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This one was even more recent. And then looking at fossils within that to say, okay, 20 million years ago, there were species around that looked like that. And then 10 million years ago, there were species that looked like that. And one example is if you look at horse-like animals. So this is right over here. We're tal... | Evidence for evolution Biology Khan Academy.mp3 |
And one example is if you look at horse-like animals. So this is right over here. We're talking about horses, zebras, donkeys, mules, things like that. The modern ones, well, this is their bone structure. But if you look at the fossil record from 12 to five million years ago, you see fossils that look like this. And th... | Evidence for evolution Biology Khan Academy.mp3 |
The modern ones, well, this is their bone structure. But if you look at the fossil record from 12 to five million years ago, you see fossils that look like this. And they're very close. So you see, it's very believable that you see you could have evolution from this to that. But then you go further back, and once again... | Evidence for evolution Biology Khan Academy.mp3 |
So you see, it's very believable that you see you could have evolution from this to that. But then you go further back, and once again, it seems like a very gradual process. And once again, this is happening over, these are from 12 to five million years ago, these are from 16 to 12 million years ago, these are from ove... | Evidence for evolution Biology Khan Academy.mp3 |
And so you can see how this is happening at a very, very gradual pace. And the mechanism, and we go into some depth in other videos in Khan Academy, you have variation species, you have the environment selecting for it. The environment might change, or different things happen, so you have different forms of selection, ... | Evidence for evolution Biology Khan Academy.mp3 |
And so you have this change, this heritable change of traits over time. And so when you look at the fossil record, it makes a lot of sense that, okay, this is strong evidence for evolution, that the animals that we see today weren't just put on, just created all of a sudden, and haven't changed since then, that there's... | Evidence for evolution Biology Khan Academy.mp3 |
And it's very, very compelling, it shows the embryonic development of a whole series of species, from a fish on the left, to a reptile, to birds, to mammals, and another mammal, to non-human mammals, and of course, to humans. And you can see at the early stages, they look eerily similar. In fact, you see proto-gill sli... | Evidence for evolution Biology Khan Academy.mp3 |
And Haeckel, he's the guy who coined ontogeny recapitulates phylogeny, which is a very fancy way of saying that your embryonic development is telling the story of the evolutionary past, which isn't true, but you'll even hear people quote that today. But his drawings and his observations, this is compelling evidence for... | Evidence for evolution Biology Khan Academy.mp3 |
The next thing I'm gonna talk about is you can think about it as micro-structures or processes, and this is microbiology. Micro-biology. And biolo-biology. Microbiology. And the more we understand about microbiology, the more compelling case of evolution. Because when we look at even, you know, one, all life forms that... | Evidence for evolution Biology Khan Academy.mp3 |
Microbiology. And the more we understand about microbiology, the more compelling case of evolution. Because when we look at even, you know, one, all life forms that we know, they involve DNA. How the DNA gets replicated and translated and transcribed is very similar from one life form to another. The idea of DNA going ... | Evidence for evolution Biology Khan Academy.mp3 |
How the DNA gets replicated and translated and transcribed is very similar from one life form to another. The idea of DNA going to, DNA coding for proteins, proteins that are made up of amino acids is something that we see throughout biology. Amino acids, which once again, hints at a common ancestry. And not only are t... | Evidence for evolution Biology Khan Academy.mp3 |
And not only are those molecular, and many of the very proteins, are very, very similar, more similar than if you looked at the macro level or even at the structural level between different species. And not just do they share these common micro-structures and processes, but the actual information stored in things like ... | Evidence for evolution Biology Khan Academy.mp3 |
But this, and you'll see other data like this that's very similar to this, which is how much genetic similarity do we have between different species? And these numbers tell us how much genetic similarity at a high level do we have with chimpanzees, mice, fruit flies, yeast, and plants. And the fact that we have 26% of ... | Evidence for evolution Biology Khan Academy.mp3 |
Because at a macro level, it doesn't seem like there's a lot in common with yeast. But when you get at a microbiological level, there's a good bit that's in common with yeast. And chimpanzees, we do relate to them. Their facial expressions often feel eerily human, their behaviors often feel eerily human, but their gene... | Evidence for evolution Biology Khan Academy.mp3 |
Their facial expressions often feel eerily human, their behaviors often feel eerily human, but their genes show just how close to human beings they actually are. And this actually shows that even, you know, mice are way closer, if you looked at the entire tree of life, based on genetic evidence, things like mice and ev... | Evidence for evolution Biology Khan Academy.mp3 |
Now the last thing that I promised I would talk about is direct evidence. Direct evidence of evolution. And I talk about this in the first evolution video. But the direct evidence we see all the time with things like bacteria, where you have bacteria, let's say growing around, and we have antibiotics that we use in our... | Evidence for evolution Biology Khan Academy.mp3 |
But the direct evidence we see all the time with things like bacteria, where you have bacteria, let's say growing around, and we have antibiotics that we use in our body to kill bacteria. But the reason why many physicians and scientists will tell you don't overuse antibiotics is because the more you use it, it causes ... | Evidence for evolution Biology Khan Academy.mp3 |
