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Once again, none of this is drawn to scale. So you're going to have the 23 chromosomes from your father. So let me do that. One, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, and 23. And then the 23 chromosomes from your mother. One, two, three, four, five, six, se... | Fertilization terminology gametes, zygotes, haploid, diploid MCAT Khan Academy.mp3 |
One, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, and 23. And then the 23 chromosomes from your mother. One, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, and 23. So you got that X chromosome from your mother.... | Fertilization terminology gametes, zygotes, haploid, diploid MCAT Khan Academy.mp3 |
So you got that X chromosome from your mother. And as you might have noticed, I've drawn them in pairs. So you now have a total, let me make it clear. You have 23 chromosomes here. 23 chromosomes, chromosomes in the sperm. You have 23 chromosomes, 23 chromosomes in the egg. And now you have 46 chromosomes in the fertil... | Fertilization terminology gametes, zygotes, haploid, diploid MCAT Khan Academy.mp3 |
You have 23 chromosomes here. 23 chromosomes, chromosomes in the sperm. You have 23 chromosomes, 23 chromosomes in the egg. And now you have 46 chromosomes in the fertilized egg. 46 chromosomes. And now that we have a full contingent of chromosomes, and then this cell can now keep replicating, keep splitting and differ... | Fertilization terminology gametes, zygotes, haploid, diploid MCAT Khan Academy.mp3 |
And now you have 46 chromosomes in the fertilized egg. 46 chromosomes. And now that we have a full contingent of chromosomes, and then this cell can now keep replicating, keep splitting and differentiating into all of what makes you you, we call this right over here, we call this a zygote. Zygote. So one way to think a... | Fertilization terminology gametes, zygotes, haploid, diploid MCAT Khan Academy.mp3 |
Zygote. So one way to think about it, the gametes are the sex cells that have half the number of chromosomes. And the zygote is the cell that's now ready to differentiate into an actual organism that has double the number, that has a full contingency of chromosomes, that has 46 chromosomes. And you see I made them in p... | Fertilization terminology gametes, zygotes, haploid, diploid MCAT Khan Academy.mp3 |
And you see I made them in pairs. And these pairs, we call these homologous pairs. And in each of these pairs, this is a pair of homologous chromosomes. Pair of homologous chromosomes. So what does that mean? Well, that means that in general, these two chromosomes, you've got one from your father, one from your mother,... | Fertilization terminology gametes, zygotes, haploid, diploid MCAT Khan Academy.mp3 |
Pair of homologous chromosomes. So what does that mean? Well, that means that in general, these two chromosomes, you've got one from your father, one from your mother, they code for the same things. They code for the same proteins. But there are different variants of how they code for those proteins, those traits that ... | Fertilization terminology gametes, zygotes, haploid, diploid MCAT Khan Academy.mp3 |
They code for the same proteins. But there are different variants of how they code for those proteins, those traits that you have. So gross oversimplification is, let's say that there was a gene, let's say that there is a gene on that one from your father that helps code for hair color. Well, there would be a similar, ... | Fertilization terminology gametes, zygotes, haploid, diploid MCAT Khan Academy.mp3 |
Well, there would be a similar, there would be another variant of that gene on the chromosome from your mother that helps code for hair color as well. So these are homologous chromosomes. These two chromosomes code in general for the same things. And so the zygote now has, you could say it has 46 chromosomes, or you co... | Fertilization terminology gametes, zygotes, haploid, diploid MCAT Khan Academy.mp3 |
And so the zygote now has, you could say it has 46 chromosomes, or you could say it has 23 pairs of homologous chromosomes. And this is, once again, this is the case for human beings. If we're talking about some other species, instead of 23 pairs of homologous chromosomes, or 46 chromosomes in total, you might be talki... | Fertilization terminology gametes, zygotes, haploid, diploid MCAT Khan Academy.mp3 |
Now, to help, biologists, to help clarify when they're talking about the number of chromosomes for a given species, they introduce two words, haploid and diploid. And haploid, haploid is referring to when you have half the full contingency of chromosomes. So for human beings, the haploid number is 23. So this is the ha... | Fertilization terminology gametes, zygotes, haploid, diploid MCAT Khan Academy.mp3 |
So this is the haploid, haploid number, haploid number, haploid number, it is 23. For another species, it would be something else. And haploid is based on the prefix hapl, or that's the prefix for single. So you have kind of a single member, I guess you could think of it, of each of the pairs. And now, you have both of... | Fertilization terminology gametes, zygotes, haploid, diploid MCAT Khan Academy.mp3 |
