just kind of the general differences. Let's get into the more specific differences between these cell types.
So these differences are broken down into five different categories. The first is in terms of size. The second is in terms of membrane-bound organelles. The third is in terms of genetic material. The fourth is in terms of ribosomes. And the fifth is in terms of cell division and cell reproduction.
All right, so let's get into the first difference here, which is in terms of size. So eukaryotic cells are approximately 10 to 100 times bigger than the size of a prokaryotic cell. So imagine your iPhone versus you. This is basically a rough estimate in difference in size between a prokaryotic cell, which is the iPhone and the eukaryotic cell, which is you. In terms of membrane bound organelles,
eukaryotic cells completely beat out prokaryotic cells. So eukaryotic cells are going to have mitochondria, Golgi apparatus, lysosome, et cetera. They're going to have all these different types of membrane-bound organelles. On the other hand, prokaryotic cells are not going to have any membrane-bound organelles.
It is important to note, though, that both eukaryotes and prokaryotes will have ribosomes, which is where proteins are made. And obviously, ribosomes are not membrane-bound organelles, but you might somehow associate ribosomes with membrane-bound organelles or whatever and start to think, OK, maybe these prokaryotes don't even have ribosomes, but that's not true.
Next, in terms of genetic material, eukaryotic cells have membrane-bound nucleuses. That's important to remember that their nucleuses are membrane-bound and that the nuclear envelope in a eukaryotic cell is covered in ribosomes and has nuclear pores. And inside of that nucleus, there's something called a nucleolus, which is a dense structure whose job it is to make ribosomes.
On the other hand, prokaryotes typically have circular bits of DNA, which is unlike us, right, because we have these chromosomes that actually have ends on them, and obviously a circle doesn't have ends, so there's no ends to their genetic material in most cases. And for prokaryotes, this genetic material is contained within the nucleoid region, which is just in the central part of the cell, and there is no membrane to this nucleoid region.
So next is the difference in ribosomes for eukaryotes and prokaryotes. So eukaryotes have a ribosome of size 80s, and prokaryotes have a ribosome of size 70s. So the eukaryotic ribosomes a bit bigger. It's got a large subunit of 60s and a small subunit of 40s. And then a prokaryotic cell has a ribosome with a large subunit of 50s and a small of 30s.
So those are a bit different. And it's a little bit whack that the small and the large subunit don't add up to equal the total size of the ribosome. So for instance, the eukaryotic ribosome is ADS, but yet the large and the small subunit together are 100s if you just add them. Doesn't really make sense, but the S in this size actually refers to Sfedberg units, which is a sedimentation rate.
And so sedimentation, obviously, is kind of a multifactorial in terms of a protein's mass density shape that all determines how fast we'll sediment. So that's, I think, why these subunits don't add up to equal the total size of the ribosome.
So that's the difference in ribosome size for eukaryotic cells versus prokaryotic cells. The next and last difference is in cell division and cell reproduction. So for eukaryotic cells, they undergo mitosis or meiosis, depending on if they are just normal somatic cells or if they are germ cells.
Prokaryotes, on the other hand, undergo binary fission, which is similar to mitosis only where you don't have all of these different stages. So you don't have a prophase, metaphase, anaphase, telophase in binary fission. On the other hand, with mitosis, you do have all those distinct stages.
All right, so those are some of the big differences between prokaryotes and eukaryotes. The next thing I want to do is look at eukaryotic cells and talk about the difference between animal cells, plant cells, fungi cells, and protozoa.
So what I'm going to do is I'm going to present these different cell types and talk about some of the unique characteristics of these cells. So let's start with plant cells. So plant cells have chloroplasts. And what goes on in a chloroplast is photosynthesis, which produces energy for plant cells. So it produces ATP, and then this ATP can then go on to make some of the sugars that the plant stores for energy.
And it's important to note that this is not the only way that plants make energy. So plants have mitochondria so they can break down these sugars through some of the same pathways that we use and produce ATP that way as well.