So we're gonna talk a little bit about DNA regulation. And this is the general idea that if you look at an organism's genome, that not all of the genes are being transcribed and translated at the same time. It could actually depend on the type of cell that DNA is inside of, or it could depend on the environment for tha... | Operons and gene regulation in bacteria.mp3 |
So for example, if you look at, say, a multicellular organism, this, maybe this is, and these are oversimplifications right over here, maybe this is some type of immune cell, immune cell. And let's say that this over here is a muscle cell. And they're not necessarily, or not likely to be these perfect circles, but this... | Operons and gene regulation in bacteria.mp3 |
And they're going to have the exact same DNA. So the DNA in both of these is the same. So DNA is the same inside, and these are going to be, these are eukaryotes, so I'll draw the nuclear membrane there, same DNA. But they have very different roles inside of this organism. So it doesn't make sense, in fact, in order fo... | Operons and gene regulation in bacteria.mp3 |
But they have very different roles inside of this organism. So it doesn't make sense, in fact, in order for them to even have different structures, they're gonna have to produce different proteins. They're gonna have different enzyme proteins inside of their cytoplasm. And so DNA regulation, one way to think about it i... | Operons and gene regulation in bacteria.mp3 |
And so DNA regulation, one way to think about it is, well, if they have the exact same genome, how do they regulate which of those genes are being transcribed and then translated, and which ones aren't? And even if you think about a unicellular organism, right here we have a bacteria. And so it's just one cell, but eve... | Operons and gene regulation in bacteria.mp3 |
For example, this over here, so this is the bacterial chromosome. This right over here might be a gene involved in the digestion of a certain type of food, if that food is present. This type of, and actually it could even be several genes that are involved in that type of food, and we will actually go into a little bit... | Operons and gene regulation in bacteria.mp3 |
So maybe that's related to digesting or consuming some type of food. Maybe you have some genes over here that are related to some type of stress mechanism. Maybe it needs to go into hibernation sometime. And so if it's not under stress, it does not have to express these genes. But if it is under stress, it does have to... | Operons and gene regulation in bacteria.mp3 |
And so if it's not under stress, it does not have to express these genes. But if it is under stress, it does have to express these. Likewise, if that thing that it needs to digest is around, it needs to transcribe these. If it's not around, it does not need to transcribe it. So that's how DNA regulation works, whether ... | Operons and gene regulation in bacteria.mp3 |
If it's not around, it does not need to transcribe it. So that's how DNA regulation works, whether you're talking about a eukaryote or a prokaryotic organism. And so what we're gonna do in this video is focus a little bit more, or a lot more, on the prokaryote side, especially we're gonna talk about this bacterium. Whe... | Operons and gene regulation in bacteria.mp3 |
When we talked about transcription in general in several videos ago and in several videos, we talked about the idea of a promoter, that you have a gene that is a sequence of DNA that's part of the broader chromosome, and we said, okay, that RNA polymerase needs to attach someplace. So that RNA polymerase needs to attac... | Operons and gene regulation in bacteria.mp3 |
And when we first talked about the idea of a promoter, we said, and this is generally true in eukaryotes, that each promoter is associated with a gene, or each gene has a promoter. But when we're talking about prokaryotes, and in this case we're talking about this bacterium, it's actually typical to have multiple genes... | Operons and gene regulation in bacteria.mp3 |
Let me write this down. So the promoter, so that's this part right over here, that's the sequence, that is a regulatory, regulatory DNA sequence. Well, that's what the RNA polymerase, which I drew as this big blob, it's a protein here, this big blob, will attach to, and then it will begin to transcribe all of these gen... | Operons and gene regulation in bacteria.mp3 |
And when you have a promoter associated with multiple genes, that combination of the promoter and the genes, and once again, when I'm talking about the promoters and the genes, I'm talking about sequences of DNA, that combination is called an operon. This is called an operon. It's a combination of that regulatory DNA s... | Operons and gene regulation in bacteria.mp3 |
And then of course, that transcription process takes that genetic information in DNA, transcribes it into messenger RNA, which can then go with the ribosomes, and we have the whole translation process, this should all be review, to produce the actual proteins that have functions within or even potentially outside of th... | Operons and gene regulation in bacteria.mp3 |
But what if we're in an environment where we don't want to transcribe this particular operon, this particular series, or maybe I should say this particular series of genes? Well then, we might, something in our environment might allow repressors to take action. So what are we talking about a repressor? Well a repressor... | Operons and gene regulation in bacteria.mp3 |
Well a repressor, a repressor, right over here, you see it attaching to a sequence of DNA after the promoter and so it blocks, it blocks the RNA polymerase from being able to do the transcription. And so this right over here, this is a protein that is called the repressor, it's literally repressing the transcription. A... | Operons and gene regulation in bacteria.mp3 |
