okay everyone we're going to resume our discussion on membrane physiology and we're going to talk about osmosis and here are the unit objectives for this lecture so osmosis is defined as the flow of water across a semi-permeable membrane due to a difference in solute concentration so let's just define some terms so what is solute so whenever you have a solute you have to have a solvent right so if you have some sugar in your coffee the sugar is the solute but the coffee is a solvent okay so um just keep that in mind for osmosis to happen you really need to have the semi-permeable membrane right so this is essential because imagine if you did not have this membrane so here you have pure water and here you have a sodium chloride solution if you did not have the membrane what's going to happen is you're not going to have movement of water you're just going to have diffusion of the sodium chloride particles all across okay but that's not what we're talking about we're talking about a scenario in which you have a semi-permeable membrane that only allows the movement of water and it does not allow the movement of any other solutes so because we have this difference in solute concentration we are going to have the movement of water now osmosis is not really just simply diffusion of water but you can think of it that way if it if it helps and you can say okay water is moving from an area of high water concentration to an area of low water concentration you may think that that's okay but the difference is osmosis happens not because of a concentration difference of water right osmosis happens because there's a pressure difference and that pressure difference is what drives the movement of water from the um from the area of low solute concentration to an area of high solute concentration as you see here um osmotic pressure so this is a pretty cool experiment right nice setup to to show you that really it's pressure that's responsible here so again here you have this container you have the semi-permeable membrane as you see here here's an area of high solute concentration and here's the area of low solute concentration you give it some time you notice that water decreased here right so you water was originally here this was your starting point and so we went down in water and we went up in water here that means water moved from this end to this end now if we perform the same experiment but we provide a piston here and this piston is going to generate some pressure that is equal to the osmotic pressure you're going to have no net movement of water right because it's the pressure difference that drives osmosis not the concentration difference of water right okay um so a little bit uh some cool facts on osmotic pressure right so for each melee osmo concentration gradient of impermeable solute you're going to get a pressure of 19.3 millimeters of mercury of osmotic pressure across the cell membrane so to put this a little bit in perspective the osmolarity of the icf is around milliosmoles per liter if the cell membrane was exposed to pure water the potential osmotic pressure can be greater than 5400 millimeters of mercury so that's pretty significant and this also stresses to you why it's one reason why it's very important to maintain proper ionic concentrations your sodium levels your potassium levels these different electrolytes they play tons of different roles one important role is we want to maintain constant physiological osmotic pressure within our different body compartments so what determines osmotic pressure molar concentration so the different concentration of the different solutes is what determines the amount of osmotic pressure present okay so um it's some it's summed up in this bullet point here that the osmotic pressure is determined by the number of particles per unit volume in a solution not by the mass okay so it doesn't so small particles and large particles right in terms of mass are going to exert the same amount of osmotic pressure the difference is how many particles we have so if you have just three small particles right versus maybe one large particle that's in terms of mass right the three are going to have a greater osmotic are going to generate greater osmotic pressure than the one it's the number of particles per unit volume not the mass of particles okay because roughly speaking they're going to exert the same pressure regardless of their mass so the term osmolarity it's the concentration of osmotically active particles expressed as osmoles per liter of solvent okay and this uh basic equation here is kind of important osmolarity equals g c or g times c right where osmolarity is the concentration of particles g is the number of particles per mole in solution and c is the chemical concentration so if you look at glucose glucose does not dissociate in solution you put it in there it's going to still be a molecule of glucose so the g or the number of particles per mole is one but sodium chloride is going to break into two particles a sodium particle and a chloride particle so therefore how many particles per mole g is two think about calcium chloride it's going to break down into three particles one calcium and two chloride ions so that therefore you're going to get three particles so you may have two solutes that can have the same concentration in solution but different osmolarity meaning so you may have 50 you may have 50 millimoles per liter glucose and you may also have 50 millimoles per liter of