Another unique characteristic of a plant cell is that it has a cell wall, which is a rigid structure, and it kind of also gives these plant cells their rectangular shape. And this cell wall is made up of polysaccharides like cellulose, for example, and other polymeric compounds. For example, lignin, which is not to be confused with, ligma, is a cross-linked polymer group that contains a phenol group.
If you're curious, you can just go Google, Lignin, and go see what that polymer looks like. And so the last thing I want to mention here is that plant cells are known to have kind of one big vacuole. With animal cells, you can have vacuoles, but they're a lot more spread out through the cell and they're a lot smaller. With plant cells, you can have these huge vacuoles that can take up to as much as like 90% of the cell.
So they take up a bunch of space within the cell and they store all kinds of different things, right? I kind of like to think of them as vacuum holes. You can put things in a hole. You can store things there. That's how I kind of like to remember what vacuoles do. But anyways, the takeaway here is that plant cells have much bigger vacuoles than animal cells and fungi cells.
All right, next, let's talk about animal cells. So animal cells, in contrast to plant cells and fungi cells, cannot synthesize all of the amino acids. So our cells can only synthesize 10 out of the 20 amino acids. And this is why there are 10 amino acids that are essential in our diet, also called the essential amino acids.
And if you've ever heard of a food called a complete protein, that is because it has all these essential amino acids. For example, quinoa is actually a complete protein. So it's got all 10 of these essential amino acids.
Yes, it tastes like dirt, but it's one of those things you've got to weigh out. How happy are my cells going to be because I'm taking in these 10 essential amino acids versus how happy are my tastes, but it's going to be as I'm shoveling dirt in my mouth. It's just one of those things you've got to weigh out. Animal cells also have centrials. This is one of the things they possess that other types of cells don't. And if you remember from the genetics podcast, these are important structures for cell division.
In addition to centrioles, animal cells have cilia. And so cilia aid in the locomotion of the cell as well as locomotion of other objects. So think about cilia that line the throat or line the airways. So as mucus goes down, they're able to kind of push that mucus back up and then you hockaloogee.
It's important to notice distinction here between cilia and flagella. So, flagella are found in eukaryotic cells, but they're also found in prokaryotic cells. Cilia, on the other hand, are only found in animal cells.
All right, lastly, let's mention some of the unique characteristics of fungi. So, fungi are kind of a mix of animal and plant cells. They share some aspects with animal cells. They share some aspects with plant cells. So, like plant cells, they contain a cell wall, and this cell wall is made up of chitin.
And chitin is an acetyl-glucosamine polymer. And also kind of interesting to note is that insect exoskeletons are made of chitin.
Like an animal cell, fungi cells are unable to do photosynthesis, so they cannot create their own energy from the sun. Instead, they have to get their energy or food from their surroundings. So let's move on now to prokaryotes. This breaks down into two different categories, which are bacteria and archaea.
There are a few characteristics that set bacteria and archaea apart. So bacteria contain a peptidoglycan cell wall. And bacteria can be further broken down into gram negative bacteria and gram positive bacteria.
The difference here is in how the cell wall is made up. So in gram negative bacteria, they have a thin outer membrane, and then they have the peptidoglycan cell wall, and then they have another cell membrane, which is called the inner cell membrane. So in other words, they have this thin peptidoglycan cell wall layer sandwiched in between two membranes. And again, it's a very thin layer of that peptidoglycan.
For gram-positive bacteria, they have this thick layer of peptidoglycan that surrounds the cell. And so this really thick layer tends to absorb this dye, which is why they're called gram-positive, because when they are exposed to this specific dye, they tend to take that up, and you can actually see that under a microscope. So gram-positive, think really thick outer layer of peptidoglycan.
All right, so back to bacteria. So again, they have a cell wall. And bacteria are found in relatively benign surroundings. So I tend to think, OK, could an animal live in this environment? OK, if so, then it's likely that a bacteria could also live there.
