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And then the products of that breakdown flow into the liver or the kidneys. And then sometimes you can even sweat these hormones out. But the idea here is that all of the time, for all of the hormones reaching the receptors, a lot are just swept up and removed from the body. And another way that concentrations of hormo... | Hormone concentration metabolism and negative feedback NCLEX-RN Khan Academy.mp3 |
And another way that concentrations of hormones in the body are controlled are through feedback loops. And the majority of feedback loops are what we consider to be negative feedback loops. And the idea behind negative feedback loops is that conditions resulting from the hormone action suppress further release of those... | Hormone concentration metabolism and negative feedback NCLEX-RN Khan Academy.mp3 |
And that can be a pretty confusing idea. So I'm going to draw out an example. So we have the hypothalamus here. I'm going to draw it in. And I'll write it down. And the hypothalamus releases a hormone, thyroid-releasing hormone, so TRH. And it releases it, and it goes down to the pituitary gland, which I'll draw in, an... | Hormone concentration metabolism and negative feedback NCLEX-RN Khan Academy.mp3 |
I'm going to draw it in. And I'll write it down. And the hypothalamus releases a hormone, thyroid-releasing hormone, so TRH. And it releases it, and it goes down to the pituitary gland, which I'll draw in, and right here. And in response to TRH, the pituitary gland releases thyroid-stimulating hormone, or TSH. And TSH ... | Hormone concentration metabolism and negative feedback NCLEX-RN Khan Academy.mp3 |
And it releases it, and it goes down to the pituitary gland, which I'll draw in, and right here. And in response to TRH, the pituitary gland releases thyroid-stimulating hormone, or TSH. And TSH goes down to the thyroid glands, which would be about right here. And the thyroid gland releases its hormones, T3, or triiodo... | Hormone concentration metabolism and negative feedback NCLEX-RN Khan Academy.mp3 |
And the thyroid gland releases its hormones, T3, or triiodothyronine, and thyroxine. And these thyroid hormones travel all throughout the body in search of the receptors in order to, let's say, upregulate metabolism. That's one of the major jobs of the thyroid glands. And so here's where the idea becomes pretty cool, b... | Hormone concentration metabolism and negative feedback NCLEX-RN Khan Academy.mp3 |
And so here's where the idea becomes pretty cool, because some of the receptors are located on the pituitary gland and the hypothalamus. And as the thyroid hormones reach the pituitary and the hypothalamus, they signal the hypothalamus and pituitary gland to stop making their hormones. And the hypothalamus and pituitar... | Hormone concentration metabolism and negative feedback NCLEX-RN Khan Academy.mp3 |
And that's the idea of evolution. And whenever we hear this word, I mean, even if we don't hear it in the biological context, we imagine that something is changing. It is evolving. And so when people use the word evolution in our everyday context, they think of this notion of change, that this is going to test my drawi... | Introduction to Evolution and Natural Selection (2).mp3 |
And so when people use the word evolution in our everyday context, they think of this notion of change, that this is going to test my drawing ability. But you see an ape, bunt over. We've all seen this picture at the Natural Museum, and he's walking hunchback like that, and his head's bent down. And I'm doing my best. ... | Introduction to Evolution and Natural Selection (2).mp3 |
And I'm doing my best. That's the ape. Maybe he's also wearing a hat. And then they show this picture where he slowly, slowly becomes more and more upright. And eventually, he turns into some dude who's just walking on his way to work, also just as happy. And now he's walking completely upright. And it's some kind of i... | Introduction to Evolution and Natural Selection (2).mp3 |
And then they show this picture where he slowly, slowly becomes more and more upright. And eventually, he turns into some dude who's just walking on his way to work, also just as happy. And now he's walking completely upright. And it's some kind of implication that walking upright is better than not walking upright, et... | Introduction to Evolution and Natural Selection (2).mp3 |
And it's some kind of implication that walking upright is better than not walking upright, et cetera, et cetera. Oh, he doesn't have a tail anymore. Let me eliminate that. This guy does have a tail. Let me do it in an appropriate width. This guy has a tail, so you're going to have to excuse my drawing skills. But we've... | Introduction to Evolution and Natural Selection (2).mp3 |
This guy does have a tail. Let me do it in an appropriate width. This guy has a tail, so you're going to have to excuse my drawing skills. But we've all seen this. If you've ever gone to a natural history museum, they'll just make more and more upright apes. And eventually, you get to a human being. And it's this idea ... | Introduction to Evolution and Natural Selection (2).mp3 |
But we've all seen this. If you've ever gone to a natural history museum, they'll just make more and more upright apes. And eventually, you get to a human being. And it's this idea that the apes somehow changed into a human being. And I've seen this in multiple contexts, even inside of biology classes and even the scie... | Introduction to Evolution and Natural Selection (2).mp3 |