sodium chloride right so their concentration is equal but what about the osmolarity right it's not going to be equal because g times c so it's going to be 1 times 50 you're going to get an osmolarity of 50. g times c is going to be 2 times 50 so you're going to get an osmolarity of 100 melee osmols per liter therefore sodium chloride is going to generate greater osmolarity than glucose another term that you just may come across as osmolality and it's very similar to osmolarity except now we're talking about kilogram of solvent as opposed to liter of solution right so it's a little bit different we're going to be using osmolarity um for the most part so here's a question um you can pause the video right and try to answer this question so tonicity is a different term that i want to introduce here and tonicity of a solution it's related to how this solution is going to affect cell volume so um and here we have three different scenarios right so let's assume um that the osmolarity of the cell is 280 milli osmos so if we place the cell in a solution that has the same osmolarity 280. what's going to happen in terms of cell volume nothing because you're not going to have any net water movement right there's no net water movement so let's write that no net movement okay so what about what happens in b so b note here that we're um the cell the solution is 360. so the osmolarity of the solution is actually much higher than the osmolarity of the cell as a result water is going to move out of the cell well what does that do that means that the cell is going to shrink we're going to call this kind of solution a hypertonic solution now in scenario c the osmolarity is lower it's 200 right so you have it you have that difference now what happens is water is going to move into the cell causing the cell to swell we call this kind of solution a hypotonic solution because it causes swelling of the cell or movement of water into the cell so tonicity takes into consideration the ability of the molecules in solution to cross the cell membrane which means you may have two solutions that are of the same osmolarity but the tonicity is different because again remember tonicity if we go back here is related to the effect of the solution on the volume of the cell so here we only care about volume changes so how can it be right how can it be that two solutions may be of the same concentration or doesn't matter concentration actually maybe you have the same osmolarity but tonicity is different here's a pretty cool example so assume solution a contains 300 millimoles per liter of sucrose solution b contains 300 millimoles per liter of urea there since both sucrose and urea um are only going to give us one particle right they're not going to there's nothing to dissociate into so only one particle for each so the osmolarity is going to be equal therefore both solutions are iso-osmotic okay osmolarity is equal now red blood cells have an intracellular osmolarity of 300 right so let's start from there now when we place red blood cells in solution a we have no change in volume why because solution a is isotonic since it's isotonic um or not i'm sorry not because it's isotonic therefore it's isotonic there's no change in volume osmolarity is equal on both sides okay however when we place it in solution b right eurea the cell is going to swell and burst right therefore we say solution b is hypotonic now why is it that the cell is going to swell and burst the answer is because urea can easily cross the cell membrane of red blood cells due to the presence of uniporters right and so if this is your container uh let's draw this out if this is your container right and then we have sorry about that let's just here we go and then here we have our red blood cell right so here is urea and we should have 300 milliosmoles here 300 milliosmoles here but because the cell membrane can actually transport urea or it has has transporters that can move urea into the cell so what's going to happen is urea is going to move in by diffusion right and since urea moves in by diffusion the concentration of urea in the solution is going to decrease therefore this 300 is going to go down to maybe 250. what's the result of that now this solution has a different osmolarity water is going to move in not just no more just urea but water moves in when water moves in the cell is going to swell and burst so the difference here between sucrose and urea was the permeability the permeability right of the membrane so that's why this bullet point is very important tonicity takes into consideration the ability of the molecules in solution to cross the cell membrane okay and so here's a pretty cool example of what happens to a red blood cell so when a red blood cell is isotonic its shape is normal if it's placed in a hypertonic solution right or if the osmolarity of blood increases it's going to shrink right and start spiking up as you see here or if it's placed in a hypotonic solution it starts swelling because water rushes in but eventually it's going to burst right lyst and it's going to die right because if it's placed in a very hypotonic solution that's why it's very very important when transferring fluids right or receiving any kind of fluid transfer or iv fluids that we take into consideration physicians take into consideration the osmolarity of that solution that the patient is going to receive so we don't mess around with with the osmotic pressures inside the inside the system thank you