[Music] Hey everybody, it's your AP Biology teacher, Mr. Poser. Today we're getting into topic 1.1 of the updated 2025 2026 AP Biology curriculum. And this is going to be on the structure of water and hydrogen bonding. So this is going to be updated in comparison to my last series that I made 5 years ago on the topic videos because uh well college board updated the AP biology curriculum, but we are starting with the same unit. This is unit one, chemistry of life. Um, and we're going to get into carbohydrates, proteins, lipids, nucleic acids. What are they made of? Uh, what do they do? What's their structure? Um, how are they built? How are they taken apart? But we got to start with actually none of those molecules. U, we got to start with water. Why? Because it's mega mega important. And in fact, it's about 70% of our body mass. Um, so it's really really important. And today we're going to talk about the structure of water and what makes it so important and unique among fluids um in the universe. And water is really really cool for a couple reasons. And it has to do u with its molecular shape. It is made of two hydrogen atoms and one oxygen atom which is why we call it H2O. Okay. So if you took a chemistry class before taking AP biology, you're going to apply some of your chemistry knowledge here um in this unit. Well, that makes sense. It's the chemistry of life, right? But uh here comes our first dose here. Um in a water molecule, right, you got two hydrogens's and one oxygen. These are what are called polar covealent bonds um that are binding the hydrogens's to the oxygen. It's an uneven sharing of electrons. And what this results in is that uh well water is what's called a polar molecule. Okay? And what polar means is that it has two opposite ends to it. written by opposite ends. I mean that the side of the molecule with the hydrogen's on it has a slight positive charge and the side of the molecule with the uh oxygen on it has a slight negative charge. Okay? And that's what makes it polar. Um polar opposites it means two different sides, right? Um so you can think of each water molecule which are tiny by the way. There's billions of them even just in one little drop of water. Um, you can kind of think of a water molecule as like a magnet, right? A magnet like this bar magnet over here has a south side and a north side. If you put the north ends of the magnet together, well, they pull away from each other. If you put the two south ends of the magnet together, they pull away from each other. But if you put the north end of one magnet and the south end of the other magnet together, they attract. Okay? Just like the positive and negative sides of um a water molecule are also going to attract. Okay? So once again, you have to conceptualize a little bit here that you have billions of billions of these water molecules in a confined space and they're all like little magnets. The north sides are going to pull apart and this the north and south are going to attract together. The positive negative attract positive positive repel, negative negative repel, right? Um so we have to keep that in mind. Now here's the gist of it. Because water is polar. All right? Because they have these opposite charges. I'm just going to label them one more time over here. Okay? because they have these opposite charges, they form what are called hydrogen bonds. And this is the difference maker when it comes to water and other polar substances on these hydrogen bonds where the positive side of one molecule attracts the negative side of another molecule. This is what they they attract to each other. And when you have a whole lot of water molecules like this, you're going to get a whole lot of pulling and pushing um between that those positive and negative sides. Okay? And especially this attraction right here, this positive and negative attraction here between the hydrogen and the oxygen, between water molecules, makes water very, very sticky. Okay? So, I'm just circling that part. That is what is going to lead us to all the other stuff we're going to talk about in this video. Hydrogen bonds, which are what these are called, uh, make water molecules hard to pull apart. Now, hydrogen bonds aren't bonds in the sense of like ionic bonds or coalent bonds. They are not bonds that are going to hold a molecule together. They are more like intermolecular forces. They are going to be bringing two different water molecules closer together and making them hard to pull apart from one another. Okay. Because of those opposite um opposites attract. Okay. Um so hydrogen bonds are the reason why water exhibits all the other properties that we're going to talk about. The first of which is what is called a high specific heat capacity. So as I put at the top here, it says because water molecules take a lot of force to pull apart, water has a high specific heat capacity. In other words, it takes a whole lot of energy to try and raise the temperature of water to heat up water. Okay? So, think about this. Uh here's another chemistry application, right? Um temperature is just