And so this is in stark contrast to archaea, which are found in very extreme conditions. For example, there are some that live in these really hot oceanic vents that can get up to 100 degrees Celsius. And they can be found in very cold places like the bottom of the ocean. There's a type of archaea called a psychrophilic archaea that lives at the bottom of the ocean, where it's very cold and at a very high pressure.
Interestingly, archaea actually have methionine as the initiator amino acid for protein synthesis, which is the same as for eukaryotic cells. Bacteria, on the other hand, have an amino acid that is actually derivative of methionine that they use as an initiator amino acid for protein synthesis.
Also, archaea are a little bit more comparable to eukaryotic cells in the fact that they have histones, and they also have multiple kinds of RNA polymerases. So the takeaway here is that archaea are transcriptionally more like eukaryotic cells. They have histones. They have these different kinds of RNA polymerases. And they also have a similar initiator amino acid for protein synthesis.
Bacteria, on the other hand, have a peptidoglycan cell wall. Again, the peptidoglycan is the differentiator here, because archaic can have cell walls. And lastly, bacteria are found in these more benign or calm environments.
The last thing we want to talk about here in terms of cell type are a very specific type of cell called erythrocytes. I've seen this come up a few times on different MCAT practice material, so I just wanted to introduce it and explain some different things about erythrocytes, just because they're interesting. They're important for obviously a lot of different physiological functions.
So erythrocytes are red blood cells, and kind of interestingly, they don't contain DNA. So let's say you are a crime scene investigator, and you're trying to solve a murder, and you recover a red blood cell, you're screwed, you're not going to be able to pull out any DNA to try to match that to a murderer. And they also do not contain any mitochondria. Instead, they produce all their energy through fermentation, which is anaerobic respiration.
And they have a cytoplasm that is rich with hemoglobin, which gives these red blood cells their red color. And functionally, they deliver and carry oxygen and carbon dioxide to different parts of the body, which is very important. And shape-wise, they're kind of have this concave disc shape, almost a little bit like a frisbee.
And if you're smart, you might be thinking, okay, what advantage does it give this cell to have this flat shape? Well, it actually increases the surface area of the cell, which allows more oxygen and carbon dioxide to be transferred. And it's also important to note that these cells are a little bit flexible, so they must be able to squeeze down into different shapes in order to make it through these really, really small capillaries.
All right, that's all I really wanted to go over for the different types of cells. You know, be able to tell the difference between eukaryotic cells, prokaryotic cells when you're given a description. You know, no, for instance, that Scram positive bacteria have this thick outer layer of peptidoglycan. These things are going to come up on the MCAT. So yeah, let's get on to the next part of this podcast, which is the different tissue types.
All right, so let's get into different tissue types. So first of all, what is a tissue? Well, a tissue is a group of cells that are found together in the same location that share the same embryonic origin and have similar morphology that enable them to carry out a specific function. And so I kind of like to think about tissue as the level of organization that's
between a cell and an organ. A cell is this single little functional unit, whereas an organ is this massive collection of cells and tissues that performs a very meaningful function in the body. It's this in-between level of organization. That's the way I like to think about it. It makes it a little bit less abstract.
And so there are four different types of tissues to know. The first is epithelial, the second is muscle, the third is connective, and the fourth is nervous. So let's break each of these down, talk about what you need to know in terms of these different tissue types.
All right, let's start first by talking about epithelial tissue. In general, epithelial tissue covers the exterior surfaces of the body, lines, internal cavities, and passageways, and forms certain glands. So with that said, epithelial tissue performs a few different functions. The first is in protecting underlying organs or tissues. You can think about the epidermis.
which protects us from pathogens that might settle on our skin or these different bad things from entering our body. And the next function is in the absorption of nutrients in the digestive tract. And in there, you can think of intestinal epithelium. So there, it's just absorbing nutrients that are passing through the gut and your body for use in whatever use it needs, whether that's the creation of energy, biosynthesis, whatever.