So you have kind of a single member, I guess you could think of it, of each of the pairs. And now, you have both of each pair, you have both chromosomes in each pair, or you have the full contingency. And this 46 chromosomes, this is called the diploid number. The diploid number for humans, diploid. The diploid number ... | Fertilization terminology gametes, zygotes, haploid, diploid MCAT Khan Academy.mp3 |
Some would argue that maybe viruses are even a more basic unit of life, but the organisms that we consider living like ourselves are made up of cells, and all living organisms that we for sure consider living are made up of at least one cell. So most basic unit of life. For example, me, this thing that's making a video... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
Now a common misconception is, well these things must be small, and they indeed are very, very, very small. Some cells are on the order of one micrometer long, and a micrometer is one millionth of a meter, or you could say one thousandth of a millimeter. And so when you think of something that small, sometimes there's ... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
But you could not be more wrong if you assume that a cell is simple. This right over here is a picture of a budding yeast cell. You can see that it's budding off right over here. But this just begins to show you some of the complexity of the cell itself, or of any cell. And in other videos, we're gonna talk about diffe... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
But this just begins to show you some of the complexity of the cell itself, or of any cell. And in other videos, we're gonna talk about different types of cells, different types of structures you'll see in some cells versus others. This right over here is a eukaryotic cell, which we will talk more about in other videos... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
Now all cells have a membrane that separate it from the outside world. You see the membrane right over here. This is just a cross-section. You could imagine a three-dimensional version of this. So this is the cell, cell membrane, kind of defines the cell in some way. And in general, the things inside the cell membrane ... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
You could imagine a three-dimensional version of this. So this is the cell, cell membrane, kind of defines the cell in some way. And in general, the things inside the cell membrane is considered the cytoplasm. Cytoplasm. Sometimes you will hear the term cytosol. A cytoplasm includes not just the fluid, but also all the... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
Cytoplasm. Sometimes you will hear the term cytosol. A cytoplasm includes not just the fluid, but also all the stuff in the fluid, while the cytosol is referring to the fluid alone. And then depending on the complexity of a cell, so this is right here, this yeast cell, this is a eukaryotic cell, which we will cover in ... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
And then depending on the complexity of a cell, so this is right here, this yeast cell, this is a eukaryotic cell, which we will cover in more depth in other videos. But one of the features of a eukaryotic cell is that you will have a membrane-bound nucleus. Now you see it in this diagram right over here. This is not a... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
This is not a common feature to all cells, but the only reason why I'm mentioning it in this video is officially the cytoplasm does not include the stuff inside the nucleus. In a eukaryotic cell, that is called the nucleoplasm, but we'll talk more about that in other videos. Now another feature that is common to all ce... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
And this picture is full of ribosomes. All these little dots right here, these little red dots, let me change my pen color, all these little red dots here, these are ribosomes. And you might assume, wow, these are so small compared to this already small thing, this cell. Surely they are simple, but they're actually fai... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
Surely they are simple, but they're actually fairly complex RNA and protein structures that their main function is producing protein. Producing protein. You could view these as almost the protein factories of living organisms. They can take genetic information in the form of RNA and produce proteins out of them. And yo... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
They can take genetic information in the form of RNA and produce proteins out of them. And you can see, this cell is full of ribosomes, and we're gonna talk about different types of ribosomes in a future video. Now another thing that is typical in most cells is genetic information. And typically, that genetic informati... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
And typically, that genetic information is stored as DNA. Now I say in most cells because it turns out that even in our own bodies, mature red blood cells don't have any DNA anymore, and there's other cells that do the same thing. But in general, in order for a cell to function and replicate, it needs some genetic info... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
That's true in both prokaryotic cells and eukaryotic cells. Prokaryotes are ones that don't have a well-defined nucleus and membrane-bound, what we call organelles, which are these substructures in cells, which we will talk more about in other videos. In a prokaryotic cell, the DNA is just floating around in the cytopl... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
