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Video transcript

in the first video on passive transport we talked about the most passive of passive transports and that is simple diffusion and we talked about how small non charged non-polar molecules would actually have the easiest time things like carbon dioxide or molecular oxygen would have the easiest time diffusing through the cellular membrane they are small enough to kind of get through the little gaps here and then since they have no charge or polarity they're going to they're going to be fairly indifferent as they pass through and we talked about in between you have things like water molecules which are small enough to pass through the gaps but they have some polarity so they're not going to be able to get through super easily but they will be able to seep through and then we talked about things that would have a tough time and that's charged particles because charged particles that we have some ions right over here sodium ion a potassium ion even though these are fairly small they're going to interact a lot with the phospho with the phosphate heads right over here with this charge which is going to keep which is going to make it hard for them to actually penetrate through the membrane what I want to talk about in this video is still passive transport remember passive transport is about not using energy it's about moving down the concentration gradient but we're going to talk about ways that passive transport can happen a little bit easier for some of these molecules over here and that's because their transport their passive transport is going to be facilitated so what we're going to talk about in this video let me figure out a place where I can write it is facilitated facilitated diffusion let me write that down facilitated facilitated diffusion so the last video was just straight up diffusion now we're going to talk about facilitating it so what do you think if you were trying to engineer something that would make it easy for these things these these types of molecules either water molecule or anion to move down its concentration gradient what would you do well the you might say well if I did if you didn't have to mess with all of this you know all the the hydrophilic heads and then the hydrophobic tails and then the hydrophilic heads here well that would make it pretty easy to move down your diffusion gradient and that's exactly what has emerged in nature essentially just tunnels through tunnels through the membrane and so one form of facilitated diffusion can happen through what we call channel proteins let me write this in orange for no good reason channel channel channel proteins channel proteins and an example of a channel protein might be this one right over here and maybe this one is specialized for being a channel for water for and so we would call this this particular one we could call an aquaporin aqua aquaporin which is just a channel protein for water and so you see it has this hole on top and let's say you had more water molecules outside the cell then you have inside the cell and you wanted to move down its concentration gradient or maybe just what you have a higher concentration of solute here and so we're going to have osmosis occurring so the water molecules are going to they're more likely to come they're more likely to come from the outside to the inside then from the inside to the outside and so you could have water molecules going there they don't even have to really mess with the membrane they're just going to go through this aqua port pro this aquaporin and then come out come out on the inside of the cell and you have similar channel proteins for ions so this might be one for ions and so let's say this is a sodium these are sodium ions right over here they're charged they would have trouble getting through but this channel protein might be might be specific to them and allows them it allows them to go through and as we'll see when you study things like neurons will see that these channel proteins especially for ions are incredibly important for amplifying an electrical signal down or a chem Oh electrical signal I guess I could say and they can also be gated they can also open and close depending on two different conditions that are in different parts of the cell so these channel proteins they could just be open or they could be open and closed gated based on different conditions which you can see that's actually key to what happens in nerve cells that we'll see in future videos now another type of facilitated diffusion can occur through what we call carrier proteins carrier proteins and I want to be clear well I'm going to talk about carrier proteins but people are still studying exactly how they work but the view is okay let me just draw the membrane here let me draw let me draw a membrane I'm going to do a carrier protein I'm going to draw a carrier protein in the membrane so this is a this is a cross-section of this is of my membrane my phospholipid bilayer here almost done and then a carrier protein the way I'm going to draw it isn't exactly how a carrier protein would actually look but it would hopefully give you the right idea so maybe it's like this maybe it's like this and things if things want to move down their concentration gradient let's say you have a higher concentration above and I'm going to say some arbitrary some arbitrary particle has a higher concentration above than it does below they can actually attach potentially or be or kind of get into a compartment over here and then that would trigger the carrier protein to change its shape so that and let me see if I can draw it's changed shape well so it could change its shape so this is when it's taking stuff from above and then and then when it sees that hey I've got stuff here it can let me it can change its shape to look something like this so it could kind of flip around let me get the other tool it could whoops really having trouble with my tools today all right all right it could flip around like this so before is open to the top but now it could flip around and the stuff that had just collected from the top could be dumped inside inside the cell and once again this is passive transport because it's all about things moving down their concentration gradient if there was no cellular membrane here these things would have moved in this direction you would have had more things moving in this direction to give an amount of time than you would have had things going in the opposite direction but the cellular membrane was getting in the way but then this carrier membrane can facilitate that passive transport it can facilitate the actual diffusion
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