And it's this idea that the apes somehow changed into a human being. And I've seen this in multiple contexts, even inside of biology classes and even the scientific community. They'll say, oh, the ape evolved into the human, or the ape evolved into the pre-human, the guy that almost stood upright. The guy that was a li... | Introduction to Evolution and Natural Selection (2).mp3 |
The guy that was a little bit hunched back, so he looked a little bit like an ape and a little bit like a human, and so on and so forth. And I want to be very clear here. Even though this process did happen, that you did have creatures that over time accumulated changes that maybe their ancestors might have looked more... | Introduction to Evolution and Natural Selection (2).mp3 |
It's not like the ape said, gee, I would like my kids to look more like this dude, so somehow I'm going to get my DNA to get enough changes to look more like this. And it's not like the DNA knew. The DNA didn't say, hey, it is better to be walking than to be kind of hunched back like an ape, and so therefore, I am goin... | Introduction to Evolution and Natural Selection (2).mp3 |
That's not what evolution is. It's not like some people imagine that maybe there's a tree, and on that tree there's a bunch of good fruit at the top of the tree. Maybe they're apples. And then maybe you have some type of cow-like creature, or maybe it's some type of horse-like creature that says, gee, I would like to g... | Introduction to Evolution and Natural Selection (2).mp3 |
And then maybe you have some type of cow-like creature, or maybe it's some type of horse-like creature that says, gee, I would like to get to those apples. And that just because they want to get there, maybe the next generation, they keep trying to raise their neck. And then after generation after generation, their nec... | Introduction to Evolution and Natural Selection (2).mp3 |
That is not what evolution is, and that's not what it implies. Although sometimes the everyday notion of the word seems to make us think that way. What evolution is, and actually this is the word that I prefer to use, it's natural selection. Natural selection, let me write that word down. And literally what it means is... | Introduction to Evolution and Natural Selection (2).mp3 |
Natural selection, let me write that word down. And literally what it means is that in any population of living organisms, you're going to have some variation. And this is an important key word here. Variation just means, look, there's just some change. If you look at the kids in your school, you'll see variation. Some... | Introduction to Evolution and Natural Selection (2).mp3 |
Variation just means, look, there's just some change. If you look at the kids in your school, you'll see variation. Some people are tall, some people are short, some people have blonde hair, some people have black hair, so on and so forth. There's always variation. And what natural selection is, is this process that so... | Introduction to Evolution and Natural Selection (2).mp3 |
There's always variation. And what natural selection is, is this process that sometimes environmental factors will select for certain variations. Some variations might not matter at all, but some variations matter a lot. One example that's given in every biology book, but it really is interesting. I believe they're cal... | Introduction to Evolution and Natural Selection (2).mp3 |
One example that's given in every biology book, but it really is interesting. I believe they're called the peppered moth, and this was pre-industrial revolution England. Most peppered moths, there was just this variation. Some of them were, I guess we could call them more peppered than others. So some of them might loo... | Introduction to Evolution and Natural Selection (2).mp3 |
Some of them were, I guess we could call them more peppered than others. So some of them might look like this. So it had spots like that. Some of them might have looked more like that. And of course, they had some black spots on them. And then some of them might have been, you know, just barely have any spots. You just... | Introduction to Evolution and Natural Selection (2).mp3 |
Some of them might have looked more like that. And of course, they had some black spots on them. And then some of them might have been, you know, just barely have any spots. You just have this natural variation. Like you'd see in any population of animals, you'll see some variation in colors. Now, they were all happy p... | Introduction to Evolution and Natural Selection (2).mp3 |
You just have this natural variation. Like you'd see in any population of animals, you'll see some variation in colors. Now, they were all happy probably for thousands of years, just this natural variation. It was a non-important trait for these peppered moths. But then all of a sudden, the Industrial Revolution happen... | Introduction to Evolution and Natural Selection (2).mp3 |
It was a non-important trait for these peppered moths. But then all of a sudden, the Industrial Revolution happens in England, and all this soot gets released from all of these factories that are running these steam engines powered by coal. And so all of a sudden, a lot of the things that once were gray or white, for e... | Introduction to Evolution and Natural Selection (2).mp3 |