The third function of epithelial tissue is in sensation. And again, you can think of the epidermis here as some of your touch receptors are located in the epidermis. And then the fourth function of epithelial tissue is in the secretion of hormones. And here you can think about the glandular epithelial.
So on the most basic level, the function of epithelial tissue is to protect, absorb, sensate, and secrete. And lucky for you guys, I actually have some of the audio from swearing in of the epithelial tissue. So let's take a listen.
Epithelial tissue do solemnly swear that I will support and defend the underlying tissues and organs, absorb nutrients, sense the presence of objects, whether foreign or domestic, and obey the orders of the glands to secrete.
So there are eight different types of epithelial tissue. The first is called simple squamous epithelium. The second is called simple cuboidal epithelium. The third is simple columnar epithelium. The fourth is called pseudo stratified columnar epithelium. The fifth is stratified squamous epithelium.
The six is stratified cuboidal epithelium, then you have stratified columnar epithelium, and then last you have transitional epithelium. And so all these different types of epithelial tissue all have different shapes and different functions. So what I want to do is I want to go through each of these eight different types, talk about their general shape and their general function.
And then at the end, I want to kind of tie together everything. And really what you need to know here is what's the difference between simple, stratified, pseudo stratified, and transitional epithelium. And by the way, what I mean when I say shape is the shape of the cells that make up this tissue layer. So as I describe these shapes, just know that that's cells, that's individual cells.
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All right, so the first type of epithelium I want to talk about here is called simple squamous epithelium. And it is essentially a single layer of flat thin cells. And this is kind of easy to remember. I mean, what comes to mind when I say the word squamous? For whatever reason, when I think the word squamous, I think of my really short, squatty friend. If I were to say, hey, what's up, man? You're looking squamous today.
You know, he might for a second in his mind go, wait, what does squamous mean? And then immediately he'd probably accept it because it just, it really describes kind of like short squatty. Anyways, single layer of thin flat cells. And functionally, it allows the quick diffusion of different molecules through the tissue layer.
Examples of simple squamous tissue are in things like capillaries in your glomerulus and in alveoli. In all three of these cases, you need really quick diffusion of different molecules. For capillaries, that's gases, like oxygen. For alveoli, again, that's gases. Then for the glomerulus, you need to be able to quickly filter the blood plasma. Go back to my renal physiology podcast if you want to know more about that.
The next type of tissue here is called simple cuboidal epithelium. And cells in this tissue layer are square-shaped, hence the cuboidal, and have a very large nuclei right in the middle of that cell. And functionally speaking, this tissue layer secretes and absorbs. So for example, simple cuboidal epithelium line the nephron and the ovaries.
And it also forms the duct portion of different glands. And again, a duct is simply a channel leading from either an exocrine gland or some kind of organ. So for example, the lactiferous duct is the channel that carries milk from the mammary gland to the nipple into a baby's mouth.
The next type of tissue is called simple columnar epithelium. And this is made up of a single layer of cells that are rectangular in shape. And they can have cilia on one end of the cells. And these cilia, again, looks kind of like these hair that stick up. Imagine like frosted tips, JT, throwback 1990 kind of a thing.
And the function of simple columnar epithelium is to absorb different molecules and then also secrete mucus and enzymes. And examples of this tissue type include the digestive tract, which are actually non-siliated simple columnar epithelium.
And they also include the upper respiratory tract, which are the ciliated form of this tissue type, and also the fallopian tube, which is also the ciliated version of this tissue. The next type of epithelial tissue is called pseudo stratified columnar epithelium.
This is a single layer of cells, although it has the appearance of multiple layers, hence its name pseudo-stratified. And I'll get into what stratified tissue is, but a little foreshadowing, it is multiple layers of cells. So this appears to be multiple layers, but is actually a single layer of cells.