So once again, promoter was a regulatory sequence where the RNA polymerase can attach, and then the operator is a regulatory sequence where a repressor can attach and keep that RNA polymerase from actually being able to perform the actual transcription and so this keeps the gene from keeping, continuing to transcribe a... | Operons and gene regulation in bacteria.mp3 |
Co, co-repressor. Repressor. And we'll go into more detail when we talk about things like the trypophoron, of how tryptophan, an amino acid, can actually act as a co-repressor. Now over here, we have the other way around, where we want even more transcription. In that case, we would have something called, we would have... | Operons and gene regulation in bacteria.mp3 |
Now over here, we have the other way around, where we want even more transcription. In that case, we would have something called, we would have an activator. And this, let me shade it in, this DNA right over here, this would be the regulatory sequence where the activator binds. And so this would be positive feedback. W... | Operons and gene regulation in bacteria.mp3 |
And so this would be positive feedback. When you have more activators, you're gonna get more transcription, while this would be, and actually, I shouldn't even call it feedback, because that implies that somehow these products produce the activator, or these products produce the repressor, but that's not necessarily th... | Operons and gene regulation in bacteria.mp3 |
I should just say that this is repressing, and this is activating. It's going to make more of the transcription actually happen. And just as we could have co-repressors, small molecules that you could think of as activating the repressor, you can also have small molecules that can turn the activator on. And these small... | Operons and gene regulation in bacteria.mp3 |
And these small molecules that turn the activator on, these are called inducers. So this right over here, these are inducers. So this protein right here couldn't activate that operon, but now that you have these inducers, and we'll study that a little bit more when we think about the lac operon, this could be a small s... | Operons and gene regulation in bacteria.mp3 |
How long are those fragments? How many base pairs long are they? Well, you might say, well, why don't I just take them out and count them, except for the fact that they're incredibly small and incredibly hard to handle. Even a fairly large fragment of DNA, let's say we're talking about something that's on the order of ... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
Even a fairly large fragment of DNA, let's say we're talking about something that's on the order of 5,000 base pairs, well, that's going to be approximately one to two micrometers long if you were to completely stretch it out, and we can't even start to think about how thin the actual diameter is. But lengthwise, the l... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
So that's not going to help us to somehow try to manipulate it physically with our hands or with kind of rough tools. So how do we do that? And we could have other vials there. How do we see how long the DNA strands that are sitting in those vials actually are? And the technique we're going to use, gel electrophoresis,... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
How do we see how long the DNA strands that are sitting in those vials actually are? And the technique we're going to use, gel electrophoresis, it actually could be used for DNA strands, it could be used for RNA, it could also be used for proteins, any of these macromolecules, to see how long are those fragments. And s... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
Gel electrophoresis. Electrophoresis. And it's called gel electrophoresis because it involves a gel, it involves electric charge, and phoresis is just referring to the fact that we are going to cause the DNA fragments to migrate through a gel because of the charge. So phoresis is referring to the migration or the movem... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
So phoresis is referring to the migration or the movement of the actual DNA. So how do we do this? Well, here is our setup right over here. We have our gel that's inside of a, that's embedded in a buffer solution. So this gel, the most typical one is agarose gel, that's a polysaccharide that we get from seaweed, and it... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
We have our gel that's inside of a, that's embedded in a buffer solution. So this gel, the most typical one is agarose gel, that's a polysaccharide that we get from seaweed, and it's literally a gel. It's a gelatinous material. And what we're going to do is, we're going to put, we're going to take samples, so we might ... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And what we're going to do is, we're going to put, we're going to take samples, so we might take a little sample from this one right over here, and we'll put it in this well right over here, and you can view these wells as little divots in the gel. You could take a little sample from here and put it into this well, and... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
So you can see the buffer I drew this fluid, and that's really just water with some salts in it, and the buffer is going to keep the pH from going too far out of bounds as we place a charge across this entire thing, because if the pH gets too far in the basic or acidic side, it might actually affect the DNA or affect t... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And the other end is going to be our positive electrode. And we're going to use the fact that DNA has a negative charge at the typical pHs, or at the pHs that we are going to be dealing with. And we could go back into previous videos, and we can see it right over here. You see these negative charges on our phosphate ba... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
You see these negative charges on our phosphate backbone. And so what is going to happen? What is going to happen once we connect both of these to a power source, and then this side is negative and this side is positive? Well, the DNA is going to want to migrate. Now, let's think about what will happen. Will shorter th... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
