hey everybody it's your ap bio teacher mr poser today we are continuing our second unit on cell structure and function by getting into topic 2.6 which is on membrane transport in topic 2.5 we talked about membrane permeability what kinds of molecules can pass through the phospholipid bilayer and in 2.4 we discussed what that phospholipid bilayer is well it's got a polar head and a two fatty acid nonpolar tails for each one of those phospholipids that kind of face each other in a bilateral like that um so we talked about the structure of the plasma membrane and we also spoke about what is able to pass through it today we're going to be able to talk about how things pass through it and the process by which the cell membrane is able to regulate what comes in and what goes out of cells and how it's able to establish something called a concentration gradient but before we get into that we have to talk about the one of the main i should say one of the main properties of fluids and by fluids i mean liquids and gases and that's diffusion diffusion as i put up here is essential for a cell to bring in nutrients and take out i should say out waste so how diffusion works it's well i'm going to get there in a second let's walk through an example of how diffusion is so let's pretend here i know it's a very complex diagram that we have a high concentration of molecules over on one side of this dotted line which we're going to pretend is a selectively permeable barrier um and on the other side we have a low concentration of this molecules now concentration you're going to hear me say that word again and again and again and again and i need to know what that means a concentration simply refers to the number of molecules in a given amount of space or a given amount of volume okay so on this side of the uh on this side of the barrier there's a lot of molecules relative to the other side um in this if they have the same amount of volume this one's going to have a higher concentration because there's more molecules that fill up that volume okay so we have a high concentration on one side and a low concentration on the other side what diffusion is it's the tendency for molecules to spread out that go from areas of high concentration to a low concentration so an example of diffusion that i always like to bring up is say you got a glass of water and you have a thing of food coloring you drop food coloring into the glass of water what's going to happen the food coloring will eventually spread out throughout the entire glass of water it's not going to stay as one tiny droplet the whole time that's diffusion the tendency for fluids like that to spread out and fill up available space all right so cells are really really dependent on this property of diffusion that fluids like gases and liquids molecules tend to spread out from one another so if we have a high concentration of molecules on one side of this barrier and a low concentration on the other side of this barrier well these molecules that are all packed together like sardines over here they're going to spread out they're going to spread out and the concentrations on either side of this barrier will eventually become equal and that's a state that we call equilibrium in fact i'm going to highlight that down here so you know that it's an important term equilibrium refers to when there's a hot or there's a equal concentration on either side of a membrane or a barrier of some kind all right so it's in equilibrium so in order to define diffusion i kind of referred to it earlier but it's the movements of particles so that they tend to spread out in available space so we can say that the food coloring diffuses in that glass of water we know that air diffuses or gas is diffused because air fills up a container or whatever container it's in right so it is diffused it is spread out there's not more air concentrated in one spot versus you know another spot in a room it's all very evenly spread out okay so that's diffusion cells are dependent on this property of diffusion and diffusion happens when there is a concentration gradient and a concentration gradient as i put up here it's a region along which the density of a substance increases or decreases so density is referring to concentration here again um so check it out this is a concentration gradient what we had before where we had more molecules on one side than we did on the other there's a higher concentration on one side and a lower concentration than the other that is what we call a concentration gradient um something you might have you might have heard the word gradient before like when it comes to i don't know colors there's a gradient of colors ranging from red to purple right um that's kind of it's kind of similar um another another word that you might hear gradient or grade is when you're going up or down a hill okay so imagine this and the area with the high concentration you know molecules are going to move to the area of low concentration until they're equal right think of this as like maybe the top of a hill okay at the top of a hill you've got a high concentration or high elevation and what things tend to do they tend to move to a low elevation so they kind of go downhill and you can kind of think of a concentration gradient in the same way that we have a high to low down here so what i like to call this is that substance diffuse down the concentration gradient molecules are naturally just going to move to where they want to go from high to low concentration just like things at the top of the hill you know if they're given the right push they're going to tend to go down um and down the gradient all right so when we're talking about diffusion with respect to the cell membrane we're talking about passive transport and passive means that like yeah we're not expending a whole lot of energy it just kind of happens naturally so we call it passive transport this because the cell does not need to use energy to move substances across so to illustrate this again we've got our fancy phospholipid bilayer here right uh we've got a higher concentration of oxygen these blue dots on one side and we've got a higher concentration of carbon dioxide on the other side so let's just say this is the inside of the cell and this is the outside of the cell now if i'm a cell i really want to get rid of my carbon dioxide because carbon dioxide is a waste product and i really really want to bring in oxygen so that you know i can do my metabolism and make my atp so what i want on this side needs to come in and what i want on this side needs to go out now how does a cell do that well a cell can rely a little bit upon diffusion in order for that to happen all right so if you look closely here there's a high concentration of blue high concentration of red and then there's an even concentration of blue and red on either side this is representing what we call equilibrium there's no more concentration gradient there's two concentration gradients here red is going to move out and blue is going to move in okay but if we just let passive transported diffusion do its thing there's going to be an even number of molecules on either side of the membrane all right so that's that's cool