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  • 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's freaking freezing out here

  • I mean, whose banana do you gotta peel in order to get into 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 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 pass through the cell's membrane.

  • Some things can pass really easily into cells without 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 really long time for them to do it.

  • Yeah. I can relate.

  • Today we're going to be talking about how substances move through cell membranes, which

  • is happening all the time, including right now, in me

  • and right now, in 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 big crowds.

  • Being parts of big crowds and people standing nearby him, breathing on him, touching him

  • accidentally and 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 bunch of hipsters invading his space.

  • That's basically what diffusion is. If everyone in the club were John Green they would try

  • and get as much space between all of them as possible until it was a uniform mass of

  • John Greens throughout the club.

  • When oxygen gets crowded, it finds places that are less crowded and moves into those spaces.

  • When water gets crowded, it does the same thing and moves to where there is less water.

  • When water does this across a 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 salt water,

  • or sugar water, or booze, which is just a solution of ethanol in water.

  • If the concentration of a solution is higher inside a cell than it is outside of the cell,

  • then that solution is called hypertonic

  • Like Powerthirst, 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.

  • Like with Charlie Sheen: we don't want the crazy, manic Charlie Sheen and 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 and in.

  • And this works in real life! We can actually show it to you.

  • This vase is full of fresh water. And we also have a sausage casing, which is actually made

  • 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 it's 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, 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 membrane without any help, but it's not particularly easy.

  • As we discussed in the last episode, some 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 water molecules have a hard time passing through 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 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, and each one

  • can pass 3 billion water molecules a second!

  • It 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 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 the was the best Dr. Who. I want to get back to my friends across the crowded bar.

  • So I transport myself against the concentration gradient 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 to do pretty much anything, including moving

  • something the wrong direction across it's concentration gradient, requires ATP.

  • ATP or adenosine tri-phosphate

  • You just want to replay that over and over again until it just rolls off the tongue because

  • it's one of the most important chemicals that you will ever, ever ever hear about.

  • Adenosine tri-phosphate, 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,

  • 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 freakin' 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.

  • Oh! Biolo-graphy! It's my favorite part of the show.

  • The sodium-potassium pump was discovered in the 1950s by a Danish medical doctor named

  • Jens Christian Skou, 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 pump 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. 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 gradients at the same time. One is the concentration

  • gradient, and the other is an electrochemical gradient. 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, 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 tri-phosphate, is an adenosine molecule with three phosphate groups attached

  • to it, but when ATP connects with the protein pump, an enzyme breaks the covalent bond of

  • one of those phosphates in a burst of excitement and 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 for potassium 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 outside with this intense desire to get inside of

  • the cell. And when something triggers the nerve cell, it lets all of those in.

  • And that gives the nerve cell a bunch of electrochemical energy which it can then use to let 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 are 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, their two bilayers rearrange

  • so that they fuse. Then the neurotransmitter spills out and -- now I remember where I left my keys!

  • Now just play that process in reverse and you'll see how material gets inside a cell.

  • That's endocytosis. There are three different ways that this happens. My personal favorite

  • is phagocytosis, and the awesome there begins with the fact that that name itself means

  • DEVOURING CELL ACTION!

  • Check this out. So this particle outside here is some dangerous bacterium in your body.

  • And this is a white blood cell. Chemical receptors on the blood cell membrane detect this 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 beatdown on it with

  • enzymes and other cool weapons.

  • Pinocytosis, or drinking action, is very simIlar to phagocytosis, except instead of surrounding

  • whole particles, it 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.

  • But 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 float 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.

  • Ah! Hey, glad you made it in too!

  • Now comes review time. You can click on any of these links and go back to the part of

  • the video where I talk about that thing if you are at all confused.

  • And you may be. This is totally, pretty complicated stuff 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 see you next time.

Oh, hey!

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