And it is part of the nucleoplasm. So I'll leave you there for now. The last thing I want you to appreciate is just the scale. As I mentioned, cells are small. This picture of a yeast cell right over here, this is a micrometer on this scale. It would be about, would be about that. That would be one micrometer. | Introduction to the cell Cells High school biology Khan Academy.mp3 |
As I mentioned, cells are small. This picture of a yeast cell right over here, this is a micrometer on this scale. It would be about, would be about that. That would be one micrometer. And to put that in context, the width of a human hair, and it actually depends on your hair, whether it's soft or whether it's more lik... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
That would be one micrometer. And to put that in context, the width of a human hair, and it actually depends on your hair, whether it's soft or whether it's more like my hair, and it kind of sticks up and you have thicker hair, but if this is a human hair right over here, this is a width of a human hair, this thing, it... | Introduction to the cell Cells High school biology Khan Academy.mp3 |
Let's talk a little bit in more depth about how DNA actually copies itself, how it actually replicates. And we're gonna talk about the actual actors in the process. Now, as I talk about it, I'm gonna talk a lot about the three prime and the five prime ends of a DNA molecule. And if that is completely unfamiliar to you,... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
And if that is completely unfamiliar to you, I encourage you to watch the video on the anti-parallel structure of DNA. And I'll give a little bit of a quick review here just in case you saw it, but it was a little while ago. This is a zoom in of DNA. It's actually the zoom in from that video. And when we talk about the... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
It's actually the zoom in from that video. And when we talk about the five prime and three prime ends, we're referring to what's happening on the riboses that form part of this phosphate sugar backbone. So we have ribose right over here, five carbon sugar, and we can number the carbons. This is the one prime carbon, th... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
This is the one prime carbon, that's the two prime carbon, that's the three prime carbon, that's the four prime carbon, and that's the five prime carbon. So this side of the ladder, you could say, it is going in the, it is going, let me draw a little line here, this is going in the three prime to five prime direction. ... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
It's going three prime to five prime. Notice, this phosphate connects to the three prime, then we go to the five prime, connects to a phosphate, this connects to a three prime, then it connects, then we go to the five prime, connects to a phosphate. Now on this end, as we said, it's anti-parallel. It's parallel, but it... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
It's parallel, but it's oriented the other way. So this is the three prime, this is the five prime, this is the three prime, this is the five prime. And so this is just what we're talking about when we talk about the anti-parallel structure. These two backbones, these two strands are parallel to each other, but they're... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
These two backbones, these two strands are parallel to each other, but they're oriented in opposite directions. So this is the three prime end, and this is the five prime end. And this is going to be really important for understanding replication, because the DNA polymerase, the things that's adding more and more nucle... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
So if we were talking about this right over here, we would only be able to add, we would only be able to add going that way. We wouldn't be able to add going, we wouldn't be able to add going that way. So one way to think about it is you can only add nucleotides on the three prime end, or you can only extend, you can o... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
If you're only adding on the three prime end, then you're going from the five prime to the three prime direction. You can't go from the three prime to the five prime direction. You can't continue to add on the five prime side using polymerase. So what am I talking about with polymerase? Well, let's look at this diagram... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
So what am I talking about with polymerase? Well, let's look at this diagram right over here that really gives us an overview of all of the different actors. So here is just our DNA strand, and you can imagine it's just somewhat natural in its natural unreplicated form. And you can see we've labeled here the three prim... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
And you can see we've labeled here the three prime and the five prime ends. And you could follow one of these backbones. This three prime, if you follow it all the way over here, it goes, this is the corresponding five prime end. So this and this are the same strand. And this one, if you follow it along, if you go all ... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
