so now that we understand the uh you know the basics of chemistry and the difference between a covalent bond an ionic bond a polar versus a nonpolar covalent bond now we're going to apply that to understanding one particular very important molecule water water is a big deal um this kind of shows just what proportion of your body is water a fetus is like 90 water that's crazy how could that be um well it turns out the cytoplasm is called of your cells that make up your body is mostly water so since cells make you up and those cells are mostly water well then you're mostly water which seems weird um you know water is obviously a large part of the earth and so living things like fish and uh you know different critters depend on water for surviving so water is really important in biology so we really need to understand it and it turns out it's also kind of a weirdo so let's get to know water a little bit and uh oh hold on sorry um this is embarrassing i'm getting a phone call hello oh okay so um apparently one of our local correspondents um is at the scene and uh wanted to thought something might be relevant to uh to this discussion so let's cut over to him all right let's check in with mr sanger at the kitchen table here and see what we got i've got two little dishes here um this first dish is simply water and this second dish is uh pure vegetable oil that's crisco brand that's a good brand um and what i'm going to do is i'm going to take some wax paper that's just you know wax paper and i'm just going to take a dropper here and i'm going to pull up some water and i'm going to add some drops to this wax paper now water remember is a polar substance that forms hydrogen bonds but wax it turns out is non-polar which means it does not form hydrogen bonds with other things and so watch what happens when i add these drops notice how they bead up like that they're not interacting with the wax paper and the reason why is because all those little water molecules are hydrogen bonding to one another um and sort of avoiding that non-polar wax and so now this is kind of fun let's see if i can do this with the camera if i take this watch i can do i can like pull it around a little bit and uh you know i can like drag it around oh there's another one and uh let's let's stick them together you ready boys hard to the camera poop look at that and then they join together and i can pull that thing and you can like see the strain of those hydrogen bonds as i pull this thing around you know hold on hydrogen bonds hold on as it gets dragged around and then if i pull it down here a little bit more right and so the hydrogen bonding allows these molecules to essentially attract to one another and that's what's allowing me to kind of pull this thing around in this way so now let's go to our vegetable oil here and do the same thing vegetable oil it turns out is non-polar um and so if i draw this in and i add it we saw how water went bloop here's the vegetable oil okay so so far maybe it kind of looks similar but you'll notice the circle isn't quite as perfect let me a little bit more on there and watch what happens when i pull try and pull on this that bring that you can kind of see how the vegetable oil it's you can't tug it right it just kind of like whatever there is no hydrogen bonding attracting between these molecules and so as a result it just kind of lets itself get tugged around like that so very stark contrast with this water molecule where you do see them getting pulled apart and moved around all right no that was really it was great yeah no you did awesome yeah no no you're the best yeah all right well thank you okay yeah that was great um so you know key points from that demonstration water is a polar molecule and you can see that in this diagram right we talked about this in the last screencast oxygen has this partial negative charge hydrogen has this partial positive charge and this is because oxygen is more electronegative so it kind of hoards the electrons close to it that gives it that slight negative charge hydrogen therefore has a partial positive charge and so what we saw in that demo was that this partial negative and this partial positive created enough attraction that we can actually like drag water molecules around on that wax paper which is pretty cool so water is polar it's continually forming and breaking hydrogen bonds so in liquid water these are not locked into place like this they're kind of forming and breaking as we go and most molecules of water are hydrogen bonded to other partners at any given instant so they're continually forming and breaking all the time so what implications does this have for life well this gives um gives a number of properties to water that are important to understand as biologists so first cohesion which is simply the attraction of water molecules to one another again because of those hydrogen bonds forming this has several uh several examples of cohesion surface tension is the attraction of molecules at the surface and and this actually can form a film that kind of is sitting on there so essentially the hydrogen bonds holding these water molecules together are strong enough that a tiny little water insect like this can actually stand on the water and just chill and i'm sure you've seen these little water bugs kind of scooting along on the surface of the water you know you could click on this video clip if you pull up the notes and watch this it's pretty entertaining you know there's this little lizard who's small enough that he can actually run or she can run across if they're moving fast enough and kind of run on that surface of water if he just stood there he would sink because he's too heavy but if he's running fast enough he can utilize the surface tension to do that another property of water is adhesion which is an attraction of water molecules to another substance so cohesion is where it's attracted to itself adhesion to another substance and an example of this is the meniscus you know in chemistry you probably learned how to measure the volume of a liquid in a graduated cylinder they talk about this weird meniscus why is there this curve here well that curve here is because the water molecules also attract to the glass that makes up this graduated cylinder and since it's attracting to the glass that kind of caused these water molecules to sort of climb up and and cling to each other in the glass and form this little meniscus you may have put a paper towel and water at some point in your life and you know the water will sort of climb up that paper towel as it goes you know or if you're ever fishing or something and part of your sweatshirt happen to hang over the boat and get in the water and then the whole thing is wet that's because these water molecules cling to one another this becomes important when we go to feed a plant or provide water for a plant water is in the roots the leaves at the top need that water and so basically this principle of water sort of climbing through hydrogen bonding and attraction to the water molecules itself an attraction to the the tube it's in causes water to sort of climb up the uh these tiny little vessels that make up a tree the interior of a tree these tubes called the xylem and so adhesion is what keeps trees alive temperature moderation is another