in this video I'll review Concepts from Chapter 3 of human anatomy and physiology 11th Edition this chapter is focused on cells in particular we'll be looking at membrane transport or the movement of substances from or between the inside and the outside of the cell there are two major methods for moving substances across the plasma membrane we have active transport and passive transport and the difference between these two is whether or not they require energy active transport requires energy to get done and usually that energy is in the form of ATP passive transport on the other hand does not require energy so let's start with passive transport there are three classes of passive transport we have simple diffusion facilitated diffusion and osmosis all three of these classes are passive because they all are movements of molecules down their concentration gradients so what down the gradient means is from high concentration to low so molecules will passively flow from high to low down their concentration gradient this means that no energy is required there's some general rules of diffusion diffusion is basically just random movement of molecules and with random movement of molecules you're going to see them move from areas of high concentration to lower until everything evens out so it'll eventually within this closed container shown in the illustration eventually we get an even distribution but it starts with a high concentration in the center so it's this this sort of natural movement of molecules energy again not required and the speed of diffusion is going to be determined by three big factors so molecule size so bigger molecule means slower diffusion temperature so as the temperature gets colder diffusion is going to slow down in distance so it's going to take a longer time for the molecules to diffuse further so the further distance the slower the diffusion so now let's look at one of those three types of passive transport and that's simple diffusion so as the name implies this is the simplest of these three this just means that it's going to be a nonpolar lipid soluble substance so this basically means it has to be very very tiny or hydrophobic basically what this is talking about is it has to be able to pass through the hydrophobic portion of the lipid bilayer so a molecule that can do that something that is sufficiently small or hydrophobic can just pass right through and that's simple diffusion so if something is high concentration outside low concentration inside is shown here you're going to see a net diffusion into the cell some examples of things that are going to travel across the membrane by simple diffusion include oxygen carbon dioxide as well as some fat soluble vitamins we also have something called facilitated diffusion this means that there are certain molecules that can be transported passively down their concentration gradients so shown here we've got some sort of solute that is going from high to low however they're going to require some sort of carrier or channel protein to get across so examples of this would be solutes like glucose amino acids and ions so things that are large and or have uh ionic charges so in those cases they can't diffuse across this lipid bilayer they can't get past the hydrophobic portion so instead we have transport proteins that are going to allow them to still diffuse down their concentration gradient but in this case we call it facilitated because the proteins are facilitating the process now finally we have osmosis so this is sort of a special case it's still diffusion but you can see here there's actually a little bit of simple diffusion going on and a little bit of facilitated diffusion what makes osmosis special is it's talking specifically about the diffusion of water so remember water is the major solvent of our bodies and so we need water to be well balanced inside and outside this is going to determine the concentration of solutes in all the different portions of our body one thing to note is that even though we have this hydrophobic portion of the lipid bilayer water is small enough that some molecules can sneak past so again they can travel by simple diffusion in this case however if you really want to speed this up we also have these transport proteins called aquaporins and you'll see these come up in in a few different places but in particular you'll see them you'll learn about them in the kidneys so these aquaporins are proteins that are going to allow for much faster transport of water down its comp or down its concentration gradient now osmosis is a little bit strange in that remember water is the solvent so it's the thing that is going to define the concentration of other molecules so when you're thinking about osmosis for me the easiest way to think about this is that it's going to move water will move from the side of the membrane with low osmolarity to the side with high osmolarity so that means that water is going to move toward the side of a membrane with more concentrated solute so here's a definition for you I use the word osmolarity here osmolarity is the concentration of solute the units are in molar so molarity molar is the unit and in living organisms that solvent is going to be water and the solute so that's solute that we're talking about here is substances dissolved in the water so if you could imagine you've got two different fluids one has water and very little substance dissolved in it one has water and a lot of substance dissolved in it now the overall volume in these two cases is equal however in the in the example that has more substances higher substances so in this case 10 molar is higher than one molar this has more of that volume being taken up by substances and less being taken up by water so water is going to move down its concentration gradient to where there's less water and in this case that is outside so that's all very complicated again the way that I like to remember this is that water moves toward higher concentration so water in this case is going to move from one toward 10. so it will always move toward the higher number this is an example showing intracellular fluid at one molar and extracellular fluid at 10 molar for example so in this case water is going to flow out of the cell until these two are equal or really until the cell basically Runs Out of Water in order for osmosis to work that membrane needs to be selectively permeable but that basically means is that it's going to allow water to pass back and forth between the inside and the outside of the cell but it's not going to allow those solutes to move back and forth so the solute stay the same and the water has to move I'll remind you again the reason for that is that osmosis is based on differences in solute concentration so if solutes can just freely move then those solutes will diffuse back and forth until they're equal on their own this becomes very important uh especially in animal cells that lack a cell wall that helps stabilize them because of this thing called tonicity so when we see a red blood cell what we really want to see is a cell that looks like kind of like a donut uh or like an inner tube that you might ride say down a river so in this case these are red blood cells that are existing within isotonic solution so your blood plasma needs to be isotonic what that really means is