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Current time:0:00Total duration:11:19

Video transcript

oh hey I didn't see you up there hell are you and waiting in this line and then they were like 15 minutes and it's frickin freezing out here I mean whose banana you got a peel to get in this club well while we're here I guess this might not be a bad time to continue our discussion about cells because cells like nightclubs have to be selectively permeable they can only work if they let in the stuff that they need to make you know kick out the stuff that they don't need like trash and ridiculously drunk people than Justin Bieber fans no matter what stuff it is it has to pass through the cell membrane some things can pass really easily into cells and without a lot of help like water or oxygen but a lot of other things that they needs like sugar or their nutrients or signaling molecules or steroids well they can't get in or it'll take a really long time for them to do it yeah I can relate [Music] today we're gonna be talking about how substances move through cell membranes which is happening all the time including right now in me and right now when you and this is vital to all life because it's not just how cells acquire what they need and get rid of what they don't it's also how cells communicate with one another different materials have different ways of crossing the cell membrane and there are basically two categories of ways there's active transport and there's passive transport passive transport doesn't require any energy which is great because important things like oxygen and water can use this to get into cells really easily and they do this through what we call diffusion let's say I'm finally in this show and I'm in the show with my brother John some of you know my brother John and I love him but he uh he's not a big fan of people I mean he likes people he doesn't like like big crowds being parts of big crowds of people standing nearby him breathing on him touching him accidentally that sort of thing because John's with me at the show we're hanging out with all of our friends near the stage but then he starts moving further and further from the stage so he doesn't get a lunch of hipsters invading a space that's basically what diffusion is if everyone in the club were John Green they would try and and get as much space between all of them as possible until it was a uniform mass of John Green throughout the club when oxygen gets crowded it finds places that are less crowded it moves into those spaces when water gets crowded it does the same thing it moves to where there is less water when water does this across membrane it's a kind of diffusion called osmosis this is how your cells regulate their water content not only does this apply to water itself which as we've discussed is the world's best solvent you're going to learn more about water in our water episode it also works with water that contains dissolved materials or solutions like solutions of salt water or solutions of sugar water or booze which is just a solution of ethanol and water if the concentration of a solution is higher inside of a cell than it is outside of the cell then that solution is called hypertonic like Brauer thirst it's got everything packed into it and if the concentration inside of the cell is lower than outside of the cell it's called hypotonic which is sort of a sad version of hypertonic so like with Charlie Sheen we don't want the crazy manic Charlie Sheen we don't like the super sad depressed Charlie Sheen we want the in the middle Charlie Sheen who can just make us laugh and be happy and that is the state that water concentrations are constantly seeking it's called isotonic when the concentration is the same on both sides outside in it and this works in real life we can actually show it to you this box is filled of fresh water and we also have a sausage casing which is actually made out of cellulose and inside of that we have salt water we've dyed it so that you can see it move through the casing which is acting as our membrane this time-lapse shows how over a few hours the salt water diffuses into the pure water it'll keep diffusing until the concentration of salt in the water is the same inside the membrane as outside when water does this attempting to become isotonic it's called moving across its concentration gradient most of my cells right now are bathed in a solution that has the same concentration as inside of them and this is important for example if you took one of my red blood cells and put it in a glass of pure water it would be so hypertonic so much stuff would be in the cell compared to outside the cell that water would rush into the red blood cell and it would literally explode so we don't want that but if the concentration of my blood plasma were too high all the water would rush out of my cell and it would shrivel up and be useless that's why your kidneys are constantly on the job regulating the concentration of your blood plasma to keep it isotonic now water can permeate a cell membrane without any help but it's not actually particularly easy as we discussed in the last episode cell membranes are made out of phospholipids and the phospholipid bilayer is hydrophilic or water-loving on the outside and hydrophobic or water hating on the inside so what are molecules have a hard time passing through these layers because they get stuck at the nonpolar hydrophobic core that is where the channel proteins come in they allow passage of stuff like water and ions without using any energy they straddle the width of the membrane and inside they have channels that are hydrophilic which draws the water through the proteins that are specifically for channeling water are called aquaporins each one can pass 3 billion water molecules a second makes me have to pee just thinking about it things like oxygen and water that cells need constantly they can get into the cell without any energy necessary but most chemicals they use what's called active transport this is especially useful if you want to move something in the opposite direction of its concentration gradient from a low concentration to a high concentration so say we're back at that show and I'm keeping company with John who's being all antisocial in his polite and charming way but after half a beer and an argument about who's the best doctor who I want to get back