Let me draw some tree trunks. Maybe there were some tree trunks that used to look like this. Maybe some tree trunks used to look something like this. And a peppered moth would be pretty OK. And maybe there were some tree trunks that were pretty dark. But all of a sudden, the Industrial Revolution happens. Everything ge... | Introduction to Evolution and Natural Selection (2).mp3 |
And a peppered moth would be pretty OK. And maybe there were some tree trunks that were pretty dark. But all of a sudden, the Industrial Revolution happens. Everything gets covered with soot from the coal being burned. And then all of a sudden, all the trees look like this. They're just completely pitch black, or they'... | Introduction to Evolution and Natural Selection (2).mp3 |
And then all of a sudden, all the trees look like this. They're just completely pitch black, or they're a lot darker than they were before. Now, all of a sudden, you've had a major change to these moths' environment. And you have to think, what is going to select for these moths? Well, one thing that might get these mo... | Introduction to Evolution and Natural Selection (2).mp3 |
And you have to think, what is going to select for these moths? Well, one thing that might get these moths are birds and the ability of the birds to see the moths. So all of a sudden, if the environment became a lot blacker than it was before, you can guess what's going to happen. The birds are going to see this dude a... | Introduction to Evolution and Natural Selection (2).mp3 |
The birds are going to see this dude a lot easier than they're going to see this dude. Because this dude on a black background, he's going to be a lot harder to see. And it's not like the birds won't catch this guy. They'll catch all of them. But they're going to catch this guy a lot more frequently. So you can imagine... | Introduction to Evolution and Natural Selection (2).mp3 |
They'll catch all of them. But they're going to catch this guy a lot more frequently. So you can imagine what happens. If the birds start catching these guys before they can reproduce, or maybe while they're reproducing, what's going to happen? This guy, the darker dudes, are going to reproduce a lot more often. And al... | Introduction to Evolution and Natural Selection (2).mp3 |
If the birds start catching these guys before they can reproduce, or maybe while they're reproducing, what's going to happen? This guy, the darker dudes, are going to reproduce a lot more often. And all of a sudden, you're going to have a lot more moths that look like this. You're going to have a lot more of these dude... | Introduction to Evolution and Natural Selection (2).mp3 |
You're going to have a lot more of these dudes. So what happened here? Was there any design, or was there any active change by any of the moths? I mean, it looks like a really smart thing to do, to become black, right? Your surrounding became black, and you wait a couple of generations of these moths, and now all of a ... | Introduction to Evolution and Natural Selection (2).mp3 |
I mean, it looks like a really smart thing to do, to become black, right? Your surrounding became black, and you wait a couple of generations of these moths, and now all of a sudden, the moths are black. And you might say, wow, those moths are geniuses. They all somehow decided to evolve into black moths in order to hi... | Introduction to Evolution and Natural Selection (2).mp3 |
They all somehow decided to evolve into black moths in order to hide from the birds more easily. But that's not what happened. You had a lot of variation in your peppered moth population. And what happened was that when everything turned darker and darker, these dudes right here, or dudettes, had a lot less success in ... | Introduction to Evolution and Natural Selection (2).mp3 |
And what happened was that when everything turned darker and darker, these dudes right here, or dudettes, had a lot less success in reproducing. These guys just reproduced more and more and more. And these guys got eaten up before they were able to do, so maybe while they were reproducing, so that they couldn't produce... | Introduction to Evolution and Natural Selection (2).mp3 |
And then this trait just became dominant. And then the peppered moth just became, you can kind of view it as a black moth. Now you might say, OK, Sal, that's one example. I need more. This is natural selection. It's purported to apply to everything. It purports to explain why we evolved from basic bacteria, or maybe ev... | Introduction to Evolution and Natural Selection (2).mp3 |
I need more. This is natural selection. It's purported to apply to everything. It purports to explain why we evolved from basic bacteria, or maybe even self-replicating RNA, which I will talk about more in the future. I need more evidence of this. I need to see it in real time. And the best example of this is really th... | Introduction to Evolution and Natural Selection (2).mp3 |
It purports to explain why we evolved from basic bacteria, or maybe even self-replicating RNA, which I will talk about more in the future. I need more evidence of this. I need to see it in real time. And the best example of this is really the flu. And I'll do other videos in the future on what viruses are and how they ... | Introduction to Evolution and Natural Selection (2).mp3 |
And the best example of this is really the flu. And I'll do other videos in the future on what viruses are and how they replicate. And viruses are actually fascinating, because it's not even clear that they're alive. They're literally just little buckets of DNA and sometimes RNA, which we'll learn is genetic informatio... | Introduction to Evolution and Natural Selection (2).mp3 |