a measure of the kinetic energy um that mass has or that matter has, right? And water, okay, since it's a fluid, these uh these water molecules are going to be constantly moving around, bouncing around, okay? Um, and for something to go to have a higher temperature, those water molecules or whatever substance it is has to be moving faster. Okay? But here's the thing. Uh, what's going to keep those water molecules from moving faster is their hydrogen bonds, their attractions between those water molecules. So, hence, it's going to take a lot more energy to try and split those water molecules up from each other because they're attracted to each other. They're trying to stick together. Um, because they are polar, they have those positive and negative ends together. All right. Um, and why is this important? Well, it makes temperature regulation really, really nice for living things. Um, it takes a lot of heat to try and bring uh our body temperatures up when it comes to a cell or an animal or a plant, whatever. Something that's made up of a lot of water is going to well, it's going to take a lot of heat for that to uh um to raise the temperature. All right. So, my bearded dragon over here um bearded dragons are ectotherms. We're going to learn about that in unit 8. Um they cannot regulate their body temperature um internally. So they have to use external factors, right? So this lizard is going to bask underneath a lamp or out in the sun for a long period of time and has to be out there for a long period of time because it has to absorb enough heat to raise its body temperature back up and it's going to take a lot of heat because well that thing is mostly made of water and water has a high specific heat capacity. Um so either way, all right, um temperature regulation is a lot easier when they don't fluctuate very much and it takes a lot of energy to raise the temperature. Speaking of which, water also has a high heat of vaporization. So, this is kind of a extension on its high specific heat. It takes a lot of energy to try to turn it into a gas. 100° C to take water from a uh liquid to a gas. Okay? So, when you sweat, you're hot, right? You're you're automatically you don't have to think about it. You are automatically releasing water from uh sweat glands from through pores in your skin. Okay? And how you're going to cool down. Water is really really good for that because it has a high specific heat capacity, right? Um, in order for water to evaporate off your skin, it needs to absorb enough heat for it to be turned into a gas for those water molecules to get moved apart from one another essentially. Um, and as a result, okay, as that water absorbs heat from the environment, um, it actually takes heat away from the surface on which the water is. And that's what's called evaporative cooling. And that's how sweating keeps you cooled down. Um, which is really, really important for temperature regulation. Our enzymes would not be able to work if our temp body temperatures were too high. We're going to talk about that in unit 3. But water is really, really good for temperature regulation. Um, and that's also why dogs go because they are trying to get some water to evaporate so that they can cool themselves down. Okay? They don't have sweat glands like we do. All right? So water um and it high specific heat capacity makes it really really good for temperature regulation. Okay, hydrogen bonds bonds bonds allow for other sticky properties as well. Uh namely those are cohesion, adhesion and surface tension. Okay, so uh this is pretty well illustrated. Cohesion and adhesion are pretty well illustrated by this picture. Uh but I'm going to come back to these and uh explain them one at a time. All right, cohesion. Okay, co means together. If you've ever heard of somebody or a team being cohesive, it means they stick together very well. They're they're they function as a unit, right? Water is cohesive and exhibits cohesion because water molecules tend to stick to one another, okay? Because of those hydrogen bonds, right? There's positive and negative sides on each of those molecules and they're going to be pulling together quite a bit. Um, so water sticks to itself very nicely. Okay? So, uh, if you've ever done this experiment before where you try to see how many drops of water that you can put on a penny, um, you can get upwards of like 50 or 60 drops of distilled water on a penny, um, because the hydrogen bonds that are present, um, in those in that water, okay, is making sure that the water sticks together and you can, uh, it forms what's called surface tension here. All right? Or if you view water on like a a window pane or like a blade of grass or some other thing like this spiderweb over here, it forms these droplets and those droplets tend to kind of clump together because once again water's cohesive properties. Water sticks to itself. It's attracted basically to itself um very well. So this picture like I said I was going to bring up this is cohesion here. Cohesion. Here's a water molecule. Here's a water molecule. They are sticking