Well, the DNA is going to want to migrate. Now, let's think about what will happen. Will shorter things migrate further, or will longer things migrate further? Well, you might say, well, longer things are going to have more negative charge, so maybe they go farther away. But then you also have to remember that they're ... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
Well, you might say, well, longer things are going to have more negative charge, so maybe they go farther away. But then you also have to remember that they're also moving more mass. So their charge per mass is going to be the same, regardless of length. And so what determines how far something gets, how much it migrat... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And so what determines how far something gets, how much it migrates over a certain amount of time, is how small it is. Remember, we have this agarose gel. And people are still studying the exact mechanism of how this DNA, or these macromolecules, actually migrate through the polysaccharide. But if you imagine this poly... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
But if you imagine this polysaccharide as kind of this mesh, this net, this sieve, well, smaller things are going to be able to go through the gaps easier than the larger things. And so if you let some time pass, if you let some time pass, some of the DNA, let's say this DNA gets around there, let's say, and I'm just c... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
Let's say that this DNA doesn't migrate. Let's say it has some that migrates that far, and let's say it has some that migrates that far. And so if you just saw this, you wait some amount of time, and you were to come back, and you were to see this migration, you were to see this migration occur, and the longer you wait... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
In fact, if you wait too long, they're gonna fall off all the way over the other edge. Is, if you just saw this, you'd say, okay, well, this strand right over here, these must be smaller DNA molecules. They must be shorter. These must be a little bit longer, and these must be even longer than that. And this grouping ri... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
These must be a little bit longer, and these must be even longer than that. And this grouping right over here is going to be the longest of all, so this was a mixture of some longer strands, and still longer ones, but not quite as long. And for example, maybe there are some really short strands. Maybe there are some re... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
Maybe there are some really short strands in that, what I'm drawing as, what I'm drawing as, that orange group right over here. So what I just did right over here, this could tell you the relative length of these strands, but how would you actually measure them? Well, that's where you can go find standardized solutions... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
So let's say you go get the DNA ladder. I'm gonna draw it in pink. So you literally could buy this. You could even buy it online. And the standard solution, let's say it separates like this. So it separates like that, goes there. Let's say some of it goes like there, and some of it goes like there. | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
You could even buy it online. And the standard solution, let's say it separates like this. So it separates like that, goes there. Let's say some of it goes like there, and some of it goes like there. Well, you would be able to know from the labeling, or whichever one you choose to buy, that this grouping here, this is ... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
Let's say some of it goes like there, and some of it goes like there. Well, you would be able to know from the labeling, or whichever one you choose to buy, that this grouping here, this is all of the DNA that is 5,000 base pairs, let's say. Let's say this right over here is 1,500 base pairs. And let's say this over he... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And let's say this over here is, let's say this over here is 500 base pairs long. And so now you can use this DNA ladder, these standardized ones, to gauge how long, how many base pairs these are. So you say, okay, this blue one here, this is a bunch of DNA that's a little bit longer than 500 base pairs, but it's short... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
You can see this green one here. Well, it's a little bit longer than 1,500 base pairs. It didn't migrate quite as far as this big bundle of 1,500 base pairs that it did. And so then you can get a better approximation. And you can choose your ladder based on what you think you are going to find there, what you're actual... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And so then you can get a better approximation. And you can choose your ladder based on what you think you are going to find there, what you're actually going to look for. Now, the other thing to appreciate is, when you see the DNA having migrated this far, you might say, okay, is this one DNA strand? Is that one DNA s... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
Is that one DNA strand that I'm looking at? And just going back to the measurements, no. That is many, many, many, many DNAs that you're looking at. And they're not all stretched out like that. Remember, even something that is 5,000 base pairs long is only going to be one to two micrometers if you stretch it out. So yo... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And they're not all stretched out like that. Remember, even something that is 5,000 base pairs long is only going to be one to two micrometers if you stretch it out. So you wouldn't even be able to see it. It's a thousandth of a millimeter. You wouldn't even be able to see it. So this is many, many, many molecules of D... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
