this is the video of for d2.3 on water potential and we'll cover the standard level content related to the movement of water water is an excellent solvent and it's water's polarity that really gives it that ability to be a great solvent to be able to dissolve solutes when water is dissolving ionic compounds like salt here the water forms shells around ions to prevent them from RE joining so you'll notice they're here in red they're teeny tiny these partially positively charged um hydrogens that are part of the water molecule surround negative ions and the partially negative um oxygens in the water molecule surround positive ions so it's able to take apart um these ionic compounds and prevent them from getting back together with polar molecules or polar compounds water forms hydrogen bonds around them um to dissolve them so here we have glucose and hydrogen bonds will actually form between the oxygen and the of the water and the hydrogens um sticking off here um of glucose so not um I'm not going to draw all of them but you can see that water is partially charged and therefore attracted to the partially charged or polar glucose molecule now if we have two solutions that have different concentrations and they are separated by a semi-permeable membrane that does not allow that solute to cross then osmosis will take place osmosis is the passive movement of water across a semi-permeable membrane towards areas of higher concentration so water always followes water always follows the higher solute concentration so here let's say these red things are solutes and they cannot cross this membrane um some of these water molecules in here will cross the membrane and will come over to this side until these are of equal concentrations so again the solute numbers or the solute amounts aren't equal but the concentrations are equal and that's what osmosis is and depending on the differences in the solute concentrations we actually have descriptive words to talk about these Solutions so an isotonic solution has this same concentration of whatever you're comparing it to so here in each of these beakers I have like let's just say this is uh dialysis tubing or a cell filled with 0.1 molar solution if it is sitting in a solution that has the same concentration we consider this solution to be isotonic the word ISO or this root word ISO means same if the solution has a lower concentration than what you're comparing it to it is said to be hypotonic so it's important to remember that these are comparative words you can't just say something is hypotonic well hypotonic compared to what so I would say the solution is hypotonic to the cell this prefix hyper means above and this refers to solutions that have a higher concentration compared to um the cell or whatever it is that you're comparing that too so these are very important terms to have moving forward now we said that water is always going to follow air um areas of higher solute concentrations so in this case water would flow into this cell water would flow towards areas of higher concentration in this scenario in a hypertonic Solutions water would exit the cell and flow towards areas of higher concentrations so I'm using this Arrow to mean water in an isotonic solution you get a small amount of movement but it's the same in both directions so we say no net movement of water so I'll erase those to avoid confusion and here's a look at how that actually happens so remember the semi-permeable membranes that surround cells allow water to pass through water is permeable but it's much less permeable to solute so that's why osmosis is occurring if there's a concentration gradient and that solute cannot pass remember that this is passive no energy is required and cells can actually control the rate of Osmosis sometimes there's a few ways that they might do that they might change their solute concentration right so we think about like contractile vacul or salt pumps or they can change um their permeability to water in regards to their membranes this is so cool embedded within membranes are sometimes we'll find these channel proteins called aquaporins These are channel porins that are channel proteins that allow water to pass through now water can go through the phospholipid by layers but it can much easier like and faster go through these aquaporins so the more aquaporins a cell produces and embeds in its membranes the more permeable that cell will be to water and the faster you'll have um osmosis happening so it's a really cool adaptation um that cells use and if you've already studied things like the kidney or ADH you probably know a little bit about this and if you haven't that will be coming soon but we want to be thinking about controlling solute concentrations or controlling permeability if cells want to control their rates of Osmosis now when we looked at this earlier we already knew the marity of like let's say this cell that I was looking at here well well what happens if I don't know the concentration of solutes of that cell and I want to find out that's something called the osmolarity it's the total solute concentration in a cell and let's say I'm looking at something like a carrot and I want to know the osmolarity of the cells in that carrot how would I do that well you can actually put that plant tissue in solutes or solutions that have different solute concentrations and you can measure the percent change in Mass if that plant tissue is gaining