together very well. You can see them joined here in the middle. Um, and although I would change one thing about this, right? Uh, this is the negative side and this is the positive side. This is the negative side. This is the positive side. The negative sides don't attract to each other. They would actually repel from each other. So, if it were more accurate, the positive side would be attracted to the negative side. And therefore, that would form the hydrogen bond that would stick those water molecules together. But I digress. Okay. Cohesion. They're sticking together. The other Whoops. The other uh main property that has to do with this is called adhesion. Water molecule not only sticks to itself but to other polar surfaces as well. Okay. So if something else is polar, water is going to stick to it very very well. All right. Here's another experiment that you could try at home or maybe you did in another previous science class or something like that. All right. If you put the stock of celery in a glass of water with some food coloring in it. All right. You can watch as the water travels up the stock of celery. Um, and why does it why does it why does it do that? Well, um, it wouldn't be possible without water's unique properties, right? So, a tree is able to bring up water all the way from the fruits to the entire tree to the tops of the leaves, whatever. Um, and how is that possible? Well, that's uh that answer is a little bit complicated, but it would not be possible without water's unique properties, right? As the tree draws in water, um the water tends to stick to itself. So, so any water it draws up, it's going to pull up more water with it. And here's the second one. It's going to stick to the polar walls of the tubes that carry up that water. Okay? This is not on in the exam, I'm pretty sure. Okay? But those tubes are called xyllem. So, the xyllem, the inner walls of those tubes that carry up water in vascular plants like trees or celery or grass, for example, um those tubes have polar surfaces so that water can stick to them. Uh this is called capillary action where it brings up water um and fights it against gravity because of its unique properties. So here's adhesion. Here's the water molecule sticking to that polar surface um and allowing it to adhere to other substances or polar substances or surfaces. Um adhesive, if you've ever heard that before, like tape is an adhesive, glue is an adhesive. Um same root word there. Okay. And surface tension is kind of an extension of uh cohesion a little bit. Okay. Um since water sticks so well together and it takes a lot of force, a lot of energy to to break apart. Um surface tension is a thing as well. Um it takes a lot of force to break the surface of water. Kind of like with the with the penny. All right? You can fit a whole lot of drops on that penny um simply because of those hydrogen bonds. Um, and it's going to take a lot more force than what this water strider is exerting to break the surface of that water, right? Um, so animals and plants and whole boatloads of different organisms can take advantage of the fact that water has surface tension and you can float on top of it because of those hydrogen bonds that are forming uh between individual water molecules. All right, so just to recap here, I'm going to read through these really quick just to sum this up. Uh, water molecules have an uneven sharing of electrons, i.e. They're polar coalent bonds. So hence, water molecules are polar. There's a slight negative charge on one side of a molecule, a slight positive charge on the other side of the molecule. These charge differences results in hydrogen bonds forming between the negative oxygen atoms and the positive hydrogen atoms on adjacent uh water molecules or water molecules that are next to one another and they're constantly forming and breaking. Um, hydrogen bonds give water its unique properties. Water molecules require a lot of energy to break apart from one another. All right. Once again, water molecules are super small and if you got billions of them in a drop of water, a billions of hydrogen bonds is going to be hard to break. Hence, water has a high specific heat and a high uh heat of vaporization. It's not going you're not going to raise the temperature of water very easily. You're not going to get those water molecules to break apart very easily um using heat. And that makes it really good for living things, temperature regulation. Um and it also water sticks to itself very well, which is what's called cohesion. And that causes what's called surface tension. Allows us to float on the top of water. Um, and allows it to well clump together. Okay? And it also sticks to other um polar surfaces which is called adhesion. Um, and this allows for plants to draw up water um from the soil and bring it to the tops of their branches or in their leaves or whatever else. Um, and that is topic 1.1, the structure of water and hydrogen bonding. We're going to get into topic 1.2, elements of life here pretty soon. Uh let me know if you have any questions and we will see you next