They're literally just little buckets of DNA and sometimes RNA, which we'll learn is genetic information. And they're just contained in these viral, these little protein containers that are these neat geometrical shapes. And that's all they are. They really don't have, you know, they're not like regular living organism... | Introduction to Evolution and Natural Selection (2).mp3 |
They really don't have, you know, they're not like regular living organisms that actively move and that actively have metabolisms and all that. What they do is they take that little DNA and they inject it into other things that can process it. And then they use that DNA to produce more viruses. But anyway, we could do ... | Introduction to Evolution and Natural Selection (2).mp3 |
But anyway, we could do a whole series of videos on viruses. But the flu is a virus. And what happens every year is you have a certain type of virus and they have some variation. And I'll just make the variation by, I don't know, how many dots they have. And they infect, let's say it's a human flu, they infect humans, ... | Introduction to Evolution and Natural Selection (2).mp3 |
And I'll just make the variation by, I don't know, how many dots they have. And they infect, let's say it's a human flu, they infect humans, and slowly our immune systems, which we can make a whole set of videos on as well, start to recognize the virus and are able to attack them before they can do a lot of damage. So ... | Introduction to Evolution and Natural Selection (2).mp3 |
Let me do all of them. They all have these little two dots and that's how, and we'll talk in the future what these dots are and how they can be recognized, but let's say that's how our immune system recognize them. They start realizing, oh, any time I get this little green dude with two dots on it, that's not a good th... | Introduction to Evolution and Natural Selection (2).mp3 |
And so you have a very strong natural selection once immune systems learn what this virus is, and we'll talk more about what learning means for an immune system, that they'll start attacking these guys. But flu, you can kind of think of them as being tricky, but they're not really tricky. They're not sentient objects, ... | Introduction to Evolution and Natural Selection (2).mp3 |
So what you have is, in any flu population, you're always having a little bit of change. So maybe the great majority of them have those two dots, but maybe every now and then one of them has one dot, one of them has three dots, and maybe that's just a random mutation. This just randomly happened. Maybe one in every mil... | Introduction to Evolution and Natural Selection (2).mp3 |
Maybe one in every million of these viruses have this only one dot instead of two dots. But what's going to happen as soon as, let's say, the human immune system gets used to attacking the virus with the two red dots? Well, then this guy isn't going to have to compete with the other virus capsules for infecting people.... | Introduction to Evolution and Natural Selection (2).mp3 |
He's going to have people's DNA all to himself. And so he or she or whatever you want to call this virus is then going to be more successful. So by next year's flu season, when people start sneezing and are able to spread it on doorknobs and whatever else again, this guy is going to be the new flu virus. So when you se... | Introduction to Evolution and Natural Selection (2).mp3 |
So when you see this process of every year there's a new flu virus, that is evolution and natural selection in real time. It is happening. It isn't this thing that only happens over eons and eons of time, although most of the substantial things that we see in our lives or even ourselves are based on these things that h... | Introduction to Evolution and Natural Selection (2).mp3 |
But it happens on a yearly basis. Another example is if you think about antibiotics and bacteria. Bacteria are these little cells that move around. And we'll talk more about them. They actually are definitely living. They have metabolisms and whatever else. And this is just a nice note. | Introduction to Evolution and Natural Selection (2).mp3 |
And we'll talk more about them. They actually are definitely living. They have metabolisms and whatever else. And this is just a nice note. When people talk about infections, it could either be a viral infection, which are these things that go and infect your DNA and then use your cell mechanisms to reproduce. Or it co... | Introduction to Evolution and Natural Selection (2).mp3 |
And this is just a nice note. When people talk about infections, it could either be a viral infection, which are these things that go and infect your DNA and then use your cell mechanisms to reproduce. Or it could be a bacterial infection, which are literally little cells that move around and they release toxins that m... | Introduction to Evolution and Natural Selection (2).mp3 |
So bacteria, these are what antibiotics kill. They attack bacteria. They kill them. Now, you probably, if you know a couple of doctors or whatever, and you say, hey, I'm sick. I think I have a bacterial infection. Give me some antibiotics. A responsible doctor says, no, I won't give you antibiotics just willy-nilly, be... | Introduction to Evolution and Natural Selection (2).mp3 |
