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And the thing that is around that it can grab electrons from is actually water. And so this is such a strong oxidizing agent that it can essentially oxidize the oxygen in water. And oxygen is itself, I mean, oxidizing is named after oxygen because oxygen is such a strong, it's so electronegative, it's such a strong, it... | Conceptual overview of light dependent reactions.mp3 |
So anyway, it grabs its electrons, once it gets this P680 plus, grabs an electron from water. And then the water essentially falls apart. So you're left just with the oxygen and then the hydrogen ions. And so those hydrogen ions also contribute, those hydrogen ions also contribute to the increased hydrogen ion concentr... | Conceptual overview of light dependent reactions.mp3 |
And so those hydrogen ions also contribute, those hydrogen ions also contribute to the increased hydrogen ion concentration on the inside. And this is where we get, this is where we get the oxygen byproduct right over here. Here we have one half of an O2, so if you do this twice, you're going to have a molecular oxygen... | Conceptual overview of light dependent reactions.mp3 |
So so far, we've talked about how the oxygen gets produced, we've talked about how the ATP gets produced, what about the NADPH? Well, we started our process in photosystem II. You might say, why is it called photosystem II if that's where we start? Well, it's actually that's because that's the second photosystem to be ... | Conceptual overview of light dependent reactions.mp3 |
Well, it's actually that's because that's the second photosystem to be discovered. You might say, what is a photosystem? Well, these photosystems and complexes, they're combinations of proteins and molecules. And photosystem in particular has chlorophyll and variations of chlorophyll and pigment molecules that are resp... | Conceptual overview of light dependent reactions.mp3 |
And photosystem in particular has chlorophyll and variations of chlorophyll and pigment molecules that are responsive to light, that are very easy, that have electrons that can get excited by light. And they can transfer that energy back down to the P680 chlorophyll A pair, which then can have its electron excited, and... | Conceptual overview of light dependent reactions.mp3 |
That electron can eventually make its way over to photosystem I, and why is it called photosystem I? Well, it's because the first one that was discovered. In photosystem I, there's another chlorophyll A pair called P700, and that's because it optimally absorbs light of a wavelength of 700 nanometers. And you have somet... | Conceptual overview of light dependent reactions.mp3 |
And you have something similar that happens, that light can either directly or indirectly excite its electron, and then that electron, as it goes to a lower energy level, it goes from one molecule to another, it can be used to reduce NAD plus into NADPH. And so that's where the NADPH comes from. And then once again, on... | Conceptual overview of light dependent reactions.mp3 |
And so that's why you'll often see these diagrams. Lights come in, electron gets excited, it goes to lower and lower energy states. As it's doing that, it's being transferred from one molecule to another, being facilitated by enzymes. That energy, part of that energy is being used to transfer hydrogen ions into the thy... | Conceptual overview of light dependent reactions.mp3 |
That energy, part of that energy is being used to transfer hydrogen ions into the thylakoid lumen, into the interior. Then in photosystem I, you have another excitation event. That thing that got excited can grab that electron that went to lower and lower energy states, and its excited electron can once again be transf... | Conceptual overview of light dependent reactions.mp3 |
And once again, the whole idea of the hydrogen ion concentration increasing here can fuel ATP synthase, which allows us to jam a phosphate onto ADP to produce ATP. So that is where we actually get all of these things, and the byproduct, of course, is our oxygen. And if you wanted to see that same idea, but kind of just... | Conceptual overview of light dependent reactions.mp3 |
Why have you done PCR? PCR was kind of the mainstay of my graduate project, where I built all sorts of different recombinant DNA molecules, and used them to learn things about plants. And so what does PCR in particular do? PCR basically makes you a lot of copies of a particular fragment of DNA that you're interested in... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
PCR basically makes you a lot of copies of a particular fragment of DNA that you're interested in. And so how does that, like why would you need to make a lot of copies of a particular fragment of DNA? So you might want to be making lots of copies so that you can clone it into a plasmid, and then do some other experime... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
That's a big use. So when we talked about cloning, and we're talking about sticking a fragment of DNA inside of a plasmid, it's not like you're just sticking one fragment into one plasmid. You're doing that with many, so you need a lot of fragments of DNA. Exactly, that is exactly it. And you might start with a very sm... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
