welcome to chapter 3 of ap biology where we're going to focus on water and why water is so important for life now when we talk about water it's as far as life as we know it it's completely embedded with life itself and this is because organisms that we're aware of that are alive are all going to be mostly water so when we look at these you know worlds these whether it's an asteroid whether it's a moon whether it's a planet elsewhere in the universe and we're trying to figure out could there be life there one of the first things that we're going to look for is is there water because we know that without water once again life as we know it because that's all we can really discuss could not exist because we're all pretty much 70 plus percent water by mass and even organisms that don't appear to be that watery like us where we walk around on land do you look around me i'm not a mermaid uh we still have where inside of our bodies there's fluid that surrounds all of our cells in this internal pond this internal environment called interstitial fluid plus we obviously have blood for some of the blood vessels and such that flow through our body too this is why if you cut yourself you might cut you might cut like a vein and you'll get blood but you might also just get that seeping where that clearish fluid just comes out that's your interstitial fluid that naturally surrounds your cells so even terrestrial organisms ones on land still have this fluid that's constantly around them that they're able to exchange waste with get nutrients from so all cells are going to be dependent upon water now with water chemistry there's certain things about water that make it kind of special chemically it's not your average molecule and so the first thing is it's very polar i mean there's plenty of molecules that are at least a little polar but in terms of water all of the bonds that it has are some of the most polar that exist and because it has this bent shape you can see it's kind of like a v or if you flipped it the opposite way it looked kind of like mickey mouse and so when you're looking at this you'll see that both of these hydrogens have low electronegativity so they are not very good at tugging on those electrons and oxygen has one of the highest electronegativities so it's very good at tugging on them so that means that these electrons tend to hover a bit closer to oxygen and so once again oxygen is going to be slightly negative it's a partial charge and hydrogen both of them are going to be slightly positive once again it's a partial charge not an ion but it's partially charged so this partial positive and partial negative allow it to attract other water molecules which we talked about these hydrogen bonds and these hydrogen bonds allow separate water molecules to stick together with a fairly powerful force you know most molecules they're covalently bonded so they're strongly connected but each individual molecule is strongly connected but the separate water molecules they're not really very well connected at all so it's kind of like me as a person i'm strongly connected but i have very weak connections physically to other people in most cases there's almost no connection at all unless you count really really really weak forces of gravity and so that's where a lot of molecules that are nonpolar are kind of like they have very very little attraction to the other molecules that are like them but with water it has about as high of a attraction that you can have without being an ion and so this allows for it to do a very good job of sticking to other water molecules as well as sticking to other substances that have any type of full or partial charge so anybody else that's ionic or polar so water's chemistry will be critical as to why it behaves the way it does why it's special so some of these characteristics that water possesses you have cohesion which is where water can stick to itself you can see here that the water droplet sticks together it's not like fragmenting in lots of tiny little droplets because these water molecules being polar like to stick to other polar water molecules and so water likes to clump together you'll also see adhesion which is water molecules sticking to other substances that are not water and so in this case you can see that it's clinging to this metal that's part of the faucet you can also take a small water droplet and make it stick to like a wall that's adhesion where water molecules because they're polar because they like to stick to other things so long as those other things aren't non-polar uh it's fairly easy to get water to stick to other stuff that's adhesion and because of these two things you can get capillarity which is where water can essentially climb against gravity if you give it a optimal situation so to test this you could take something like a cup filled with water and you could put a straw in it when you put that strawn it you'll see the water level inside the straw can actually climb up just a little bit above the water level in the glass and so it allows for it to kind of lift up a little bit against gravity you could also take a piece of paper toweling and you could stick it in that water and so you'll see the water you know creep up and you'll see that the paper towel will start to darken as it goes that's capillarity this is critical for plants because that's largely how they get water around their structures is they use these thin tubes that allow for water to climb up without needing some complex pump or something else to push it we also have high surface tension because water likes to stick to other waters we get where it's a little bit difficult to fall into water if you're small enough and light enough like this water strider you can ultimately climb around on water we've seen that there's certain items that will float so in class we'll do probably some basic demos with this if you haven't seen them where you can float like paper clips on water and that's because of this surface tension that does not mean you can jump out of a boat and expect to make it you're way too massive for this to work but if you were a very tiny insect you very well might be able to kind of walk out under that water and cruise along that's because of the cohesion that gives it this high surface tension high specific heat what this means is it takes quite a bit of energy to make water change temperature so if you want to make water evaporate that's vaporization if you want to make it freeze you're going to have to make sure that you're either extracting taking a bunch of heat from it to make it freeze or adding a bunch of heat to it to make it actually boil this is useful for us for homeostasis because when we go out and we're primarily composed of water it means you don't walk into a walk-in freezer at a fast food place and die you know it's going to take a while for your body to cool because water resists temperature change this is great if you're in like a lake and you're an organism and it starts to have like a really cold night it's not like the lake is going to drop from 80 degrees to 50 degrees overnight the lake