oh, hey! i didn't see you up there. how long have you been waiting in this line? i've been here for like 15 minutes and it'sfreaking freezing out here i mean, whose banana do you gotta peel inorder to get into this club?
crash diets for models, well, while we're here i guess this mightnot 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 stuffthat they need and they kick out the stuff that they don't need
like trash and ridiculously drunk people and justin bieber fans. no matter what stuff it is it has to passthrough the cell's membrane. some things can pass really easily into cellswithout a lot of help, like water or oxygen. but a lot of other things that they need,like sugar, other nutrients, signaling molecules or steroids they can't get in or it will take a reallylong time for them to do it. yeah. i can relate. today we're going to be talking about howsubstances move through cell membranes, which
is happening all the time, including rightnow, in me and right now, in you. and this is vital to all life, because it'snot just how cells acquire what they need and get rid of what they don't, it's alsohow cells communicate with one another. different materials have different ways ofcrossing the cell membrane. and there are basically two categories of ways: there'sactive 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 intocells 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. someof 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 big crowds. being parts of big crowds and people standingnearby him, breathing on him, touching him accidentally and that sort of thing because john's with me at the show, we'rehanging out with all of our friends near the stage. but then he starts moving further andfurther from the stage so he doesn't get
a bunch of hipsters invading his space. that's basically what diffusion is. if everyonein the club were john green they would try and get as much space between all of themas possible until it was a uniform mass of john greens throughout the club. when oxygen gets crowded, it finds placesthat are less crowded and moves into those spaces. when water gets crowded, it does the samething and moves to where there is less water. when water does this across a membrane, it'sa 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 inour water episode. it also works with water that contains dissolvedmaterials, or solutions, like salt water, or sugar water, or booze, which is just asolution of ethanol in water. if the concentration of a solution is higherinside a cell than it is outside of the cell, then that solution is called hypertonic like powerthirst, it's got everything packedinto it! and if the concentration inside of the cellis lower than outside of the cell, it's called hypotonic. which is sort of a sad version of hypertonic.
like with charlie sheen: we don't want thecrazy, manic charlie sheen and we don't like the super sad, depressed charlie sheen. we want the "in the middle" charlie sheenwho can just make us laugh and be happy. and that is the state that water concentrationsare constantly seeking. it's called isotonic. when the concentration is the same on bothsides, outside and in. and this works in real life! we can actuallyshow it to you. this vase is full of fresh water. and we alsohave a sausage casing, which is actually made of cellulose, and inside of that we have saltwater. we've dyed it so that you can see it movethrough 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 concentrationof salt in the water is the same inside the membrane as outside. when water does this, attempting to becomeisotonic, it's called moving across it's concentration gradient. most of my cells right now are bathed in asolution that has the same concentration as inside of them, and this is important. for example, if you took one of my red bloodcells and put it in a glass of pure water,
it would be so hypertonic so much stuff would be in the cell comparedto outside the cell that water would rush into the red blood celland it would literally explode. so, we don't want that! but if the concentration of my blood plasmawere too high, water would rush out of my cell, and it would shrivel up and be useless. that's why your kidneys are constantly onthe job, regulating the concentration of your blood plasma to keep it isotonic. now, water can permeate a membrane withoutany help, but it's not particularly easy.
as we discussed in the last episode, somemembranes are made out of phospholipids, and the phospholipid bilayer is hydrophilic, orwater-loving, on the outside and hydrophobic, or water-hating, on the inside. so water molecules have a hard time passingthrough these layers because they get stuck at that 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 straddlethe width of the membrane and inside they have channels that are hydrophilic, whichdraws the water through. the proteins that are specifically for channelingwater are called aquaporins, and each one
can pass 3 billion water molecules a second! it makes me have to pee just thinking aboutit. things like oxygen and water, that cells needconstantly, they can get into the cell without any energy necessary but most chemicals use what's called activetransport. this is especially useful if you want to movesomething in the opposite direction of its concentration gradient, from a low concentrationto a high concentration. so, say we're back at that show, and i'mkeeping company with john who's being all antisocial in his polite and charming way,but after half a beer and an argument about
who the was the best dr. who. i want to getback to my friends across the crowded bar. so i transport myself against the concentrationgradient of humans, 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 todo pretty much anything, including moving something the wrong direction across it'sconcentration gradient, requires atp. atp or adenosine tri-phosphate you just want to replay that over and overagain until it just rolls off the tongue because it's one of the most important chemicals thatyou will ever, ever ever hear about. adenosine tri-phosphate, atp.