Exactly, that is exactly it. And you might start with a very small sample of DNA. And so you just really need to, where else would you have to do PCR? PCR is used a lot in forensics. It's also used a lot in medical diagnostics. So this could actually be your DNA that was being checked to see if you have a gene that wou... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
PCR is used a lot in forensics. It's also used a lot in medical diagnostics. So this could actually be your DNA that was being checked to see if you have a gene that would predispose you to a particular condition. All sorts of really practical applications. Because it's hard to identify just one, one fragment of that g... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
All sorts of really practical applications. Because it's hard to identify just one, one fragment of that gene. So you'd want to make copies, or as they say, amplify it, so that you could run it in gels and stuff and see how all of those molecules, how big they are or something like that. Exactly, if you were just looki... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
Exactly, if you were just looking in your DNA pulled out of your cell, that would be a needle in a haystack. So this is how you can really zoom in and look at just the thing you need to see. Okay, so you've drawn some diagrams here, and I actually have never done a PCR, but you have, so I'm going to tell you how I unde... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
So what you drew over here, this is double-stranded DNA, and this could have been from a sample of someone's hair or whatever else. And let's say we want to replicate or make many, many copies of a fragment of this. And so let's say the fragment that we really care about is the fragment roughly from there to there, thi... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
And so this first step, denaturation, I have trouble pronouncing things. It's a weird word. It's a weird word. You have 96 degrees Celsius, so this is almost at the boiling point, so it's quite hot, and that separates the two strands. Precisely. And so once they're separated, then you can cool things down, although thi... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
You have 96 degrees Celsius, so this is almost at the boiling point, so it's quite hot, and that separates the two strands. Precisely. And so once they're separated, then you can cool things down, although this still isn't that cool, 55 degrees Celsius would be very uncomfortable. But you would cool it down to this, an... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
But you would cool it down to this, and then these primers show up. And so one thing to remind ourselves is this process is happening inside of a test tube or in a big solution. So you heat it up, the DNA, the two strands separate, and do you just have this primer lying around? So the primer is something that you've or... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
So the primer is something that you've ordered from a company and you've ordered a lot of it. So you put in a ton of primer in your reaction so that there's a really good chance that when you get to this step here called annealing, that a primer is going to bind to many of your pieces of DNA. So this is our solution. I... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
Is this all happening in water? Water with some salts and stuff floating around, yeah. Okay, so we have our solution right over here. You'd put whatever your initial DNA sample is in there, and once again, it's a very small amount. You'd put a lot of that primer, so you'd want to put that in a lot of surplus. Let me do... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
You'd put whatever your initial DNA sample is in there, and once again, it's a very small amount. You'd put a lot of that primer, so you'd want to put that in a lot of surplus. Let me do that in this magenta color. You obviously wouldn't see it in real life. It would just all dissolve. It would just look like a drop of... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
You obviously wouldn't see it in real life. It would just all dissolve. It would just look like a drop of liquid. It would look like, but for visualization, you put a lot of primer, and so you heat it up, the DNA strands separate, and then when you cool it back down, this primer's going to be specific to the ends of th... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
It would look like, but for visualization, you put a lot of primer, and so you heat it up, the DNA strands separate, and then when you cool it back down, this primer's going to be specific to the ends of the region that you want to copy. Exactly. And so when you order online or wherever that you want a certain primer, ... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
Exactly, that's super important. Okay, and so when you cool it back down, the primer attaches, and then you heat it back up, not quite to the 96 degrees Celsius, but to the 72 degrees Celsius, where you extend those, and I'm assuming since it's called polymerase chain reaction, that this is where the polymerase is invo... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
So the polymerase is what is actually extending this, and is it, so I'll just draw a polymerase enzyme right over there doing the extending, and is it any type of polymerase enzyme? Could I just take the polymerase from my cells and throw it in there? So you actually need a special polymerase because you need one that ... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
