in this video we're going to start discussing um tonicity and um a look at the difference between tunisia and osmolarity um it's really important to see what happens to the volume of a cell um when placed in hypotonic and hypertonic solution so to kind of understand the concept of tonicity and what the hypertronic and hypotonic solution is we're going to start with some basics at first and then we're going to move to those solutions and see how we change the volume of the cell so we're going to start with some string definitions in the beginning so tonicity is the ability of extracellular fluid so tonicity is ability of extracellular fluid right ecf to um make water move in and out of a cell to make water move either in or out of cell due to osmosis in comparison to tunisia osmolarity is the total concentration of solute in a solution so total concentration of solute in a solution so to understand tunisia um we'll start talking about osmolarity and then we are going to review what osmosis is so um in this diagram what i have is a cell and this cell is a red blood cell we know that inside the cell we have a fluid and we refer to that fluid as intracellular fluid we're going to place this cell into a solution right so around here which is considered ecf ecf is a solution and what the solution is is a gas or liquid which is known as a solvent in which another particle called the solute is dissolved right so a solution is made of solvent which most of the time is water which is the case here and the solute which um for our intents and purposes we can use a couple of examples it would be proteins it could be sodium i can be so the concentration of a solute in solution is known as osmolarity right um so osmolate is going to tell us how much of that solute we have we are going to compare it to extracellular fluid right so let's look in the red blood cell and we're obviously going to use um small numbers um but usually this concentration is measured in lots and lots and lots of molecules right so let's assume that inside the red blood cell we have a solute and let's assume it's area okay and we have four molecules of urea if we place this red blood cell into a solution that has the same concentration of urea right so i'm going to do four of this here this solution will be called isotonic right so an isotonic solution has same concentration of solute as intracellular fluid remember we are comparing this concentration between icf and ecf now we also know that inside the cell we have water obviously um let me draw i'll erase this red blood cell so we have more space let's say we have four waters and let's say we have four waters outside and here are a couple of things that we need to take into account when we talk about solids when we talk about water is water permeable to the plasma membrane another question will water be able to move through the plasma membrane and we know that water is able to move across plasma membrane with ease okay it's small even though it's polar it's pretty small so it passes easily from the plasma membrane the solute is impermeable right so the plasma membrane is impermeable to the solute so let's write here plasma membrane permeable to water impermeable to the solute right so that means that whether it's rare or examples of proteins or sodium this solute that we have represented in orange cannot pass either into the cell we're out of the cell right so the solute that we have in the cell is going to stay in the cell the soluble would have outside the cell is going to stay outside the cell so there is no movement across the plasma membrane for solute because it's impermeable to it now let's look at water is water able to move through the membrane we already said that you can because the plasma membrane is permeable to it so water moves in and out because by definition uh osmosis is movement of water but we do not see a change in the volume of a cell because there is no net movement of water right here no net movement because we have the same amount of solute on both sides of the membrane the amount of water on both sides of the membrane is also going to be the same right and we know that even though water is moving there is no net movement there is no more water moving in or out of the cell so if a plane if a cell is placed into an isotonic solution its volume is not going to change so right here volume of red blood cell does not change and its volume does not change because we have the same concentration of solute on both sides of a plasma membrane so we have the same volume of water on both sides of the plasma membrane so the bottom line the most important thing that we have to remember here is one plasma membrane is impermeable to solute but is permeable to water and when we talk about tunicity we are comparing the ability of this extracellular fluid to make water move either in or out so we're talking about net movement of water due to responses and we know that in an isotronic solution there is no net movement of water just because we have the same concentration of solvent