Now, you probably, if you know a couple of doctors or whatever, and you say, hey, I'm sick. I think I have a bacterial infection. Give me some antibiotics. A responsible doctor says, no, I won't give you antibiotics just willy-nilly, because what happens is the more antibiotics you use, you're more likely to create ver... | Introduction to Evolution and Natural Selection (2).mp3 |
A responsible doctor says, no, I won't give you antibiotics just willy-nilly, because what happens is the more antibiotics you use, you're more likely to create versions and I want to be very careful about the word create, because you're not actively creating them. But let's say, and let me finish my sentence, you're v... | Introduction to Evolution and Natural Selection (2).mp3 |
So let's say that these are all bacteria and you have gazillions of them, right? And every now and then you get one that's slightly different. Now, in a population of bacteria, these all will make you equally sick, and this is just some random difference in the bacteria, maybe on its DNA some slight different changes h... | Introduction to Evolution and Natural Selection (2).mp3 |
These all are the kind of bacteria, you don't want to get a lot of them in your system. Your immune system can attack them and fight them off, but if you get a lot of them, they might kill you or make you sick or whatever else. Now, if everyone just starts using antibiotics when they're not sick or when they don't real... | Introduction to Evolution and Natural Selection (2).mp3 |
But what happens if you all of a sudden kill a lot of the green bacteria? Well, now the blue bacteria have the whole ecosystem that before it was competing with all these green dudes to get all the good stuff inside of your body, but now he's all alone, and now he can replicate willy-nilly. So now he's going to replica... | Introduction to Evolution and Natural Selection (2).mp3 |
And this is, once again, it wasn't like there was any design, there was any intelligent process here that said, look, some bacteria said, I'm going to be a little bit smarter and design myself to resist this antibiotic threat. No. There's just these random changes that happen, and mutations and viruses and bacteria hap... | Introduction to Evolution and Natural Selection (2).mp3 |
And there are these random changes that happen. And this might be a one in a billion change. But all of a sudden, if you start killing off all of the people that it's competing with, this guy can start replicating really fast and then become the dominant bacteria. And then all of a sudden, that antibiotic that you had ... | Introduction to Evolution and Natural Selection (2).mp3 |
And then all of a sudden, that antibiotic that you had developed very carefully to destroy the green dudes is useless. And you have this superbug. You might have heard the word superbug. That's what a superbug is. It's not like it designed itself somehow. It's just that we got very good at killing its competition, and ... | Introduction to Evolution and Natural Selection (2).mp3 |
That's what a superbug is. It's not like it designed itself somehow. It's just that we got very good at killing its competition, and so we allowed it to take over. And we can't kill it because all of the drugs were just good at killing its competition. That these bacteria just keep mutating and keep mutating. And if we... | Introduction to Evolution and Natural Selection (2).mp3 |
And we can't kill it because all of the drugs were just good at killing its competition. That these bacteria just keep mutating and keep mutating. And if we use these antibiotics a little bit too heavily, we'll always be selecting for the things that won't be affected by the antibiotics. Well, anyway, I think I've spok... | Introduction to Evolution and Natural Selection (2).mp3 |
Well, anyway, I think I've spoken long enough. But this is a fascinating, fascinating topic. And I really wanted to make this my very first video or lecture, if you will, on biology. Because if you really went to, you know, biology is the study of life, and we can talk about what life is, whether viruses are living, wh... | Introduction to Evolution and Natural Selection (2).mp3 |
Because if you really went to, you know, biology is the study of life, and we can talk about what life is, whether viruses are living, whatnot. But if you really want to study living systems, you really can't make any assumptions other than natural selection. We could go to another planet where the creatures don't have... | Introduction to Evolution and Natural Selection (2).mp3 |
Or maybe they have some other type of hereditary information stored in their cells. Or they replicate some other way. Or they're not even carbon-based. Maybe they're silicon-based. And if we went to that type of a planet in order to study the biology on that planet, everything else we know about biology, about viruses ... | Introduction to Evolution and Natural Selection (2).mp3 |
Maybe they're silicon-based. And if we went to that type of a planet in order to study the biology on that planet, everything else we know about biology, about viruses and DNA, would be useless. But if we do understand this one concept of natural selection, that your environment will select. And there's no active proce... | Introduction to Evolution and Natural Selection (2).mp3 |