So as you were mentioning, even the cool step of this process is not something that your body would want to be hanging out in. So the polymerase is actually from a really heat-tolerant microorganism. And what is that? It's called a TAC polymerase? Thermophilus aquaticus, I think? Makes quite a mouthful. And they found ... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
It's called a TAC polymerase? Thermophilus aquaticus, I think? Makes quite a mouthful. And they found it at heated vents, this organism that is able to stand these high temperatures. But that, I guess, leads to another question. Why do you have to heat it up to begin with? I guess just to separate the two strands? | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
And they found it at heated vents, this organism that is able to stand these high temperatures. But that, I guess, leads to another question. Why do you have to heat it up to begin with? I guess just to separate the two strands? That's really the key reason. You just have to get them apart. You don't have an enzyme to ... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
I guess just to separate the two strands? That's really the key reason. You just have to get them apart. You don't have an enzyme to do it the way you might in a cell, so heat does the trick. Okay, so I get it. So this is one step. I'm guessing I'm getting at least the polymerase part of the PCR, where you heat it up, ... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
You don't have an enzyme to do it the way you might in a cell, so heat does the trick. Okay, so I get it. So this is one step. I'm guessing I'm getting at least the polymerase part of the PCR, where you heat it up, the strands separate, then you have all of this extra primer there. The primer, because there's so much p... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
I'm guessing I'm getting at least the polymerase part of the PCR, where you heat it up, the strands separate, then you have all of this extra primer there. The primer, because there's so much primer, the primer's much more likely to bind to at least at this part of the sequence than for these two strands to get back to... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
And you would have added that at the beginning, you know, the TAC polymerase. I guess I'll put it in this, I'll do it in a yellow color. So you would also put all that TAC polymerase in there. And once again, these things aren't robots. They don't know exactly what they need to do. They just bump into things in the rig... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
And once again, these things aren't robots. They don't know exactly what they need to do. They just bump into things in the right way and react in the right way. And then you would also have to add a bunch of nucleotides. Yes, absolutely. Your reaction is not going to work if you forget the nucleotides. So the TAC poly... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
And then you would also have to add a bunch of nucleotides. Yes, absolutely. Your reaction is not going to work if you forget the nucleotides. So the TAC polymerase, when you heat it back up again after the primers have been attached, is going to start adding all of these nucleotides. And what, do you just wait a certa... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
So the TAC polymerase, when you heat it back up again after the primers have been attached, is going to start adding all of these nucleotides. And what, do you just wait a certain amount of time or will it just keep going on forever? It'll keep going on for a while. Usually you do pick the length of that step to match ... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
Usually you do pick the length of that step to match how much time you expect the polymerase to need to complete your fragment. But it kind of will stop, either it'll fall off or it'll stop when you go on to the next step. Okay, so this, I get this is so far. So, so far we have, after one cycle, let me, what you've wri... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
So, so far we have, after one cycle, let me, what you've written here, after one cycle we would have doubled at least that part, that part of the sequence that we care about. Although we might even have, we might have copied even beyond that sequence. So where does the chain reaction come into this? So I guess you can ... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
So I guess you can interpret chain reaction in two ways. And one is, that's sort of what the polymerase does, is it, you know, adds things to make a chain. But there's actually even more of a chain reaction dimension here. And that's that we're actually getting this kind of exponential process going on. So you do it on... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
And that's that we're actually getting this kind of exponential process going on. So you do it one cycle, you get to this situation right here. You heat it up, the strands separate, you cool it down, the primers attach, you heat it up again, the Taq polymerase does its job. And like all polymerase, it goes from the fiv... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