So this and this are the same strand. And this one, if you follow it along, if you go all the way over here, is the same strand. So this is the three prime end of it, and then this is the five prime end of it. Now the first thing, and we've talked about this in previous videos where we gave an overview of replication, ... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
Now the first thing, and we've talked about this in previous videos where we gave an overview of replication, is the general idea is that the two sides of our helix, the two DNA, the double helix, needs to get split, and then we can build another, we can build another side of the ladder on each of those two split ends.... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
It involves a whole bunch of enzymes and all sorts of things, and even in this diagram, we're not showing all of the different actors, but we're showing you the primary actors, at least the ones that you'll hear discussed when people talk about DNA replication. So the first thing that needs to happen, right over here, ... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
It is tightly, tightly wound. And it actually turns out, the more that we unwind it on one side, the more tightly wound it gets on this side. So in order for us to unzip the zipper, we need to have an enzyme that helps us unwind this tightly wound helix. And that enzyme is the topoisomerase. And the way that it actuall... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
And that enzyme is the topoisomerase. And the way that it actually works is it breaks up parts of the backbones temporarily so that it can unwind, and then they get back together. But the general high-level idea is it unwinds it, so then the helicase enzyme, and the helicase really doesn't look like this little triangl... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
These things are actually far more fascinating if you were to actually see the molecular structure of helicase. But what helicase is doing is it's breaking those hydrogen bonds between our nitrogenous bases. In this case, this is an adenine here, this is a thymine. It would break that hydrogen bond between these two. S... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
It would break that hydrogen bond between these two. So first, you unwind it. Then the topoisomerase unwinds it. Then the helicase breaks them up. And then we actually think about these two strands differently, because as I mentioned, you can only add nucleotides going from the five prime to the three prime direction. ... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
Then the helicase breaks them up. And then we actually think about these two strands differently, because as I mentioned, you can only add nucleotides going from the five prime to the three prime direction. So this strand on the bottom right over here, which we will call our leading strand, this one actually has it pre... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
Remember, this is the five prime end right over here, so it can add going in that direction. It can add going in that direction right over here. This is the five prime to three prime. So what needs to happen here is to start the process, you need an RNA primer. And the character that puts an RNA primer, that is DNA pri... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
So what needs to happen here is to start the process, you need an RNA primer. And the character that puts an RNA primer, that is DNA primase. We'll talk a little bit more about these characters up here on the lagging strand. But they'll add an RNA, let me do this in a color you can see. An RNA primer will be added here... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
But they'll add an RNA, let me do this in a color you can see. An RNA primer will be added here. And then once there's a primer, then DNA polymerase can just start adding nucleotides. It can start adding nucleotides at the three prime end. And the reason why the leading strand, it has it pretty easy, is this DNA polyme... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
It can start adding nucleotides at the three prime end. And the reason why the leading strand, it has it pretty easy, is this DNA polymerase right over here, this polymerase. And once again, they aren't these perfect rectangles as on this diagram, they're actually much more fascinating than that. You see DNA polymerase... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
You see DNA polymerase up there, you also see one over here, polymerase. This polymerase can just, you can kind of think of it as following the open zipper, and then just keep adding, keep adding nucleotides at the three prime end. And so this one seems pretty straightforward. Now, you might say, well, wouldn't it be e... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
Now, you might say, well, wouldn't it be easy if we could just add nucleotides at a five prime end? Because then we could say, look, this is going from three prime to five prime. Well, maybe that polymerase or different polymerase could just keep adding nucleotides like that, and then everything would be easy. Well, it... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
Well, it turns out that that is not the case. You cannot add nucleotides at the five prime end. And let me be clear, this three prime right over here, I'm talking about this strand. This strand over here, let me do this in another color, this strand right over here, this is the three prime end, this is the five prime e... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