And in terms of shape, the cells that make up this pseudo stratified columnar epithelium are roughly rectangular in shape, although they're pretty irregularly shaped. Maybe some of them are kind of squatty down low and get skinny up top, and then some are really top heavy and skinny on the bottom. They're just a lot more irregularly shaped than the simple epithelial case. And these cells can also be either ciliated or non-ciliated.
And the main function of this tissue layer is to secrete mucus and to also move mucus. So think about your upper respiratory tract. You got this mucus coming down and so these cells kind of act to push that mucus back up where it gets spit out by you.
And so examples of the ciliated case of pseudostratified columnar epithelium are in the upper airway in trachea. And then the non-ciliated case examples include the vast deferens, which is the duct that carries sperm from the testicle to the urethra, which then gets ejaculated.
All right, the next epithelium type is called stratified squamous epithelium. So in terms of cell types, these are all these really squatty thin cells that make up this layer. And what's different in this case is that it's stratified tissue. So you have more than one layer of cells. So you got multiple layers of flat cells, and there is no intracellular space. In other words, this looks kind of like a brick wall.
And similarly to a brick wall, the function of stratified squamous epithelium is to protect against a wear and abrasion. And it's also important to note that this tissue type must be kept moist. Examples of this tissue include tissue in the oral cavity, vagina, and esophagus.
The next type of tissue is called stratified cuboidal epithelium. Again, this is multiple layers of cells. And in this case, these cells are cube-shaped. And again, there is no intracellular space. So this time imagine a brick wall, but now instead of these like flat thin bricks, you've got these kind of squarish bricks. And the function of
The stratified cuboidal epithelial layer is to protect this house. And by this house, I mean glands. So they protect glands. And so what glands do they protect? Well, they protect sweat glands, salivary glands, and mammary glands.
All right, so now on to the seventh type of epithelial tissue to know. That is the stratified columnar epithelium. And as you can imagine, this tissue is made up of cells that are multiple layers thick and are rectangular in shape. And apparently, this is a fairly rare type of epithelial tissue.
And the function of this tissue is similar to that of the stratified cuboidal epithelium. It's to protect. But they also do a little bit of secretion, so it's something to throw in there. Examples are the male urethra, parts of the eye, and also ducks in the salivary glands.
The last type of epithelial tissue to know is called transitional epithelium. And this is a little bit different than the others, kind of interesting. It is made up of multiple layers of cells that appear to either be cuboidal or a columnar or squamous in shape. So these cells could be square, rectangular, or flat. And what it depends on is it depends on if this tissue layer is stretched or if it's compressed.
So this gets into the function of transitional epithelium. The function of this tissue is to either contract or expand. In other words, the function of this tissue is to be flexible. So when this tissue is contracted, let's say we're talking about tissue in the bladder, you just peed, now it's contracted, then this tissue layer is actually going to look cuboidal in appearance.
However, when you have a full bladder in these cells in this tissue are all kind of stretched out, then they're going to look squamous. And that generally makes sense when you think about it. And so as I kind of alluded to, this tissue exists in places where you need either expansion or contraction. So that's in places like the bladder, the ureter, the urethra, et cetera.
All right, so that is all eight of the different types of epithelial tissue. What I want you to take away from this is the fact that simple epithelial tissue is a single layer of cells and mostly participates in either absorption or secretion.
then pseudo stratified tissue looks like it's two layers, but really it's only one. And a lot of times it functions in secretion of different things. And then stratified tissue is multiple cell layers thick and provides protection for different things. And then lastly, we have transitional epithelial tissue, which is multiple cell layers thick and kind of acts as this flexible tissue in places where it's needed.
All right, so the second type of tissue now that I want to get into is called muscle tissue. And there are three different types of muscle tissue. There is smooth muscle, skeletal muscle, and cardiac muscle. What I want to do here is I want to talk about the differences in similarities between smooth skeletal and cardiac muscle. And then I'll also briefly throw in a little bit about slow versus fast twitch muscle, which are also called type 1 and type 2 muscle fibers.
So for most people, this should be review. Never a bad thing to review stuff. But if not, then I hope this helps for you for the MCAT and you learn it's something new.