to my friends across crowded bar so I've transport myself against the concentration gradient of human spending a lot of energy dodging stomping feet throwing an elbow to get to them that is high energy transport in a Cell getting the energy necessary to do pretty much anything including moving something the wrong direction across its concentration gradient requires ATP ATP adenosine triphosphate you just want to replay that over and over again until it rolls off the tongue because it's one of the most important chemicals that you will ever ever ever hear about adenosine triphosphate ATP if our bodies were America ATP would be credit cards it's such an important form of information currency that we're going to do an entire separate episode about it which will be here I'll go into the wrong direction but it'll be here when we've done it but for now here's what you need to know when a cell requires active transport it basically has to pay a fee in the form of ATP to a transport protein a particularly important kind of frickin sweet transport protein is called the sodium potassium pump most cells have them but they're especially vital to cells that need lots of energy like muscle cells and brain cells ha ha biography it's my favorite part Oh sodium potassium pump was discovered in the 1950s by a Danish medical doctor named yen's Christian school who was studying how anesthetics work on membranes he noticed that there was a protein in cell membranes that could pump sodium out of a cell and the way he got to know this bump was by studying the nerves of crabs because crab nerves are huge compared to humans nerves and are easier to dissect and observe but crabs are still small so he needed a lot of them he struck a deal with a local fisherman and over the years studied approximately 25,000 crabs each of which he boiled to study their fresh nerve fibers he published his findings on the sodium potassium pump in 1957 and in the meantime became known for the distinct odor that filled the halls of the Department of physiology at the University where he worked 40 years after making his discovery school was awarded the Nobel Prize in Chemistry and here's what he taught us turns out these pumps work against two gradients at the same time one is the concentration gradient and the other is the electric chemical gradient that's the difference in electrical charge on either side of a cell's membrane so then ourselves at school is studying like nerve cells in your brain typically have a negative charge inside relative to the outside they also usually have a low concentration of sodium ions inside the pump works against both of these conditions collecting three positively charged sodium ions and pushing them out into the positively charged sodium ion rich environment to get the energy to do this the protein pump breaks up a molecule of ATP ATP adenosine triphosphate an adenosine molecule with three phosphate groups attached to it so when ATP connects with a protein pump an enzyme breaks the covalent bond on one of those phosphates in a burst of excitement and energy the split releases enough energy to change the shape of the pump so that it opens outward and releases three sodium ions this new shape also makes it a good fit for potassium ions that are outside the cells so it upon plots two of those in so what you end up with us a nerve cell that is literally and metaphorically charged it has all those sodium ions waiting outside with this intense desire to get inside of the cell and when something triggers the nerve cell it lets all those in and that gives the nerve cell a bunch of electrochemical energy which you can then use to help you feel things or touch or smell or taste or have a thought there is still yet another way that stuff gets inside of cells and this also requires energy it's also a form of active transport it's called vesicular transport and the heavy lifting is done by vesicles which are tiny sacs made of phospholipids just like the cell membrane this kind of active transport is also called cytosis from the Greek for cell action when vesicles transport materials outside of a cell it's called exocytosis or outside cell action a great example of this is going on in your brain right now it's how your nerve cells release neurotransmitters you've heard of neurotransmitters they're very important in helping you feel different ways like dopamine and serotonin after neurotransmitters are synthesized and packaged into vesicles they're transported until the vesicle reaches the membrane when that happens the two bilayers rearrange so that they fuse and then the neurotransmitter spills out and now I remember where I left my keys I'll just play that process in Reverse and you'll see how material gets inside the cell and that's endocytosis there are three different ways that this happens my personal favorite is phagocytosis and the awesome that begins with the fact that that name itself means devouring cell action check this out so this particle outside here is some kind of dangerous bacterium in your body and this is a white blood cell chemical receptors on the blood cell membrane detective punk invader and attach to it actually reaching out around it and engulfing it then the membrane forms a vesicle to carry it inside where it lays a total unholy beat down on it with enzymes and other cool weapons pinocytosis or drinking action is very similar to phagocytosis except instead of surrounding whole particles it just surrounds things that have already been dissolved here the membrane just folds in a little to form the beginning of a channel and then pinches off to form a vesicle that holds the fluid most of your cells are doing this right now because it's how our cells absorb nutrients what if a cell needs something that only occurs in very small concentrations that's when cells use clusters of specialized receptor proteins in the membrane that form a vesicle when receptors connect with the molecule that they're looking for for example your cells have specialized cholesterol receptors that allow you to absorb cholesterol if those receptors don't work which can happen with some genetic conditions cholesterol is left to flow it around in your blood and eventually causes heart disease so that's just one of many reasons to appreciate what's called receptor mediated endocytosis
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