This strand over here, let me do this in another color, this strand right over here, this is the three prime end, this is the five prime end. And so you can't just keep adding nucleotides just like that. And so how does biology handle this? Well, it handles this by adding primers right, as this opening happens, it'll a... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
Well, it handles this by adding primers right, as this opening happens, it'll add primers. And this diagram shows a primer is just one nucleotide, but a primer is typically several nucleotides, roughly 10 nucleotides. So it'll add roughly 10 RNA nucleotides right over here, and that's done by the DNA primase. So the DN... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
So the DNA primase is going along, the lagging is going along this side, this, I could say the top strand, and it's adding the RNA primer, which won't be just one nucleotide, it tends to be several of them. And then once you have that RNA primer, then the polymerase can add in the five prime to three prime direction. I... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
So then it can just start adding DNA like that. And so you can imagine this process, it kind of, you add a, the primase puts some primer here, and then you start building from the five prime to the three prime direction. You start building just like that, and then you skip a little bit, and then that happens again. So ... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
So you end up with all these fragments of DNA, and those fragments are called Okazaki fragments. So the Okazaki fragments. And so what you have happening here on the lagging strand, you can think of it as, why is it called the lagging strand? Well, you have to do it in this kind of, it feels like a suboptimal way, wher... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
Well, you have to do it in this kind of, it feels like a suboptimal way, where you have to keep creating these Okazaki fragments as you follow this opening. And so it lags, it's going to be a slower process. But then all of these strands can be put together using the DNA ligase. The DNA ligase. Not only will the strand... | Leading and lagging strands in DNA replication MCAT Khan Academy.mp3 |
So here you are sitting in your classroom, and you desperately want to tell your friend here something. So you write a little note, and you pass it to them from your hand directly to their hand. So they see the note, but no one else sees it. It's just for them. But let's say you want to tell your friend across the room... | Cellular communication Cells MCAT Khan Academy.mp3 |
It's just for them. But let's say you want to tell your friend across the room something. I don't know about you, but when I was in elementary school, I used to write notes on pieces of paper, fold it up into a paper airplane, and I had pretty good aim back then, so I'd throw my paper airplane over to my friend across ... | Cellular communication Cells MCAT Khan Academy.mp3 |
And again, they'd be the only one who got the message. No one else got my little paper airplane message. And if I wanted to tell a few friends something, I might call a little huddle and actually say a few things just to this group of friends here. So my voice would cross this small distance between us. Now let's kick ... | Cellular communication Cells MCAT Khan Academy.mp3 |
So my voice would cross this small distance between us. Now let's kick things up a notch. I was kind of a rebel in elementary school, and sometimes I'd go to the secretary's desk and take over the intercom to say funny things to my friends, maybe to tell them to meet me at the flagpole or on the playground at recess or... | Cellular communication Cells MCAT Khan Academy.mp3 |
And so this intercom message that I'd send out, that would go to everyone. It would be broadcasted to the whole school. And so those who wanted to come meet me would do that, and those who wouldn't, wouldn't. And we can think of cells as little people that do really similar things because you might not always think abo... | Cellular communication Cells MCAT Khan Academy.mp3 |
And we can think of cells as little people that do really similar things because you might not always think about it, but it's really, really important that cells are able to talk to each other. And evolutionarily, cells being able to communicate with each other are a major reason why we're as complex as we are as huma... | Cellular communication Cells MCAT Khan Academy.mp3 |
So how do cells pass notes to each other? How do they directly communicate with each other? Well, one way is by actual direct contact. So cells have lots of proteins stuck into their plasma membranes here that serve a lot of functions. And the most important one is for communication. So let's look at a macrophage here.... | Cellular communication Cells MCAT Khan Academy.mp3 |
So cells have lots of proteins stuck into their plasma membranes here that serve a lot of functions. And the most important one is for communication. So let's look at a macrophage here. This is a type of white blood cell that's a part of your immune system. So when these macrophages see a foreign invader, maybe a littl... | Cellular communication Cells MCAT Khan Academy.mp3 |