that the concentration in the extracellular fluid needs to equal the concentration of substrate or of of substance in the intracellular fluid when that's true the cell volume is going to be stable and your cells will look like this nice donut shape that can fit very neatly through your your little tiny capillaries to all the tissues of your body if you were to place that blood cell into a hypertonic solution what that would mean is that the cell volume is going to shrink so that's what's shown here this cell has been placed into a hypertonic solution and that is caused the cell to shrivel and that's because the substance concentration in extracellular fluid is greater than intracellular fluid so this is this hypertonic solution is the same as the example I used in this previous slide so this would be a hypertonic solution 10 molar versus one molar intracellular so you can see here hypertonic solution causes a cell to shrink and finally we have a hypotonic solution so this is an example where the extracellular fluid concentration is lower than the intracellular concentration and when you put a cell into a hypotonic solution you'll see that it's going to actually enlarge so it's going to take all that water and the water is going to move by osmosis into the cell and the cell will blow up and eventually burst so these changes in shape can drastically affect a cell's function so for instance when this red blood cell shrinks or shrivels it is no longer going to be as capable of binding and releasing oxygen additionally it's no longer going to be as capable of moving through capillaries and it might get caught on capillaries or it might actually get caught on other cells when it's blown up again can't fit through all those capillaries as it's moving through the body and eventually it might burst and in that case it's no longer an intact cell so these changes in shape can be very important so what we really want is an isotonic solution for our cells so now that was passive transport so we talked about three types of passive transport simple diffusion facilitated diffusion and osmosis now we're going to talk about two classes of active transport so those include what's called active transport as well as something called vesicular transport so for these two classes of active transport molecules can now be moved against their concentration gradient so that means that you can take a molecule and move it from a side of the membrane that has a load concentration you can actually move it to the side that has a higher concentration this means that it's going to require energy so this is going to allow you to move even larger molecules additionally it allows you to move polar molecules and it allows you most importantly to move them against their concentration gradients so here's an example so active transport that first type of active transport and in fact this is also called primary active transport this is going to require carrier proteins often these are called pumps so they are going to bind specifically and reversibly with substances that are being moved so what's shown here is a sodium potassium pump so it's going to be able to bind to both sodium and potassium and move both of those molecules against their concentration gradients so from low concentration too high because this is going really against physics this means that you have to put energy into these pumps so in this case this pump is an ATP Ace meaning it it is going to hydrolyze ATP it's going to use that energy from that phosphate bond in ATP to power the pumping of these ions from one side of the membrane to the other so the sodium potassium pump the reason it's chosen for this example is it's actually the most studied example of active transport it is vital to the function of all of your cells it moves sodium out of the cell and it moves potassium into the cell both ions are being moved against their concentration gradient so you're building up a lot of potassium inside the cell and you're building up a lot of sodium outside the cell and that's why it requires ATP the reason that it's found in all of your cells is that it's vital for maintaining what's called a resting membrane potential so I'll talk about what is a membrane potential so this is something vital for cell function uh it's a voltage so it's a a difference a separation of ions so you'll have maybe more positive ions outside the cell and more negative ions inside the cell and that's going to be what creates the membrane potential it's especially important for what we call electrical cells and these are nerve and muscle cells they're going to use those membrane potentials and changes in membrane potentials to trigger signals and to trigger contractions so in addition to that primary active transport we also have vesicular transport this is going to involve transport of large particles and fluids across the membrane and as the name implies the defining feature here is that it's going to use vesicles so these are membranous sacs so here we see uh some endocytosis happening this is transport of substances into the cell so we have substances coming from the outside buying it they're getting taken up here by this vesicle and moved into the cell so that movement into the cell in vesicles is called endocytosis on the bottom you can see the opposite is exocytosis so EXO means out and in this case those substances that were moved into the cell previously are now being released into the outside via these vesicles so it's transport out of the cell by vesicles we can break endocytosis that movement of substances into the cells in vesicles into three different types so we have phagocytosis this is endocytosis of large molecules or even microorganisms I think that's what's being shown here or some sort of large particle is being shown here we have pinocytosis this is endocytosis of fluids with very small solutes so phago is large Pinot is small but both of these are bringing substances into the cell in vesicles and then finally we have receptor-mediated endocytosis and this this is going to be substances are going to bind to specific receptors and then those receptors are going to trigger the import of those substances into the cell and so what makes this different from these other types is that these are going to be very specific substances binding to specific receptors that only respond to those substances whereas phagocytosis and pinocytosis are going to respond to a much broader range of substances now we can look at the opposite exocytosis movement out of the cell there are four major steps we have first that vesicle is going to move toward the plasma membrane then through some steps that we won't get into in this class it is going to fuse with that membrane so the vesicle fuses with the membrane shown down here next up a pore is going to open so you can see here that that pore is now opening so that the substances within the vesicle are now open to the outside and then the final step is that those substances are actually released to the other side of the membrane so the extracellular space so those are the four steps of exocytosis and I'll leave you with a question about membrane transport the maintenance of the resting membrane potential by the sodium potassium pump is an example of and I've listed here four different types of membrane transport