So smooth muscle is muscle that is under involuntary control. So this is things like breathing, secretion of gut enzymes, blood vessel constriction, et cetera. And smooth muscle tissue is not striated. Hence the term smooth, right? It's smooth in appearance. It doesn't have these striations. And this is because smooth muscle fibers are not arranged in orderly sarcomers like skeletal muscle and cardiac muscle.
It's also important to note that smooth muscle cells have a single nucleus. In contract to smooth muscle tissue, skeletal muscle tissue is under voluntary control. So, you know, moving your arms around, moving your legs around, peeing, that's all skeletal muscle. They also function to protect internal organs. And skeletal muscle is striated. They have these striations, and that is due to the orderly arrangement of fibers into these things called sarcomeres.
And they are also, in contrast to smooth muscle, multinucleated. So these skeletal muscle cells can have multiple nucleuses. And this is because these muscle cells must be able to produce large amounts of enzymes and proteins needed for muscle contraction since they're being used almost constantly. And so in order to do this, you gotta have more genetic material around that can then be transcribed and translated into these enzymes and proteins.
The last type of muscle tissue I want to talk about here is cardiac muscle tissue. As you could probably guess from the name, cardiac muscle is only found in the heart. It contracts in a very coordinated manner, obviously, to make sure your heart is beating altogether. It's kind of scary if different parts of your heart were beating at different rates and you probably wouldn't live very long.
So like skeletal muscle, cardiac muscle is striated. However, like smooth muscle, it has a single nucleus and is involuntarily controlled. And while I was writing this podcast, it got me kind of thinking, what if you had voluntary control of your heart muscle?
Probably be pretty bad, right? You know, you get people trying to play like drinking games where it was whoever can hold their heart the longest is, you know, the winner and doesn't have to take a shot or, you know, you'd have people doing competitions between different friends of saying, you know, who can hold their heart the longest, this, this and that. It probably wouldn't be great.
All right, before I leave cardiac tissue, I want to talk about slow versus fast twitch muscle fibers. So they're technically called type 1 versus type 2 muscle fibers. So type 1 muscle fibers are slow twitch muscle fibers. And these muscle fibers have more mitochondria, which means they can do more oxidative phosphorylation.
They are also more red in color and they tend to have a slower contractile speed, i.e. their name, slow twitch. And these are the type of muscle fibers that will be like firing if you're just standing in line for coffee or just standing there trying to hold your pose. So maybe in your back, these muscles are firing pretty much constantly, right, as you're trying to keep your shoulders back.
and keep it straight back for good posture. On the other hand, you have type 2 muscle fibers, which are called fast twitch muscle fibers. They are much paler in color and have less mitochondria, and they rely more on anaerobic respiration because they lack this high density of mitochondria.
And they also have a faster contractile speed. And you can think about more explosive movements like sprinting and running that trigger these fast-twitch muscle fibers. And this is a bit counterintuitive to me because I think fast-twitch muscle, I think, okay, this must need a ton of energy in order to be firing super fast, super hard, then it must have more mitochondria to produce more ATP.
But that's just not the case. The mitochondria enable the slow twitch muscle to be more resistant to fatigue. But my assumption is that the anaerobic pathway that the fast twitch muscle uses is a bit faster, and you can produce energy at a faster rate. Therefore, you get a faster contractile speed. Now, kind of just putting together pieces here, but I'm pretty sure that's what happens.
That's not super important to know for the MCAT though. What's really important to know and take away from this is that type 1 or slow twitch muscles have more mitochondria. They are more red in color and they do more oxidative phosphorylation in comparison to type 2 or fast twitch muscle fibers.
So the next type of tissue I want to talk about is called connective tissue. And connective tissue provides structure and support and is also the most abundant tissue type. And there are a few different ways in which to classify connective tissue. And the one that I found that I like the best is categorizing it into three different categories, the connective tissue proper.
the supportive connective tissue, and then the fluid connective tissue. Connective tissue proper is probably the hardest to understand out of these three, and these are tissue types that have the three following characteristics. First, they are made up of dispersed cells.