And like all polymerase, it goes from the five prime to the three prime direction. We talked about in that in the application. So now you have two strands. But now, since all of that stuff is in that solution, you can just keep, you can heat it up again. Now each of, now these two strands can turn into, or these two, t... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
But now, since all of that stuff is in that solution, you can just keep, you can heat it up again. Now each of, now these two strands can turn into, or these two, these two double strands can now turn into four single strands. Then you can cool it down again. Now they get primers attached to them. And they're still the... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
Now they get primers attached to them. And they're still the same primer because we still care about the same sequence. And then that can keep, and so now you go from one to two to four. And so you keep repeating this. And so how many times would it be typical for you to repeat this cycle? So like 35 might be a pretty ... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
And so you keep repeating this. And so how many times would it be typical for you to repeat this cycle? So like 35 might be a pretty typical number of cycles to do. It depends a little what you're doing. But you're gonna do it a lot of times. And so if you do this 35 times, I mean each time you're multiplying by two. S... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
It depends a little what you're doing. But you're gonna do it a lot of times. And so if you do this 35 times, I mean each time you're multiplying by two. So it's gonna be two to the 35th power, which is well over a billion times. So, and how long would that take? You've done this before. Um, depends on the length of yo... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
So it's gonna be two to the 35th power, which is well over a billion times. So, and how long would that take? You've done this before. Um, depends on the length of your fragment, but usually like two to three hours. So in two to three hours, you can start with one fragment and get into the billions. If it's perfectly e... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
Um, depends on the length of your fragment, but usually like two to three hours. So in two to three hours, you can start with one fragment and get into the billions. If it's perfectly efficient, which I wish it always were, but you usually get quite a few pieces made. And one thing that I was, that I've always wondered... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
And one thing that I was, that I've always wondered when I first learned about this, and I'd like to go into a lab and do this with you, is, is, okay, I get that you have your primer and then the, and then the, the polymerase is just going to extend it like that. But I was like, well, it's, you know, it's going to be, ... | Polymerase chain reaction (PCR) Biomolecules MCAT Khan Academy.mp3 |
And you might be able to predict from its name what it does. It synthesizes ATP. Now you've probably seen it before. We saw it when we looked at respiration, or you will see it when you look at respiration, which is going on in most of the cells of your body. And you also see it when you study photosynthesis. The gener... | ATP synthase Cellular energetics AP Biology Khan Academy.mp3 |
We saw it when we looked at respiration, or you will see it when you look at respiration, which is going on in most of the cells of your body. And you also see it when you study photosynthesis. The general thing that it does is, is it sits across a phospholipid membrane, and through other processes, you will have hydro... | ATP synthase Cellular energetics AP Biology Khan Academy.mp3 |
You still might have a few over here. And a hydrogen ion is essentially a proton. So on this side of the membrane, it'll be more positive, so there will be a electromotive force to go to the other side. And also, you just have a higher concentration, so there's a chemical gradient, a concentration gradient, where if th... | ATP synthase Cellular energetics AP Biology Khan Academy.mp3 |
And also, you just have a higher concentration, so there's a chemical gradient, a concentration gradient, where if there's some way for these protons to get to this side, they would wanna get there. So there's an electrochemical gradient that they would wanna go down. And ATP synthase provides a channel for those proto... | ATP synthase Cellular energetics AP Biology Khan Academy.mp3 |
But as those protons travel through the ATP synthase, they turn this part of it, which drives this axle, and then this axle nudges these parts of the protein so that they jam together an ADP with a phosphate group to produce ATP. So down here, going into this part of the complex, you'll have an ADP and a phosphate grou... | ATP synthase Cellular energetics AP Biology Khan Academy.mp3 |
And that's going to be the case both in respiration, which occurs in the mitochondria, and in photosynthesis, which occur in chloroplasts. Now, there's a few differences. In mitochondria, the hydrogen ions, these protons, the concentration builds up in the intermembrane space right over here because of the electron tra... | ATP synthase Cellular energetics AP Biology Khan Academy.mp3 |