It's a thousandth of a millimeter. You wouldn't even be able to see it. So this is many, many, many molecules of DNA is migrating that far. And they would have to be that small to be able to migrate through that polysaccharide gel. Now, the last thing you're probably saying is, okay, wait, but how am I even seeing it o... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And they would have to be that small to be able to migrate through that polysaccharide gel. Now, the last thing you're probably saying is, okay, wait, but how am I even seeing it over here? How do I actually see this DNA, especially if they're these super, super small molecules? And the answer is you put some type of m... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And the answer is you put some type of marker on the DNA that will make them visible, some type of dye or something that might become fluorescent. And one of the typical things that people often use is ethidium bromide. And ethidium bromide is called an intercalating agent. And it's a molecule, you can see the ethidium... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And it's a molecule, you can see the ethidium right over here. These are two DNA, two backbones of DNA. You can see the base pairs bonding here. And then this right over here, that is ethidium that has fit itself. That's why we call it intercalating. It has fit itself in between the rungs of the ladder. And when it doe... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And then this right over here, that is ethidium that has fit itself. That's why we call it intercalating. It has fit itself in between the rungs of the ladder. And when it does so inside of DNA, it actually becomes fluorescent when you apply UV light to it. So if you put this ethidium bromide into all of your DNA right... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And when it does so inside of DNA, it actually becomes fluorescent when you apply UV light to it. So if you put this ethidium bromide into all of your DNA right over here, and then as it migrates, and then if you were to turn on the UV light, it would become fluorescent. And you would actually see these things. And so ... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And so if you wanted to see what it actually would look like in real life, well, this is what it would look like if you were to look at it straight on. Where this would have been a well, let me make it a little bit easier to read. So right over here would have been the well where you would put the DNA ladder. And it wo... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And it would come up with standardized measurements. Maybe that's our 5,000 base pairs. This right over here is our 1,500 base pairs. And this right over here is our 500 base pairs. And then let's say you had some solution of some other DNA. And you wait a little while, and you see, look, it migrated not quite as far a... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And this right over here is our 500 base pairs. And then let's say you had some solution of some other DNA. And you wait a little while, and you see, look, it migrated not quite as far as the 500 base pairs. So it must be a little bit, this must be a bundle of things a little bit longer than 500 base pairs, but for sur... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
So it must be a little bit, this must be a bundle of things a little bit longer than 500 base pairs, but for sure, a lot shorter than 1,500 base pairs. And once again, it doesn't have to have just one fragment length. You could have had another group that was maybe right at 1,500 base pairs. And you've probably seen th... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
And you've probably seen this. Whenever you see people talking about genetic analysis and things like this, you're often seeing people looking at one of these readouts from gel electrophoresis. So now you know what's actually going on here. This isn't a strand of DNA. This is a bunch of DNA that has been tagged with so... | Gel electrophoresis Biomolecules MCAT Khan Academy.mp3 |
Now what do I mean by a semi-permeable membrane? That means they allow some things to go through and not other things. And let's say this semi-permeable membrane, it does allow water molecules to pass. And in a few seconds we'll talk about what it does not allow to pass, which makes it semi-permeable. But let's just th... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
And in a few seconds we'll talk about what it does not allow to pass, which makes it semi-permeable. But let's just think about what would happen if we just had water molecules on either side. Well, we've already talked about it in the videos on diffusion. The water molecules, since we have an equal concentration on ei... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
The water molecules, since we have an equal concentration on either side, the probability that one of these water molecules goes this way in a certain amount of time is equal to the probability that a water molecule goes from right to left in the same amount of time. And that's because we have equal concentrations. And... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
They all have, they all are, they all have different velocities. They have different speeds and in different directions. And we just have to think about it probabilistically. The probability of going from left to right through one of these gaps is going to be equal to the probability of going right to left in any given... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
The probability of going from left to right through one of these gaps is going to be equal to the probability of going right to left in any given period of time. But now let's make this interesting. Let's treat our water as a solvent and let's put some solute in it. So let's dissolve some solute. So let's throw some so... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
So let's dissolve some solute. So let's throw some solute particles here. And I'm gonna make it, make them bigger so you can see they would physically have trouble passing through these gaps. There's other ways where you could have semipermeable membranes that use charge to allow certain things to pass through and not ... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
There's other ways where you could have semipermeable membranes that use charge to allow certain things to pass through and not others. But it's easier to visualize the size. And thinking about the membrane as only allowing certain things of certain size to pass through. So let's throw some solute there. And actually I... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