In this video, we are going to talk about the evolution of photosynthesis on Earth, because that's the only place that, at least so far, we're aware of photosynthesis occurring. I personally believe that it's occurring in many places in the universe, but we don't really know just yet. But first, a reminder of what phot... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
It's the process where organisms are able to take carbon dioxide in the atmosphere, in the presence of water, and then use the sun's energy, so sunlight, to then produce sugars, and I'll C6H12O6, and free oxygen, so O2. Now, the way I've written this chemical equation right over here, it hasn't balanced. So, see, I hav... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
But now, let's see, I have 12 hydrogens here. I only have two hydrogens here, so let me multiply this by six as well. And let's see, on the left-hand side, I have two oxygens times six is 12, plus another six oxygens, 18 oxygens. So I need 18 oxygens over here. I already have six right over here, so I'll need another 1... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
So I need 18 oxygens over here. I already have six right over here, so I'll need another 12 right over here, so I'll put a six out front, so I have balanced that. And it's important for you to realize that this is a really big deal. You would not exist without photosynthesis. And I know what you're thinking. You don't ... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
You would not exist without photosynthesis. And I know what you're thinking. You don't photosynthesize things. But the things that you eat, or the things that you eat, the things that they eat, they do some sort of photosynthesis. At the end of the day, our energy is coming from the sun. Even if you're eating an animal... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
But the things that you eat, or the things that you eat, the things that they eat, they do some sort of photosynthesis. At the end of the day, our energy is coming from the sun. Even if you're eating an animal, that animal might be eating another animal that eats a plant, and that plant is using the sun's energy and it... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
But an interesting question is, where does this come from? And a straightforward answer is, we don't have all the answers, but scientists have a reasonably good idea of where it probably came from. Today, we can observe cyanobacteria. This is what cyanobacteria looks like. It is, like all bacteria, prokaryotic, and it ... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
This is what cyanobacteria looks like. It is, like all bacteria, prokaryotic, and it is able to conduct photosynthesis. And scientists believe that organisms not too different from cyanobacteria, probably an ancestor of cyanobacteria, existed on Earth 2 1⁄2, 3 billion, maybe even older, maybe even further back in time,... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
And that's early in Earth's history. Earth has only been around for about 4 1⁄2 billion years. And that ancestor of cyanobacteria, like cyanobacteria, was able to take carbon dioxide in the atmosphere in the presence of water and produce oxygen. And even though this is bacteria and each of these organisms are very smal... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
And even though this is bacteria and each of these organisms are very small, in aggregate, they can have a pretty significant impact. For example, this is a cyanobacteria plume near Fiji, and you can see that these are pretty significant things that can be a significant contributor to oxygen in the atmosphere. And scie... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
So what you see in this chart right over here is our best sense of what the oxygen content in the atmosphere was if we go back in time. And we can start to figure this out based by looking at rock samples that are very, very old, by looking at the fossil record, very, very old. So just so you understand what's going on... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
This is one billion years ago, two billion years ago, three billion years ago, 3.8 billion years ago. So it covers most of Earth's history. If you just wanna put things in context, humans, modern humans, have only been around for about two or 300,000 years, so we wouldn't even show up as a pixel on this diagram right o... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
So we're going back into deep time. And scientists believe that the first photosynthetic organisms might have evolved approaching three billion years ago, although we're not exactly sure. And those organisms might have been producing oxygen from the photosynthesis, but it might have been absorbed by things like the oce... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
But eventually, those organisms, probably these ancestors of cyanobacteria, became significant enough that the oxygen started pouring in the atmosphere. And we start to see that right over here, where before this point, the atmospheric oxygen was pretty close to 0%. And then right over here, at the great oxygenation ev... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
And these two lines represent two different estimates of what percentage of the atmosphere oxygen was at these various times. And it grew all the way to modern times, where oxygen is roughly 20 or 21% of our atmosphere. And the reason why this is sometimes called an oxygen catastrophe is that there were a lot of anaero... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