And we studied that in other videos. And then the protons travel through the ATP synthase. You can see a little mini version right over here. You could imagine that what we see really big, that is a blown-up version of this part of the mitochondria. And this, of course, is not to scale. So in the case of a mitochondria... | ATP synthase Cellular energetics AP Biology Khan Academy.mp3 |
You could imagine that what we see really big, that is a blown-up version of this part of the mitochondria. And this, of course, is not to scale. So in the case of a mitochondria, this would be the inner membrane. Right over here would be the intermembrane space between the inner and the outer membrane, intermembrane s... | ATP synthase Cellular energetics AP Biology Khan Academy.mp3 |
Right over here would be the intermembrane space between the inner and the outer membrane, intermembrane space. And right over here would be the matrix of the mitochondria. And so as the protons go through, they're able to produce ATP in the matrix. Now, in chloroplasts, the hydrogen protons build up inside the thylako... | ATP synthase Cellular energetics AP Biology Khan Academy.mp3 |
Now, in chloroplasts, the hydrogen protons build up inside the thylakoids, which are these parts of the chloroplast. That space inside the thylakoids, often called the thylakoid space, sometimes called the lumen, that proton buildup inside the thylakoids happens because of the light reactions, the first phase of photos... | ATP synthase Cellular energetics AP Biology Khan Academy.mp3 |
And to understand how this works, it's really the reverse of endocytosis, we're going to go and produce some proteins in the endoplasmic reticulum. This is our classic example. Those proteins are then going to bud off in their own little vesicles, which then merge with the Golgi apparatus, where they are further proces... | Exocytosis Membranes and transport Biology Khan Academy.mp3 |
So they're processed in the Golgi apparatus right over here. And then eventually they're going to bud off of the Golgi apparatus in new vesicles, and those vesicles are going to make their way over to the cell's outer membrane, the plasma membrane, and the membranes of the vesicles are going to merge with the membrane ... | Exocytosis Membranes and transport Biology Khan Academy.mp3 |
There are other cases where maybe it merges partially, releases the contents, and then buds back. It's called the kiss-and-run method of exocytosis, but the classic one is it merges with the membrane. We can look at this membrane. After the vesicle's membrane has merged with the plasma membrane, the membrane might look... | Exocytosis Membranes and transport Biology Khan Academy.mp3 |
After the vesicle's membrane has merged with the plasma membrane, the membrane might look like this. It might look like this. So if the vesicle, let me do the vesicle's membrane in orange. The vesicle's membrane is that in orange. Well, now it has merged like this, and it has released its contents. It has released the ... | Exocytosis Membranes and transport Biology Khan Academy.mp3 |
The vesicle's membrane is that in orange. Well, now it has merged like this, and it has released its contents. It has released the protein to be used someplace else, someplace else in the body. And I want to be clear, this membrane, and I've talked about it many times in many other videos, even though I've drawn it as ... | Exocytosis Membranes and transport Biology Khan Academy.mp3 |
And I want to be clear, this membrane, and I've talked about it many times in many other videos, even though I've drawn it as one line right over here, this is going to be, this is going to be a phospholipid bilayer. So if we were to zoom in, if we were to zoom in, it would look like this. It's a phospholipid bilayer. ... | Exocytosis Membranes and transport Biology Khan Academy.mp3 |
So these are some of the phospholipids that were part of the original, or part of the original membrane. And then we also, in my little box, I get some of the ones that are part of, or that were part of the vesicle that was holding that protein, that were part of the vesicle that was holding the protein. So I really wa... | Exocytosis Membranes and transport Biology Khan Academy.mp3 |
And all of these lines, these membranes that you see, these are all bilayers of phospholipids, just to make sure that we are visualizing this correct. And that's what exocytosis is. And one thing that I find interesting is when you first learn about it, you see a diagram like this, and you just assume, okay, well these... | Exocytosis Membranes and transport Biology Khan Academy.mp3 |
But it actually isn't that chaotic. They actually can sit on tracks. So they actually can sit on tracks. Remember, we talk about the cytoskeleton, which isn't drawn enough, probably because it makes drawings really messy. But whenever we think about a cell, there's all this structure to it. There's all this structure t... | Exocytosis Membranes and transport Biology Khan Academy.mp3 |
Remember, we talk about the cytoskeleton, which isn't drawn enough, probably because it makes drawings really messy. But whenever we think about a cell, there's all this structure to it. There's all this structure to it. Microtubules, microfilaments, intermediate filaments, all of these things over here that not only p... | Exocytosis Membranes and transport Biology Khan Academy.mp3 |
