water is the most important solvent within our body so we need to look at how water behaves with regards to its ability to move across membranes we've talked about diffusion and we've talked about diffusion in too simple or in two different ways that being simple diffusion and facilitated diffusion and to this point when we've used these terms we've been talking about the diffusion of solute molecules across the membrane a variety of diffusion that deals with water is called osmosis osmosis is the diffusion or the passive movement of water across membranes and the reason that it moves is due to differences in solute concentration on either side of the membrane so when we use the term hydrophilic realize that means water loving so in essence water is the ultimate hydrophilic molecule interestingly though it does seem to have the ability to move across most cell membranes in an unrestricted manner so water appears to be able to move directly through the membrane in most cells recall that we stress the fact that the phospholipid bilayer membrane we treat that as a lipophilic substance and so we had previously indicated that lipid molecules can move through the membrane quite easily but water here is an exception water seems to be able to move through unrestricted in most cells so in some locations water requires additional channels to allow it to move so in most cells these channels are not necessary in the way I've seen this described is that water is such a small molecule in the surface area over which it's diffusing in the cell membrane is so large that simply based on the concentration of water its small size in the large surface area over which it's moving water is able to make its way across that membrane relatively unrestricted there are some places in the body though that water can't move across that membrane easily and in those areas it does require special channels in 2003 the Nobel Prize was awarded for the identification of these water channels that are necessary in some tissues and these water channels are called aquaporins so the direction that water is going to move whether it's through directly through the cell membrane or through aquaporins will be determined by something called osmotic pressure which is related to the solute concentration on either side of the membrane now that half equation is used to calculate osmotic pressure now we will not be using any equation really throughout the semester to actually calculate numbers but the reason that we'll use equations is to show relationships so osmotic pressure is Def is typically denoted by the pi symbol and this is a description of the eventoff equation there's several variables on the right side of the equation the r stands for gas constant and T stands for absolute temperature now both of these are basically going to be stable in our body so those are not ones that we really need to worry about the other two though are things that can change so C stands for the concentration of a solute and then Sigma represents what we call the reflection coefficient so let's see what reflection coefficient means reflection coefficient is a unitless number that ranges from zero to one a reflection coefficient of zero means that the solute is able to freely penetrate through a membrane so zero percent of the solute is reflected by the membrane so in this case if the green dots represent a solute or the reflection coefficient of zero and we put two green dots on the left side of this container that solute molecule will then equilibrate because it can freely pass through that membrane and in this situation we will have developed then equal concentrations on either side of the membrane now compare that to a solute that has a reflection coefficient of one in this case the solute molecule cannot cross across the membrane so one hundred percent of the solute is reflected so non-penetrating solutes or solutes that have a reflection coefficient of one meaning one hundred percent of them are reflected in those situations we will see a solute concentration develop across the membrane and then we would expect to see that to cause water to move so let's take an example with a reflection coefficient of zero and we're going to pour an amount of solute on the left side of this container that has a reflection coefficient of zero so this means that it is freely permeable through the membrane zero percent will be reflected so it will then equilibrate across the membrane and we will have no difference in solute concentration remember that water moves because of a difference in solute concentration but in a solute with a reflection coefficient of zero it immediately distributes across the membrane so there will be no difference in solute concentration so there will be no osmosis resulting after we add this penetrating solute to the solution so we say that penetrating solute is ineffective in causing osmosis let's go to the complete other side of the spectrum okay so in this example we're going to have a reflection coefficient of one so if we add solute to the left side of the container that has a reflection coefficient of one that means one hundred percent of that solute is being reflected which means none of that solute moves across the membrane now in this case we absolutely have a solute concentration difference and that difference in solute concentration is going to cause movement of water and in this case the water is going to be drawn to the side of the solute so that's going to result in water moving from the right side to the left so this example shows how non-penetrating solute creates a gradient and therefore it is effective in causing osmosis so let's look at that same situation again where we have non-penetrating solute added to the left side of the container and as we've established this creates a solute gradient which in turn is going to cause water to want to move from the right to the left now if we can take a piston and we can apply pressure to the left side of this system we can prevent that water from moving the magnitude of force that that piston applies would represent the osmotic pressure of that Solution on the left side so one way to think of osmotic pressure is the magnitude of pressure which is necessary to apply to that system to prevent osmosis from occurring another way to think of Osmosis from the perspective of this solution itself is the tendency of a solution to cause osmosis so the more non-penetrating solute or solute with a reflection coefficient of greater than zero or in this case one the greater the concentration of that non-penetrating solute the greater osmotic pressure that solution will have reflection coefficients range from zero to one if we have a reflection coefficient of one that means that one hundred percent of that solute is reflected so we say that solute is non-penetrating now if we have two solutions separated by a membrane if there is a difference in the concentration of this non-penetrating solute then osmosis will occur if solute is penetrating then we will have no concentration difference across the membrane and osmosis will not occur so in this class we'll talk and think about Solutions as having a reflection coefficient of either 0 or 1 so we'll clearly be able to define whether or not water will be caused to move or not but realize in reality most substances have a reflection coefficient somewhere between 0 and 1.