This is a type of white blood cell that's a part of your immune system. So when these macrophages see a foreign invader, maybe a little bacteria or a virus, they can ingest it. They can ingest it, then they break it down, and then they display a little piece of it, which is now called an antigen, on their surface. So t... | Cellular communication Cells MCAT Khan Academy.mp3 |
So they show it off on their surface with one of these cell surface proteins here. And this one in particular is called an MHC2 protein. So now this little antigen here has become the note that they want to pass on. This antigen is the message. And so another white blood cell, maybe a helper T cell might come along and... | Cellular communication Cells MCAT Khan Academy.mp3 |
This antigen is the message. And so another white blood cell, maybe a helper T cell might come along and then grab hold of this antigen here with one of its cell membrane proteins, in this case a T cell receptor. So just by doing this, the macrophage here managed to pass a message onto the helper T cell here. And now, ... | Cellular communication Cells MCAT Khan Academy.mp3 |
And now, based on which antigen this is, the T cell can decide whether to start a full-blown immune response. Maybe it'll go off and ring more alarm bells by activating other antibody cells, which are called B cells. Or not, maybe they'll just do nothing. It just depends on what type of note this is. So when cells dire... | Cellular communication Cells MCAT Khan Academy.mp3 |
It just depends on what type of note this is. So when cells directly touch to communicate, sort of unsurprisingly, this is called direct cell-cell communication, or just direct binding. Now, what about our other methods of communication? Cells can also communicate over short distances. This is our paper airplane here. ... | Cellular communication Cells MCAT Khan Academy.mp3 |
Cells can also communicate over short distances. This is our paper airplane here. So for example, let's look at two neurons. So they're in close approximation, but one end of a neuron doesn't quite touch the start of the next neuron here. There's a little gap there called the synaptic cleft. So what neurons do is they ... | Cellular communication Cells MCAT Khan Academy.mp3 |
So they're in close approximation, but one end of a neuron doesn't quite touch the start of the next neuron here. There's a little gap there called the synaptic cleft. So what neurons do is they release little signals called neurotransmitters to communicate with each other. So neurotransmitters get released from the en... | Cellular communication Cells MCAT Khan Academy.mp3 |
So neurotransmitters get released from the end of this neuron, and they'll diffuse across this little distance here until they bind onto one of the dendrites of this next neuron. And that effectively passes the message on from this neuron to this next neuron. The paper airplane is thrown from here to here, and this is ... | Cellular communication Cells MCAT Khan Academy.mp3 |
And over here, just like calling a huddle, one cell can talk to a small group of cells locally as well. So for example, just underneath our skin, let's say the skin inside our nose, we have these immune cells called mast cells, and they're really important in mediating allergic reactions. That's why I purposely picked ... | Cellular communication Cells MCAT Khan Academy.mp3 |
So let's say you're allergic to certain pollens, and one day in the spring you found yourself walking through the park. Inevitably, you'd breathe in some pollen, and then the pollen would go on to attach to these antibodies stuck to our mast cells. And what happens as a response to this is that the mast cells release l... | Cellular communication Cells MCAT Khan Academy.mp3 |
So histamine acts as our short-range message. It travels around to cells in the area to let them know that an allergen has been found, and lets them know to start preparing for an allergic reaction to take place. And this type of communication is called paracrine signaling, paracrine meaning nearby. Finally, the interc... | Cellular communication Cells MCAT Khan Academy.mp3 |
Finally, the intercom takeover. How do our cells talk to huge groups of cells at once? Well, they do that by endocrine signaling. So for example, cells in our pituitary gland in our brain make a lot of the important hormones in our bodies. So let's say that they're making growth hormone, GH, to send around to all the c... | Cellular communication Cells MCAT Khan Academy.mp3 |
So for example, cells in our pituitary gland in our brain make a lot of the important hormones in our bodies. So let's say that they're making growth hormone, GH, to send around to all the cells in the body. Sending this hormone is their form of communication here. So they'll create the growth hormone inside their cell... | Cellular communication Cells MCAT Khan Academy.mp3 |
How do you maintain a steady body temperature when you're exposed to ice packs or hot water bottles? Healthy body temperature is 37 degrees Celsius or 98.6 degrees Fahrenheit. I need to keep a steady temperature near 98.6 degrees Fahrenheit or else crucial molecules in my body will change shape and stop working and I'l... | Homeostasis.mp3 |
Homeostasis is a scientific term for my body's ability to maintain its proper equilibrium temperature. But what if I'm exposed to steaming hot water or freezing cold ice? How does my body maintain its equilibrium temperature then? Let's see. I'll cover myself with ice packs and see how my body reacts. Five cold minutes... | Homeostasis.mp3 |