Microtubules, microfilaments, intermediate filaments, all of these things over here that not only provide structure to the cell, but they can be used to transport. And these vesicles, these vesicles can actually ride, can actually ride on these structures. And you could actually have motor protons, motor, not protons, ... | Exocytosis Membranes and transport Biology Khan Academy.mp3 |
So this is kind of a transportation. It's really like a factory to push them towards the membrane so that they can be released. So whenever I think about it, it's fascinating, because I always talk about these cells being a universe unto themselves. And they aren't just these blobs. They have all of these structures. T... | Exocytosis Membranes and transport Biology Khan Academy.mp3 |
And they aren't just these blobs. They have all of these structures. There's all of these proteins that are, really, at an unbelievably small scale, able to do these fairly intricate processes. So what I just showed you, once again, this is classic exocytosis. You'll see it when you have proteins, lipids being produced... | Exocytosis Membranes and transport Biology Khan Academy.mp3 |
So hopefully we can make some headway. So a good place to start, let's just imagine that I have some type of container here. Let's say that's my container. And inside of that container, I have a bunch of water molecules. They're all rubbing against each other. It's in its liquid form. This is liquid water. | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
And inside of that container, I have a bunch of water molecules. They're all rubbing against each other. It's in its liquid form. This is liquid water. And then inside of the water molecules, I have some sugar molecules. Maybe I'll do sugar in this pink color. So I have a bunch of sugar molecules right here. | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
This is liquid water. And then inside of the water molecules, I have some sugar molecules. Maybe I'll do sugar in this pink color. So I have a bunch of sugar molecules right here. I have many, many more water molecules, though. I want to make that clear. I have many, many more water molecules in this container we're de... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
So I have a bunch of sugar molecules right here. I have many, many more water molecules, though. I want to make that clear. I have many, many more water molecules in this container we're dealing with. Now, in this type of situation, we call the thing that there is more of the solvent. So in this case, there's more wate... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
I have many, many more water molecules in this container we're dealing with. Now, in this type of situation, we call the thing that there is more of the solvent. So in this case, there's more water molecules. And you can literally just view more as the number of molecules. I'm not going to go into a whole discussion of... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
And you can literally just view more as the number of molecules. I'm not going to go into a whole discussion of moles and all of that, because you may or may not have been exposed to that yet. But just imagine whatever there's more of, that we're going to call the solvent. So in this case, water is the solvent. And wha... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
So in this case, water is the solvent. And whatever there is less of, so the more water is the solvent, and in this case, that is the sugar, that is considered the solute. So the sugar, it doesn't have to be sugar. It could be any molecule that there's less of than the water in this case. Sugar is the solute. And we sa... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
It could be any molecule that there's less of than the water in this case. Sugar is the solute. And we say that the sugar has been dissolved into the water. And this whole thing right here, the combination of the water and the sugar molecules, we call a solution. We call this whole thing a solution. And a solution has ... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
And this whole thing right here, the combination of the water and the sugar molecules, we call a solution. We call this whole thing a solution. And a solution has a solvent and a solute. The solvent is water. That's the thing doing the dissolving. And the thing that is dissolved is the sugar. That's the solute. | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
The solvent is water. That's the thing doing the dissolving. And the thing that is dissolved is the sugar. That's the solute. Now all of this may or may not be review for you, but I'm doing it for a reason. Because I want to talk about the idea of diffusion. And the idea is actually pretty straightforward. | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
That's the solute. Now all of this may or may not be review for you, but I'm doing it for a reason. Because I want to talk about the idea of diffusion. And the idea is actually pretty straightforward. If I have, let's say the same container. Let me do a slightly different container here, just to talk about diffusion. W... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
And the idea is actually pretty straightforward. If I have, let's say the same container. Let me do a slightly different container here, just to talk about diffusion. We'll go back to water and sugar, especially back to water, but let's say we have a container here. And let's say it just has some air particles in it. I... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
