so in this lecture we're going to talk about osmosis which is the diffusion of water of water h2o across a membrane and it would need to be permeable an impermeable membrane to water you'd have no movement of anything and we wouldn't be talking about osmosis it's the movement of water across a permeable membrane right and osmosis is still just the diffusion of a molecule and we just talked about how diffusion um passive diffusion facilitated fusion is going to happen when molecules move from an area of high concentration to an area of low concentration okay and that's the exact same thing happening with osmosis except that we're focusing on the movement of of water and not the movement of solutes and that's a big distinction right so let's take a scenario where we have a condition where we have this youtube which probably maybe you've seen many years ago or at some point in in the past but we have this youtube and we fill it with water and it's got a semi-permeable membrane that is not permeable to and let's say we're dealing with is let's say we're dealing with glucose so it's not permeable to glucose and on one side we add some glucose right let's say we had 10 glucose molecules and on the right side we're going to add let's say we just add three glucose molecules so if we were talking about diffusion in what direction would the glucose molecules move from side a to b or from side b to a right if the if this membrane was permeable to glucose then we would see glucose moving from a to b but as this membrane is not permeable to glucose there is no movement of glucose so the answer would be there is no movement right glucose can't go through that membrane that permeable membrane right so what's going to end up happening is that in order to help the system reach equilibrium glucose can't move so the only other thing that can move is water and so water would be the molecule that moves and so we would end up with a scenario where we have our youth tube and after several hours right after some time we would find that the left side has way more water than the right side the number of the number of glucose molecules would still be the same on both sides but the volume of water the amount of water would be different so the concentration of our glucose would be changed because now we have more water molecules than when we started even though the same number of glucose molecules are present right so why is that right we just talked about how molecules naturally on their own with no energy expenditure go from an area of high concentration to an area of low concentration and in this case that's exactly what water has done water has moved from this side to that sorry water has not moved from that side water has moved from side b to side a okay down its concentration gradient okay so let's think about why that is so let's zoom in on this area right here so if i zoom in let me just create this little scenario right here where we've zoomed in and here's our semi-permeable membrane that is not permeable to glucose and we said that the left side had more glucose molecules compared to the right side okay so this is glucose okay and we'd said at the initial setup that the volume of water was the same on both sides okay so if the volume of water is the same on both sides in our initial setup let's say we put 10 h2o molecules right on both sides one two three four five six seven eight all right and we would have 10 molecules of water on both sides and we said that glucose cannot go through the membrane the semi-permeable membrane so there's no diffusion of glucose happening right that means water is going to end up having to move or osmosis is going to occur so what direction is water going to move right well water is going to move from an area of high concentration to low concentration right the other way to think about it is that it's going to move from an area where there is a high concentration of free h2o molecules to an area that has low concentration of free h2o molecules so really it's the movement of these free water molecules that we're talking about we're observing on the phenomenon of osmosis right so what does this really mean right what do we mean by free are there water molecules that are not free yes those would be molecules that are associating with the solute why because if a solute is a solute in an aqueous solution it must be hydrophilic what does that mean well water is polar it's got partial charges so solutes must be polar or charged in order to be hydrophilic and interact with water molecules via hydrogen bonds okay and hopefully that is bringing some bells essentially what that means is that solutes are able to form hydrogen bonds with water molecules and when they do that those water molecules are no longer free so let's say in our scenario that glucose in this case can form hypothetically right two hydrogen bonds with uh water molecules two different water molecules right so this glucose forms a hydrogen bond with that water molecule and with that water molecule and the same thing with this glucose molecule here and this these two water molecules right and the same thing would be true this glucose with this water molecule and that water molecule so let's circle our water molecules that are in height or hydrogen bonding with the solute right and so we see that we've got four sorry six um water molecules that are now associating with our glucose molecule so these six water molecules are not free okay the free water molecules would be i'm just drawing them a little bit bigger so that we could see them right would be these molecules that are not associating with solutes and are therefore free and now if we look at the number right or the concentration of our water mole our free water molecules we see that on this side we have four free h2o and on this side we have eight free h2o and we said that water in osmosis is still going to go from an area of high concentration to low concentration and so given our little scenario right water is going to move from high to low which is exactly what we saw in our previous scenario where we had this a bigger youtube okay so osmosis is still diffusion it's still the movement of water from an area of high concentration to an area of low concentration however the distinction here is that we're talking about water water molecules when we're talking about osmosis and we're talking about free water molecules and these are free because they are not in association they're not forming any intramolecular interactions with the solutes themselves okay so the higher the concentration of solutes on one side the more interactions with water is are going to be formed on that side reducing effectively the number of free water molecules and then water just moves down its concentration gradient and that is osmosis okay and this is important for bacterial cells especially when we take a look at their cell wall right so gram-positive bacteria have a really thick peptidoglycan layer and so this peptidoglycan layer is going to help structurally protect the bacteria from osmotic stress changes in osmotic conditions in its environment right if all of a sudden there is a ton of glucose that you poured into your culture well compared to the glucose that's inside the cell that's going to cause some kind of um change in the move the way water moves right all of a sudden water is going to be drawn out of the cells and so they'll shrivel up right what if you added put your cells into a condition where there is relatively little amounts of solute compared to the inside of the bacterium well water would be drawn into the bacteria in that case and without its um cell wall the bacterium would continue to swell until eventually it pops but because of the cell wall the bacterium can take take in water but it it's kept from lysing due to osmotic stress so the question might be well what about gram negative bacteria we know that gram-negative bacteria have the plasma membrane they have a thin peptidoglycan layer which i guess i was drawing green previously right then pepto-glycan layer this is terrible and then they have a an outer membrane let's draw that a little thicker right are gram-negative bacteria as resistant to osmotic changes compared to gram-positive bacteria right well no their their spell wall is structurally not as strong as the gram of positive bacteria right so they're a little bit more susceptible to changes in osmotic and or osmotic changes in their environment right so why wouldn't all bacteria eventually become gram positive right and that's part of hopefully what you would get from reading the rest of the chapter right but the point is that these two strategies have different functions different properties that are complementary in some sense right gram-positive bacteria have a strong outer cell wall that helps with structure but it turns out that the cell wall of gram-negative bacteria is good for protection against chemical degradation right antibiotics enzymes right so gram-negative bacteria have an upper hand when it comes to environments where the cell wall is is um being challenged by chemical agents right antibiotics enzymes it's less permeable to these uh molecules and so it's it's more resistant right gram positive bacteria not so much right the cell wall is not as impermeable to antibiotics and enzymes so they are much more susceptible okay however gram-positive bacteria are going to be or have this cell wall that helps with structural integrity right a stronger cell wall which is going to protect from mechanical stress all right so gram-positive bacteria have protection from some forms of mechanical stress more so than gram-negative and gram-negative are a little bit more resistant to chemical stress or as grand positive or more susceptible to chemical stress and in this lecture i did not talk about hypotonic isotonic and hypertonic solutions um but you should be familiar uh with these terms and how to interpret and use these terms in a question um for simplicity of explaining i didn't get into that jargon but you are going to see this in a clinical setting um and in the literature so it's important for you to have a good understanding of what hypo and hypertonic and isotonic means and how to apply that in a situation right