They are more extracellular material than actual cells themselves. And then third, they have extensive protein fibers in their extracellular matrix. And to me, this definition is a bit convoluted. What I think you should understand is that the connective tissue proper is broken down into two different types, either loose or dense. So the dense connective tissue proper is tissue that gives support and structure.
And these are things like ligaments and tendons. And then the loose connective tissue proper is kind of like a weaker, I think of it more of like a gelatinous tissue layer. These are adipose tissue and also the hypodermis.
So I think just connect those four things to the connective tissue proper, ligaments, tendons, adipose tissue, and hypodermis. The supportive connective tissue, on the other hand, is tissue that supports and shapes the body and protects internal organisms. And this is comprised of bone
and cartilage. And so bone, as I'm sure you know, is collagen fibers embedded in a mineralized ground substance containing hydroxyapatite, which is a mineral. And the collagen component of the bone is what makes it a little bit flexible, less rigid, and the hydroxyapatite component adds hardness to the bone.
So if you didn't have hydroxyapatite, you might be a little bit like a less cool elastic girl from The Incredibles. So bone is pretty common knowledge. Odds are you probably understood what bone is made of before you listen to this podcast. But if you're like me, you probably have no idea what cartilage is made of. You know, you might be thinking, okay, that's the stuff on my ears and my nose. But what exactly is cartilage?
Well, there are three different kinds of cartilage. There is hyaline cartilage, fibro cartilage, and elastic cartilage. In terms of this podcast, I'm going to focus on the first and most abundant form of cartilage, which is hyaline cartilage. And hyaline cartilage is found in the lining of bones and in joints. So it kind of pads different joints. It's also found in the ribs to provide the ribs with a little bit of flexibility.
And then it's also found in the trachea. And so it's made of primarily collagen, but also has a chondroitin sulfate component to it. And so what is chondroitin sulfate? It's this glucosaminoglycan polymer.
So, high lean cartilage then is made up of collagen, which makes it really flexible. And then this sugar polymer that's called chondroitin sulfate that actually adds kind of a compression resistance factor to the cartilage. And combined together, they make a great combo for a strong, yet kind of flexible tissue.
So moving on to the third category of connective tissue, which is fluid connective tissue. And so there are two types of fluid connective tissue. The first is blood, and the second is lymph.
And to be honest, it doesn't make much sense to me why these are considered connective tissue. It doesn't seem like they're really connecting much. But I guess you can say, OK, there are connections between different organs, right? Your blood is the connection between your lungs and the other parts of your body. So maybe that's fine. So anyways, blood.
has a few different components within it. So it's got red blood cells, which I talked about earlier. They're called erythrocytes, and they carry gases. It also has white blood cells in it, which are called leukocytes, and they're important for immunity. And then lastly, blood has platelets in it. And platelets are important for clotting. And then next, the other fluid connective tissue is called lymph.
And so lymph contains a lot of white blood cells, which you can have to do with immunity. And it also carries some of the fats that you eat in your food that it get absorbed in through the gut. All right, so here are the takeaways for you. So there are three main types of connective tissue. You have connective tissue proper, which is made up of varying cell types. And ligaments and tendons are types of connective tissue proper.
Next, you have structural connective tissue. These are cartilage and bone, and those provide the kind of structure to the human body as their name connotates. And then lastly, you have fluid connective tissue, which is blood and lymph. And just kind of really don't let that slip your brain because it's easy to look past the fact that blood is a connective tissue.
All right, on to the very last type of tissue that is nervous tissue. And so nervous tissue is the main component of the nervous system. And there are two main classes of cells that make up nervous tissue. The first is the neuron, which I'm sure you're very familiar with, and the second are neuroglia, or maybe as you've read in your book or heard others pronounce, neuroglia.