if our bodies were america, atp would be creditcards it's such an important form of information currency that we're going to do an entireseparate episode about it, which will 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 ofatp, to a transport protein. a particularly important kind of freakin' sweet transportprotein is called the sodium-potassium pump. most cells have them, but they're especiallyvital to cells that need lots of energy, like muscle cells and brain cells. oh! biolo-graphy! it's my favorite part ofthe show.
the sodium-potassium pump was discovered inthe 1950s by a danish medical doctor named jens christian skou, who was studying howanesthetics work on membranes. he noticed that there was a protein in cell membranesthat could pump sodium out of a cell. and the way he got to know this pump was by studyingthe nerves of crabs, because crab nerves are huge compared to humans' nerves and areeasier to dissect and observe. but crabs are still small, so he needed a lot of them. hestruck a deal with a local fisherman and, over the years, studied approximately 25,000crabs, each of which he boiled to study their fresh nerve fibers. he published his findingson the sodium-potassium pump in 1957 and in the meantime became known for the distinctodor that filled the halls of the department
of physiology at the university where he worked.forty years after making his discovery, skou was awarded the nobel prize in chemistry. and here's what he taught us: turns out these pumps work against two gradientsat the same time. one is the concentration gradient, and the other is an electrochemicalgradient. that's the difference in electrical charge on either side of a cell's membrane.so the nerve cells that skou was studying, like the nerve cells in your brain, typicallyhave a negative charge inside relative to the outside. they also usually have a lowconcentration of sodium ions inside. the pump works against both of these conditions,collecting three positively-charged sodium
ions and pushing them out into the positivelycharged, sodium ion-rich environment. to get the energy to do this, the proteinpump breaks up a molecule of atp. atp, adenosine tri-phosphate, is an adenosinemolecule with three phosphate groups attached to it, but when atp connects with the proteinpump, an enzyme breaks the covalent bond of one of those phosphates in a burst of excitementand energy. this split releases enough energy to change the shape of the pump so it "opens"outward and releases the three sodium ions. this new shape also makes it a good fit forpotassium ions that are outside the cell, so the pump lets two of those in.so what you end up with is a nerve cell that is literally and metaphorically charged.
it has all those sodium ions waiting outsidewith this intense desire to get inside of the cell. and when something triggers thenerve cell, it lets all of those in. and that gives the nerve cell a bunch of electrochemicalenergy which it can then use to let you feel things, or touch, or smell, or taste, or havea thought. there is still yet another way that stuffgets 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 tinysacs made of phospholipids just like the cell membrane. this kind of active transport is also calledcytosis, from the greek for "cell action" when vesicles transport materials outsideof a cell it's called exocytosis, or outside
cell action. a great example of this is goingon in your brain right now. it's how your nerve cells release neurotransmitters. you've heard of neurotransmitters. they arevery important in helping you feel different ways. like dopamine and serotonin. after neurotransmitters are synthesized andpackaged into vesicles, they're transported until the vesicle reaches the membrane. whenthat happens, their two bilayers rearrange so that they fuse. then the neurotransmitterspills out and -- now i remember where i left my keys! now just play that process in reverse andyou'll see how material gets inside a cell. that's endocytosis. there are three differentways that this happens. my personal favorite
is phagocytosis, and the awesome there beginswith the fact that that name itself means devouring cell action! check this out. so this particle outside hereis some dangerous bacterium in your body. and this is a white blood cell. chemical receptorson the blood cell membrane detect this punk invader and attach to it, actually reachingout around it and engulfing it. then the membrane forms a vesicle to carry it inside, whereit lays a total, unholy beatdown on it with enzymes and other cool weapons. pinocytosis, or drinking action, is very similarto phagocytosis, except instead of surrounding whole particles, it surrounds things thathave already been dissolved. here the membrane
just folds in a little to form the beginningof a channel and then pinches off to form a vesicle that holds the fluid. most of yourcells are doing this right now, because it's how our cells absorb nutrients. but what if a cell needs something that onlyoccurs in very small concentrations? that's when cells use clusters of specialized receptorproteins in the membrane that form a vesicle when receptors connect with the molecule thatthey're looking for. for example, your cells have specialized cholesterol receptors thatallow you to absorb cholesterol; if those receptors don't work, which can happen withsome genetic conditions, cholesterol is left to float around in your blood and eventuallycauses heart disease. so that's just one of
many reasons to appreciate what's calledreceptor-mediated endocytosis. ah! hey, glad you made it in too! now comes review time. you can click on anyof these links and go back to the part of the video where i talk about that thing ifyou are at all confused. and you may be. this is totally, pretty complicatedstuff we're dealing with right now, so you just go ahead and watch all that. and if you have any questions, of course,we'll be down below in the comments and on twitter and facebook as well and we'll seeyou next time.
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