We'll go back to water and sugar, especially back to water, but let's say we have a container here. And let's say it just has some air particles in it. It could be anything, oxygen or carbon dioxide. So let me just draw a couple of air molecules here. So let's say that that is a gaseous, just for the sake of argument, ... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
So let me just draw a couple of air molecules here. So let's say that that is a gaseous, just for the sake of argument, oxygen. So each of this is an O2. Each of those, right? And let's say that this is the current configuration, that all of this is a vacuum here. And that there's some temperature. So these water molec... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
Each of those, right? And let's say that this is the current configuration, that all of this is a vacuum here. And that there's some temperature. So these water molecules, they have some type of kinetic energy, they're moving in some type of random directions right there. So my question is, what is going to happen? Wha... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
So these water molecules, they have some type of kinetic energy, they're moving in some type of random directions right there. So my question is, what is going to happen? What is going to happen in this type of container? Well, any of these guys are going to be randomly bumping into each other. They're more likely to b... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
Well, any of these guys are going to be randomly bumping into each other. They're more likely to bump into things in this down left direction than they are in the upright direction. So if this guy was happening to go in this down left direction, he's going to bump into something and then ricochet into the upright direc... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
But in the upright direction, there's nothing to bounce into. So in general, everything is moving in random directions, but you're more likely to be able to move in the rightward direction. When you go to the left, you're more likely to bump into something. So it's almost common sense. Over time, if you just let this s... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
So it's almost common sense. Over time, if you just let this system come to some type of equilibrium, and I'm not going to go into detail on what that means. You can watch the thermodynamics videos if you'd like to see that. You'll eventually see the container will look something like this. I can't guarantee it. There'... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
You'll eventually see the container will look something like this. I can't guarantee it. There's some probability it would actually stay like this. But very likely that those five particles are going to get relatively spread out. This is diffusion. And so it's really just the spreading of particles or molecules from hi... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
But very likely that those five particles are going to get relatively spread out. This is diffusion. And so it's really just the spreading of particles or molecules from high concentration to low concentration areas. In this case, the molecules are going to spread in that direction, from a high concentration to low con... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
In this case, the molecules are going to spread in that direction, from a high concentration to low concentration area. Now you're saying, Sal, what is concentration? And there's many ways to measure concentration. And you can go into molarity and molality and all of that. But the very simple idea is, how much of that ... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
And you can go into molarity and molality and all of that. But the very simple idea is, how much of that particle do you have per unit space? So here you have a lot of those particles per unit space. And here you have very few of those particles per unit space. So this is a high concentration, and that's a low concentr... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
And here you have very few of those particles per unit space. So this is a high concentration, and that's a low concentration. So you can imagine other experiments like this. You could imagine a solution like, let's do something like this. Let's say I have two containers. Let's go back to the solution situation. So thi... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
You could imagine a solution like, let's do something like this. Let's say I have two containers. Let's go back to the solution situation. So this was a gas, but I started off with that example. So let's stay with that example. Let's say that I have a door right there that's larger than either the water or the sugar mo... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
So this was a gas, but I started off with that example. So let's stay with that example. Let's say that I have a door right there that's larger than either the water or the sugar molecules. On either side, I have a bunch of water molecules on either side, so I have a lot of water molecules. I just had water molecules h... | Diffusion and osmosis Membranes and transport Biology Khan Academy.mp3 |
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