mass that must be because it is in a hypotonic solution so that's telling me whatever the osmolarity of the carrot is it must be greater than 0.1 because when I put it in a zero molar or a 0.1 molar solution it gains Mass which likely indicates well definitely indicates that it was in a hypotonic solution we see that movement of water into the cell on the other hand when I'm noticing that that plant tissue is losing mass that means that it is in a hypertonic solution so remember that cells that are sitting in hypertonic Solutions are going to lose water and therefore lose Mass what I'm looking for in order to find the osmolarity of that plant tissue is the isotonic solution so in an isotonic solution there's no net movement of water and therefore no net gain or net loss of mass so that means I have found where the solution and the plant tissue or the carrot are equal in concentration and I can say that that carrot has an osmolarity of 0.2 molar if you don't have an exact dot on here you can see where it crosses um your horizontal axis that's where there's zero net gain in Mass there are some really cool things that you can do here you can see um that we've calculated um and Visually represented variation in our data so you can have a look at either standard deviation or standard errors okay um and so you should be able to put error bars on your graph you should be able to talk about controlled variables so things like getting the plant tissue sample from the same carrot from the inside of the carrot not including the skin having the same surface area to volume ratio allowing the same amount of time controlling the temperature of the solution lots of things like that that I recommend that you investigate now speaking of plants plants have a cell wall and that cell wall does a lot of things but one of the things that it can do is prevent excess water from entering even when a plant is placed in a very hyperonic hypotonic solution and lots of water wants to enter the cell via osmosis there's a limit to how much water can actually enter due to the rigidity of that cell wall so it prevents excess water entry and it prevents the cell from bursting animal cells do not have a cell wall so when they are placed in a hypotonic solution water enters the cell and if enough water enters the cell that cell can actually burst and then that cell would die on the other hand if placed in a hypertonic solution then these animal cells will lose water and will shrivel up okay so they will shrink and you'll see them actually get smaller some freshwater ukar like this parium here have adaptations that prevent this so this does not have a cell wall so if it's placed in a hypotonic environment um lots of water could ere and then that cell could burst but they had these really cool adaptations called contractile vacul that can kind of pump some of of that excess water out um it requires energy but it's a great example of um homeostasis and um an adaptation that this organism has to prevent things like bursting so let's focus for a minute on just plants okay or things with the cell wall we'll talk about plants specifically plant cell walls are made of cellulose they're very strong they can handle lots of pressure and it is that pressure that helps plants stand up and be erect right so plants don't have a skeleton let's say they rely on turgidity inside of their cells to remain upright so when we say turgid that means a cell that has a lot of internal pressure and that internal pressure is coming from water this will only be the case when you put a plant cell in a hypotonic medium that's going to drive water to enter the cell it kind of blows up this cell and creates a lot of water pressure on the inside of the cell again the cell wall prevents it from bursting so this is great this is when plants are actually very um happy we'll say if the pressure of the inside of the cell drops due to the exit of water the plant becomes flaccid and you get a plant that looks like this it looks like wilting that's if it's in an isotonic environment so even when the solute concentrations are equal the plant is flaccid we really need it to be hyp phonic um in order to remain upright if you put a plant in a hypertonic solution then the cell membrane can actually shrink away from the cell wall due to the excessive amount of water that is lost and we call that plasmolysis and this is going to result in plant death now whereas plants really need to be in a hypotonic environment animal cells um need to be bathed in isotonic Solutions so we talked a little bit about um if you put an animal cell in a hypotonic solution it could burst in a hypertonic solution it could shrink so we need our this is a blood cell our blood cells and other cells to be bathed in isotonic Solutions so often if you're getting um an IV like this it's um going to be connected to a bag with saline solution so saline solution is a salt solution at we use this to rehydrate someone because it is isotonic to human cells so we want the blood plasma to be isotonic when compared to the blood cells or other tissues so great application there also if you are preparing an organ for transplant um between the patient that it is harvested from and the patient that it will go into you want to make sure that that organ is bathed in an isotonic solution again to keep those cells um uh in an isotonic solution means to prevent them from bursting or prevent them from shrinking