But it was not a catastrophe for what eventually would be us, because we are dependent not just on the sugars from photosynthesis, but we're also dependent on the oxygen from photosynthesis. We use this oxygen to conduct respiration. You can almost view respiration as a backwards process. You take your sugars, and in t... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
You take your sugars, and in the presence of oxygen, you are able to extract that energy from those sugars so that we can live. Now, an interesting question is, at what point did we go from these prokaryotic bacteria organisms to eukaryotic organisms that are able to perform photosynthesis, most notably plants? Well, t... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
We have a whole other video on that, but it's this idea that the ancestors of the cyanobacteria might have lived in symbiosis with another eukaryotic cell where the cyanobacteria ancestor was able to harness light energy to produce sugars for the larger cell, and in return, the larger cell was able to give protection o... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
And good evidence that these might have ancestors that used to live independently is that they have DNA that is very similar to the DNA of cyanobacteria. They have ribosomes, their own ribosomes, that are very similar to the ribosomes of cyanobacteria. And so like mitochondria, we believe chloroplasts originated as ind... | Photosynthesis evolution Cellular energetics AP Biology Khan Academy.mp3 |
And at its essence, we can call this metabolism. Metabolism. And this is the taking energy in different forms, breaking it down into its more fundamental components, and then building it up in ways that we would find useful, useful for energy, useful for structure, so that we can actually live our lives, we can grow, w... | Introduction to metabolism anabolism and catabolism Khan Academy.mp3 |
So as I just said, metabolism, and we're gonna go into a bunch of examples of this, metabolism at its heart is really two different processes. There's the breaking down of substances for energy or for structure, to get back to the building blocks, and we call that catabolism. Catabolism. So this is the breaking down of... | Introduction to metabolism anabolism and catabolism Khan Academy.mp3 |
So this is the breaking down of things. And then once we've broken down things, we're ready to rebuild them in ways that we would find useful. And we call this anabolism. Anabolism. Or anabolism. Anabolism, just like that. And one way to think about it is, imagine that someone had built something with Legos, and you wa... | Introduction to metabolism anabolism and catabolism Khan Academy.mp3 |
Anabolism. Or anabolism. Anabolism, just like that. And one way to think about it is, imagine that someone had built something with Legos, and you wanna build something with Legos. Well, you could go to those Legos, and you'd wanna break it down, but not break it down too much, you wouldn't melt the plastic, you would ... | Introduction to metabolism anabolism and catabolism Khan Academy.mp3 |
And one way to think about it is, imagine that someone had built something with Legos, and you wanna build something with Legos. Well, you could go to those Legos, and you'd wanna break it down, but not break it down too much, you wouldn't melt the plastic, you would break it down into the individual Lego pieces, and t... | Introduction to metabolism anabolism and catabolism Khan Academy.mp3 |
So let's just think about how all of this gets started. And what's exciting is that all of this got started, or gets started, from stars, from fusion reactions in stars. And this right over here is a picture of a star, a star that we are very familiar with, this is the sun. But you may or may not realize that the sun i... | Introduction to metabolism anabolism and catabolism Khan Academy.mp3 |
But you may or may not realize that the sun is only one of probably several stars that have been involved in life as we know it. The sun is our most direct source of energy for most of life as we know it, there are some bacteria and things that are able to live off of vents at the bottom of the ocean because of the hea... | Introduction to metabolism anabolism and catabolism Khan Academy.mp3 |
But when I say that other stars might have been involved, including dead stars that existed billions of years ago, it's because the heavier elements that we're composed of, or that are around us in the environment, the carbon, the oxygen, we could just keep going, pretty much everything other than hydrogen, it was cons... | Introduction to metabolism anabolism and catabolism Khan Academy.mp3 |
And you have radiation, you have energy from the sun, and once again, that energy is coming from fusion reactions. And it's fusing lighter elements into heavier elements. So the sun is also constructing more heavy elements. But that energy, that energy makes its way to the Earth. And you have organisms, like plants, th... | Introduction to metabolism anabolism and catabolism Khan Academy.mp3 |
But that energy, that energy makes its way to the Earth. And you have organisms, like plants, that are able to use that energy to construct the material, the food, we could say, that is eventually going to get around to us. And so this process, you may or may not be familiar with it, this is photosynthesis. And we're g... | Introduction to metabolism anabolism and catabolism Khan Academy.mp3 |
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