Let's see. I'll cover myself with ice packs and see how my body reacts. Five cold minutes later, let me check my body temperature. Sure enough, it's still near normal body temperature, homeostasis in action. Within a degree or so of 98.6 is still considered normal and despite how cold I feel, I haven't actually gotten ... | Homeostasis.mp3 |
Sure enough, it's still near normal body temperature, homeostasis in action. Within a degree or so of 98.6 is still considered normal and despite how cold I feel, I haven't actually gotten any colder. How did my body do this? It made me feel cold and want to warm myself up by shivering, little muscle movements that gen... | Homeostasis.mp3 |
It made me feel cold and want to warm myself up by shivering, little muscle movements that generate heat. See how pale my arm looks? After noticing the cold, my body directed my blood to my core and less to my skin and extremities. My arm quickly loses heat to the cold environment, but the temperature stays constant in... | Homeostasis.mp3 |
My arm quickly loses heat to the cold environment, but the temperature stays constant in my core, which is thicker, so it loses less heat to the environment. I also get goosebumps where my hair stands on end, creating an insulating layer like the jacket my body wishes I were wearing. So my body uses a lot of tools to k... | Homeostasis.mp3 |
When my body senses that it's cold, homeostasis mechanisms make me shiver, draw blood away from my skin, and give me goosebumps. These make me warmer, so my core temperature isn't changed. My body uses some of the opposite tools to cool down. It directs blood to the surface to cool down, making me a bit pink. It needs ... | Homeostasis.mp3 |
It directs blood to the surface to cool down, making me a bit pink. It needs to resort to more extreme measures if I want to be active in the heat, because moving my muscles uses energy and lets off heat, sort of like shivering to keep warm in the cold, but in this case, my body needs to counteract the warmth that the ... | Homeostasis.mp3 |
It also makes me sweat. In order to get the energy to evaporate into the air, sweat pulls heat from my body, and this helps me cool down. Not all animals have as effective sweat glands as people do, so people can endure longer periods of intense activity than many other animals. When I got warm, homeostasis mechanisms ... | Homeostasis.mp3 |
When you take an AP biology exam, it is likely that it will include a formula sheet that will include formulas like this on it. And it can be a little bit intimidating at first because we're not used to seeing formulas like this that involve, in fact, this is formerly calculus notation in a biology class. But what we'l... | Population growth rate based on birth and death rates Ecology AP Biology Khan Academy.mp3 |
So putting this aside, let me just ask you a simple question. Let's say we're studying a population and we see that the birth rate, the birth rate of this population is equal to 60, let's say we're studying a population of bunnies, 60 bunnies, bunnies per year. And let's say we know that the death rate of bunnies, deat... | Population growth rate based on birth and death rates Ecology AP Biology Khan Academy.mp3 |
Now without even paying attention to this formula sheet up there, what do you think, given this data, is the population, population growth rate for this population of bunnies? Pause this video and see if you can answer that. Well, your population growth rate, if you think about just even say a given year, in that year,... | Population growth rate based on birth and death rates Ecology AP Biology Khan Academy.mp3 |
So you will grow by 60 bunnies per year, but then you would shrink by the 15 that died. So it would shrink by 15 bunnies, bunnies per year. And so in that year, you would net out 45 bunnies. And that's a rate, because you're saying per year. So you would grow by 45 bunnies, bunnies in that year. And that would be your ... | Population growth rate based on birth and death rates Ecology AP Biology Khan Academy.mp3 |
And that's a rate, because you're saying per year. So you would grow by 45 bunnies, bunnies in that year. And that would be your population growth rate. Now the thing that we just did very intuitively, you don't need advanced math to think through what we just did. That's exactly what this formula is saying. This notat... | Population growth rate based on birth and death rates Ecology AP Biology Khan Academy.mp3 |
Now the thing that we just did very intuitively, you don't need advanced math to think through what we just did. That's exactly what this formula is saying. This notation, where you say D something DT, this is the rate at which this something is changing with respect to time. So this is just a fancy way of saying, what... | Population growth rate based on birth and death rates Ecology AP Biology Khan Academy.mp3 |
So this is just a fancy way of saying, what is the rate at which our population is changing with respect to time? There's other ways that you could have written that. If you didn't wanna use calculus notation, you could have written change in population for a given change in time. The Greek letter delta often denotes c... | Population growth rate based on birth and death rates Ecology AP Biology Khan Academy.mp3 |
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