So let's briefly cover these two different cell types. So neurons are the functional unit of the nervous system. They propagate signals. They have these things called synapses. These are kind of how your brain functions, how your brain communicates. And that's all I'm really going to say about neurons. You can go listen to my podcast on the nervous system if you're interested in those more. I'm going to now going to switch and I'm going to talk about neuroglia or glial cells.
So, neuroglia or glial cells are cells that support neurons. And there are a lot of different kinds of support. And so, this is kind of the point in which I combine tissue and cell type. So, if you remember, I'm talking about neural tissue. And so, neural tissue is made up of two different cell types, so I'm going to go into the different types of neuroglia cells.
So the first type is called astrocytes, and these provide nutrients to neurons, maintain ion balance, and remove unneeded or access neurotransmitters from the synaptic cleft. The next type of glial cells are called epindedimal cells, and these can either be ciliated or they can be non-ciliated.
In non-siliated ependidymal cells, form cerebrospinal fluid, while the ciliated ependidymal cells help the cerebrospinal fluid circulate. And what is a function of cerebrospinal fluid again? Well, it just cushions the brain and the e-spinal cord. Oligo dendrocytes are the next type of gliosel, and they form a myelin sheath around some neurons in the central nervous system.
And the myelin sheath is a fatty substance wrapped around the axon of neurons that basically helps the signal propagate down the axon faster. Like oligodendrocytes, swan cells also form myelin sheaths, but they do it in the peripheral nervous system. The big difference there is that oligodendrocytes operate in the central nervous system, swan cells operate in the peripheral nervous system.
The last type of cells I want to talk about here that are glial cells are called microglia. And they are small macrophage cells in the central nervous system that protect against different diseases by engulfing pathogens in the brain and then phagocytosing these pathogens, basically eating them. And interestingly enough, microglia have actually been determined to play some kind of role in the pathogenesis of Alzheimer's disease.
So a while ago, I went to a talk given by Dr. Kim Greene, who is a scientist who studies Alzheimer's disease and the role that microglia play in Alzheimer's disease. And so it's important to know here that microglia are activated in response to neuronal damage or to invaders.
And so in the case of Alzheimer's disease, microglia are activated by the deposition of A-beta peptides. So these are the peptides that form the amyloid plaques that is kind of like the classic pathogenesis that occurs in Alzheimer's. And so what his research showed was that microglia can actually get stuck on this on our active mode, leading to basically long-term inflammation within the brain.
And so lately, there's been a lot of interest in terms of chronic inflammation in the brain and Alzheimer's disease. What Dr. Green was really interested in was essentially asking the question, do these microglia actually help in Alzheimer's disease? He knew and he saw that when they became activated,
For long periods of time, this would lead to long-term inflammation, which was bad for Alzheimer's disease. But you wanted to know, okay, do these actually help against Alzheimer's disease? I think the classic thinking is that these microglia will digest these A-beta plaques, which then leads to the mitigation of Alzheimer's disease in the long-term.
And so what Kim Green did is he developed a way to actually deplete mouse brains of microglia. And basically he showed that when you deplete one mouse's brain of microglia and you have the other mouse in which you don't deplete any of the microglia, the A beta plaques form at just the same rates.
In other words, these microglia aren't necessarily eating up these plaques, as I think a lot of people thought previously. And so really the takeaway of the talk was, you know, these cells that we originally thought were the big protectors of the brain and protected people from getting Alzheimer's may actually play a role in the pathogenesis of Alzheimer's disease. They might become activated permanently by these abated plaques, and then that leads to chronic inflammation, which then may sprawl on Alzheimer's disease.
So it's kind of interesting, you know, I took away that, wow, these cells may not be exactly what we've thought for a long time. So kind of interesting little piece of research. Each episode of MCAT Basics is brought to you by Med School Coach. To access Med School Coach services, including MCAT tutoring and medical school admissions advising, visit our website at medschoolcoach.com. Good luck as you prepare for the MCAT.
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