So let's throw some solute there. And actually I'll throw a little bit of solute here too. I'll do one or two particles right over here. But I'm gonna do many more. I'm gonna do many more over there on the right hand side. So we have a higher concentration of solute on the right hand side. And this is a semipermeable m... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
But I'm gonna do many more. I'm gonna do many more over there on the right hand side. So we have a higher concentration of solute on the right hand side. And this is a semipermeable membrane. And you can see even from the size where I drew these gaps, these big particles aren't going to be able to go through the membra... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
And this is a semipermeable membrane. And you can see even from the size where I drew these gaps, these big particles aren't going to be able to go through the membrane. They aren't going to be able to diffuse. If they were allowed to diffuse, then they would just go down their concentration gradient. And in any given ... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
If they were allowed to diffuse, then they would just go down their concentration gradient. And in any given moment of time, you would have a higher chance of one of these big particles moving from the right to the left than from the left to the right. Because you just have more on the right hand side. But this is a se... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
But this is a semipermeable membrane. And these things aren't just going to be allowed to naturally diffuse. Now all of these big particles, they all have their own unique velocities. So they all have their unique velocities. What do we think is going to happen? Well let's just think about the problem. We know that the... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
So they all have their unique velocities. What do we think is going to happen? Well let's just think about the problem. We know that the big particles can't diffuse from one side to another. But what's going to happen to the water molecules? Well the water molecules on the left hand side, they're not gonna be stopped. ... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
We know that the big particles can't diffuse from one side to another. But what's going to happen to the water molecules? Well the water molecules on the left hand side, they're not gonna be stopped. If they're bouncing in the right way, they can bounce from the left to the right, or they can move from the left to the ... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
If they're bouncing in the right way, they can bounce from the left to the right, or they can move from the left to the right through one of these gaps. But what about the ones on the right side? Well if they're the just right conditions, if they're the just right conditions, maybe this character could move through thi... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
So you're definitely going to have water molecules going back and forth. But I'd argue that the ones on the right hand side, there's a lower probability of water molecules from the right hand side moving to the left as from the left hand side moving to the right. And why is that? Well there's all this interference at p... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
Well there's all this interference at play from these big molecules that aren't able to diffuse. These are gonna be bouncing around. Sometimes they're going to be even, sometimes you could imagine them even blocking, they're going to be blocking the approach to these openings. If this membrane wasn't here, they wouldn'... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
If this membrane wasn't here, they wouldn't block the approach, they would just keep on going. But since that membrane is there, they might block it or they might ricochet off. And while they ricochet off, they might push on some water molecules. They might push on some water molecules going in this direction right ove... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
They might push on some water molecules going in this direction right over there. And so an argument can be made that these water molecules, some of them will still make it from right to left, but you have a lower probability of going from right to left as you have from going to left to right. And so because of this, y... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
Remember the solute is the thing that's dissolved in the water. And in general, we always consider the solvent to be whatever there's more of. In this case, it's water, and water is probably the most typical solvent. And the solute is whatever there's less of. So the solute is dissolved in the solvent. And so we have a... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
And the solute is whatever there's less of. So the solute is dissolved in the solvent. And so we have a net migration of the water molecules from this solution that has a low solute concentration to one that has a higher solute concentration. And this phenomenon we call osmosis. We call this osmosis. And there's other ... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
And this phenomenon we call osmosis. We call this osmosis. And there's other arguments for osmosis. And it's something that we've observed many, many, many times. If you put something that's used to fresh water and it has skin or it has membranes that allows water to pass through it, put it in salt water. You know, kin... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
And it's something that we've observed many, many, many times. If you put something that's used to fresh water and it has skin or it has membranes that allows water to pass through it, put it in salt water. You know, kind of the famous things like slugs will not do well in the presence of salt because the water inside ... | Osmosis Membranes and transport Biology Khan Academy.mp3 |
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