you know we brought some of that carbon dioxide out we brought some of the oxygen in that that that's great that's great you know but as a cell you need more oxygen and you need to get rid of your carbon dioxide so how exactly do you do that a cell is able to do something called active transport and it has to use some of its energy in the form of the molecule atp to move solute against that should say against the concentration gradient all right so check it out we're at equilibrium right now we got an equal amount of oxygen and carbon dioxide on either side of this membrane but we want to bring more oxygen in and we want to take more carbon dioxide out so what are we going to do well a cell can use some of its atp and use some of those proteins that are embedded in the phospholipid bilayer in order to get this done okay it's not going to be easy to move things against the concentration gradient but the cell's got to do it's got to use some energy a metaphor i like to use like if we go back to calling a concentration gradient like a hill right if you're going from high concentration to low concentration and you're on your bike or something you can just coast down the hill right it's not going to it's kind of passive you don't need to expend that much energy but if you want to go from low elevation to high elevation yeah then you've got to use some energy right it's the same kind of idea here another metaphor is like if you're paddling down river with the stream right you can just kind of let the stream take you um down the river but if you want to go up river you've got to expend some energy okay it's the same concept here all right even if we're at equilibrium if we got to move things against the concentration gradient or against you know diffusion then that means that you know we got to expend some energy so what a cell might do and i just want to make a note here that carbon dioxide and oxygen are shipped out in of the cell membranes chipped in and out of the cell membrane via facilitated diffusion which is our next topic one of our topics coming up um but we're just gonna pretend it's active transport for the sake of this video all right so disclaimer there um so if my cell wants it can use some atp to power up this protein over here and take more stuff in and move more stuff out that's called active transport it has to use a little bit of energy in order to re-establish a concentration gradient so check it out we had mostly blue on this side to begin with and mostly red on that side to begin with now it's kind of flip-flopped we brought more oxygen in and more carbon dioxide out thanks to active transport and here's a really important thing to note in fact i am going to underline it ready underlined it's important active transport and selective permeability allow a cell to make concentration gradients okay so a cell can manipulate using active transport and using its membrane proteins it can manipulate what concentrations are on either side of the membrane so we can control what's going in and what's going out that's where the selective permeability of the cell membrane comes in for example your neurons so the the cells that make up your nervous system your brain your spinal cord all that stuff they need to have a particular concentration of sodium on one side and potassium on the other side in order for it to be at what's called the resting potential um so that it can actually send signals from one cell to the other and so that your body can function as a whole and you can your brain can work okay so without being able to uh establish these concentration gradients your nerve cells would not work and many cells need to be able to control their concentration gradients and manipulate them in such a way that they can control what comes in and what comes out all right so that's active transport all right but another type of active transport that we got to talk about has to do with a really big molecule okay so this is my very detailed protein over here a protein can't just pass through the bilayer and it can't just go through you know another protein like that it's really big um so what does the cell do for that it's a one second it's a form of active transport that we're going to talk about they're called exocytosis and endocytosis now you we may have been exposed to this term before but i'm going to highlight it here again membranes in and out of the cell can form vesicles which are sacs made of membrane um so that it can import and export large molecules so remember how the consistency of the cell membrane it's like a bubble it's like a soap bubble right and a bubble can change shape bubbles can form smaller bubbles within them right that's the same idea with the cell membrane so um the plasma membrane as i put down here works with the what we call the endomembrane system and we might have talked about this in an earlier uh topic but the endomembrane system is like the membranes on the inside of the cell right so the er it synthesizes proteins the smooth er synthesizes lipids and hormones and that kind of stuff but how does it ship it out of the cell well it's able to you know take a little bit of its membrane and pinch it off and form a little bubble okay so if whatever the uh cell membrane or excuse me the uh er or maybe the golgi apparatus made um it's gonna be pinched off in this little bubble in a bubble it's like a it's a vesicle and it's made of membrane and it's a little membrane bubble that's going to travel out to the plasma memory to the outside and it can fuse with the rest of the plasma membrane and thus release those particles that are you know the products that the cell made it can release it and send it to other cells if need be um and endocytosis let's just say this is like some kind of food molecule it's a big big food molecule um the plasma membrane can kind of form a bubble around it and then import it into the cell and that's called endocytosis okay so it's able to form a vesicle just by you know changing the shape of that cell membrane it's pretty sweet so as i said stated before exocytosis is when the cell secretes molecules by the fusion of vesicles with the plasma membrane to release it to the outside okay so the er the golgi forms a vesicle sends it out i'm in endocytosis is when the cell secretes molecules um by the fusion that's not right um the cell takes in molecules by the fusion of vesicles with the plasma membrane glad we fixed that um so it's able to the cell membrane is able to pinch off and form a little bubble um that's called a vesicle and it's able to transport that around the cell pretty sweet um two more types of endocytosis that i want to talk about phagocytosis is what we call the endocytosis of the solid food particles so if we were to be specific about what kind of endocytosis this is that's called phagocytosis um and another type of endocytosis this is called pinocytosis which is endocytosis of extracellular fluids so this is kind of like the cells eating and this is the cell drinking all right um that'll be it for 2.6 this is an important topic and it's going to lead you know into the rest of the unit for sure let me know if you have any questions and we'll see you next time bye