might drop a couple degrees but that's normally manageable for the organisms living there you know it doesn't change too rapidly for them to survive the other cool thing is water floats when it's solid this is really really rare amongst any chemicals typically you become solid you're more dense you would sink in whatever liquid version of yourself there is so liquids generally speaking across the board are going to be less dense than solids but with water because these hydrogen bonds as the water molecules slow down they actually are able to stretch out as much as possible these hydrogen bonds and lock into this lattice formation so it's kind of like if you were in a room with a bunch of people that were kind of bouncing around together as you tried to move and bump into people imagine you all kind of stopped and spread out your arms to make sure no one was within that distance so that way you were able to spread out further because you weren't moving as rapidly you know you were able to kind of take the time if you will and arrange yourself to be ideally spread apart that's what happens with ice and so really cold water is actually going to be more dense because they're still moving so you can think of it as there's still these hydrogen bonds that exist but a lot of those hydrogen bonds just think of them as being more like bent so it's kind of like you might still be holding hands with somebody to represent the hydrogen bond but because you're both moving your arms are both bent so you're closer together than if you just sat there with your arms all outstretched that's kind of what we're seeing with this process here and so this is going to allow for ice to float which is critical because things that are living underneath it in the lake are now going to still have water that they're in so they can survive the winter versus having the top freeze and fall and crush them and then the whole lake freezes solid and they all die so it's kind of important more important than you'd think that ice floats and lastly it's called the universal solvent because it has that partial charge and because we said it's so friendly with things that are also partially or fully charged water does a great job of dissolving or dissociating things that you put into it that have any propensity to become at least partially charged now the one thing that it won't do is go through and dissolve with things that are going to be nonpolar so if you try to mix oil and water they don't mix and that's why so we say universal solvent because it mixes and with pretty much anything but that is pretty much anything not actually everything now ph is a critical concept with water water can naturally break down to form two ions h plus and o h minus and just in case you see it sometimes this h plus binds to a water and becomes h3o plus so we'll consider these essentially the same thing if you see either or don't freak out they're essentially the same thing so this happens naturally but you should be able to see that water splits into both of these so ultimately there should be equal amounts of these things if it was pure water and we call equal amounts of those things would be a ph of seven that's called neutral right if you have more h plus because i put an acid in there and acids typically are going to have more h plus that's going to move you down the scale it's towards the bottom of the scale that we have acids and what this means is these numbers represent the concentration of h plus ions so 1 on the ph scale means that the concentration is 10 to the negative 1 or essentially 0.1 when we go to 2 it's 10 to the negative 2 or 0.01 so you can see this is a logarithmic scale meaning it goes by tens so a ph of 2 is 10 times less acidic than a ph of 1. a ph of three is a hundred times less acidic than a ph of one and so as we go up we eventually get to neutral and then eventually at the top here we get two basic and when we talk about basic that means you have more o h minus than h plus and so our h plus concentrations are exceedingly small you know 10 to the negative 13 10 to the negative 12. these are incredibly small numbers you know point zero zero zero zero lots of zeros before you get to a one aquatic organisms which is most of the earth is aquatic they have to really balance their ph or else they can die so think of it kind of like for us where we have to watch our temperature because if it changes too much it can kill us for aquatic organisms they have to worry to some extent about temperature but for them might be even a bigger deal to worry about ph if the ph shifts it can completely screw them up so this is just some of the math stuff that we won't really do much of but i figured i'd at least let you see it but it just lets you see what i just told you where h plus is whatever the ph is right 10 to the negative whatever so we said 2 is 10 to the negative 2 that's the concentration and ph the log idea just means by tense so if we change from seven to ten that's one two three so that would be a change of one thousand three zeros ten a hundred a thousand so just kind of get used to the fact that even what seems to be small ph changes of like one is actually 10 times different so that can be a huge deal for an animal or a plant that's living in those conditions and lastly with ph then we have buffers buffers resist change in ph so they can help organisms maintain homeostasis and stay alive and the way they do this is if i add a bunch of acid they can absorb that h plus and if i add a bunch of bases they tend to release the h pluses that they may have absorbed earlier and so they can help balance out the concentration of h plus to prevent the ph from spiking and becoming too acidic or becoming too basic now they're not a cure-all you know they can't work no matter what you do if i add enough acid you will die if i add enough base to you and like inject it in your veins you're gonna die but they can make it where you don't have like some lemon juice and then you die you know yes lemon juice is very acidic but you have buffers that are capable of dealing with that just like your stomach has acids but ultimately you have buffers that you can secrete that help neutralize those in your intestines so you don't just have your intestines get eaten away by the gastric acid that's in your stomach and so even though many organisms are very sensitive to ph they typically have some durability so long as they stay within the range that the buffers that are naturally present in their body can help them and this is one of the buffers that you might see so this is the carbonate ion co32 minus this is one of its versions called bicarbonate which is hcl31 minus you'll notice that this one just has one extra h h plus and then we've got h2 co3 which would be carbonic acid and so we have where these forms can shift within stuff like our blood so that if we add acid they can ultimately have some of these carbonate ions and bicarbonate ions absorb it to kind of move this direction and if i were to add bases we could have them move to the opposite direction and release h pluses that's how these guys can