property of water water has a very high specific heat which means it takes a lot of energy to increase the temperature of water you probably know this inherently if you put an empty pot on a hot stove that metal pot is going to heat up like instantly very quickly it is going to be very hot whereas if you put water on a hot stove it takes a long time to get it to boil and it's kind of annoying because you want to make your pasta but it's taken a long time the reason for this is because when you add water add heat energy to water you have to first break the hydrogen bonds you got to get it to move you know to make the temperature of something go up you got to get those molecules to wiggle right but if there's hydrogen bonds holding them together you kind of have to first break those hydrogen bonds before they'll start to wiggle more so it takes more energy than you might expect to heat up water because you have to deal with these hydrogen bonds continually causing those molecules to attract to one another so water is kind of unusual and then it has a very high specific heat and that's important again to living things organisms that live in water don't experience the dramatic temperature changes that land organisms do and you know i'm guessing um you know if you've been on a beach and you're sitting on land you know that beach gets really really hot when the sun is pounding on it the water doesn't warm up quite as much and then at night the sand gets much cooler but the water doesn't cool out as much so you know the land temperature is like up and down up and down throughout the day where's the water towers and so lake critters and ocean critters don't have to deal with temperature regulation quite as much as land critters do so go fish um evaporative cooling it turns out that water absorbs energy when it evaporates resulting in cooling so sweating and panting both cool us down essentially uh when this liquid water from either sweat or saliva on the tongue turns to gas it takes a lot of energy to break that hydrogen bond so that this molecule can escape as a gas particle and you know that energy's got to come from somewhere and the energy is coming from your own body heat and so that's why sweat cools you down and dogs pant by the way when they pant don't feel bad for them they're breathing just fine they kind of have the normal pattern of breathing but superimpose on this so it's like i said you know so there's like little tiny ha has among the bigger but it doesn't matter um this is also why it feels cold when you get out of the shower you get out of the shower your body's wet all this water is evaporating and to evaporate it's got to take your body heat to allow these molecules to escape and then you feel cold but as soon as you towel off you feel warmer um and here's just kind of a nicer picture of that so oh wait no it's not evaporative cooling sorry no that is the same gosh i'm going crazy all right um another interesting property ice floats on water this is kind of a weird thing now it doesn't seem weird to you because your entire life you've seen ice cubes floating on water but in most cases in most substances solids are more dense than liquids and so the solid would sink for most substances but water it's the opposite the solid is less dense why is that the reason why again is hydrogen bonding so the hydrogen bonding causes these molecules um to sort of lock in this sort of honeycomb pattern as they slow down so as water freezes the molecules slow down slow down and then kind of form this little stable pattern here and so if you compare your ice frozen water with your liquid water this is actually less dense there's more space between the molecules than the liquid water and as a result this is less this is less dense and therefore going to float the relevance of this of course fish get to live in minnesota if ice sunk then lakes would freeze solid in the winter and all living things inside those lakes would die but because ice floats as it cools down it kind of floats up to the top and then eventually you get this nice insulating area of ice on top and the fish can chillax underneath it all until some jerk drops a hook in there and pulls it out water is also an excellent solvent a lot of molecules dissolve in this and again it ties back to hydrogen bonding a polar molecule so remember a polar molecule is one that has these partial negative partial positive ends well water molecules will interact with those polar parts the partial positive hydrogens in water will interact with partial negative ends of a polar molecule the partial negative portion of water will interact with the partial positive parts of a polar molecule and so it can kind of help break apart and dissolve polar substances ionic substances same thing so serious table salt sodium and chlorine chloride ions and so again the oxygen atoms are partially negative they're going to attract to the sodium the hydrogen atoms are partially positive they're going to attract to the chlorine and so by kind of interacting with those they can pull these ions apart and dissolve table salt that is why salt dissolves in water there's some terms i just kind of want to toss out to you hydrophilic and hydrophobic hydrophilic simply means water loving it's a substance that dissolves in water while hydrophobic is water fearing which is a substance that does not dissolve in water and so based on that video or vegetable oil hydrophilic or hydrophobic what do you think what do you think hopefully you said hydrophobic it does not attract to water and this is why oil and water separate if you've ever mixed oil and water together they kind of separate out finally water ionizes to form h plus noh minus in small amounts and then the h plus immediately joins with another water to form this h3o plus business and it creates this kind of equilibrium so um you know because of this we have something called ph which is a measure of the h plus ion concentration that is it's a measure of acidity so here's the equation from uh boy that's grainy sorry here's the equation that you learned about from chemistry we don't really do these calculations in ap bio but we do need to understand what ph is acids have a high concentration of h plus so their ph is going to be less than seven bases have a low concentration of h plus and the ph is greater than seven um and the reason why is because um basically it's the exponent so if it's 10 to the minus 14 then ph would just be 14. so 10 to the minus 14 is a very small number so 14 would be a base indicative of a low concentration of h plus whereas 10 to the minus 2 its ph would be 2 which means that's a high concentration of h plus and so we call that an acid so there you go um the importance to this many biological molecules are held together by hydrogen bonds as we're going to see proteins for example are these ribbon-like molecules that twist and fold in this very specific three-dimensional shape and so if we change the ph that is if we change the h plus concentration that's going to disrupt the way that these molecules hydrogen bond and interact and then it's going to fall apart and unfold so ph is really really important to biological things so that's it for properties of water and with our next screencast we're going to take a look at the molecules of life