In Da Club – Membranes & Transport: Crash Course Biology #5

In Da Club – Membranes & Transport: Crash Course Biology #5

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.

100 Replies to “In Da Club – Membranes & Transport: Crash Course Biology #5”

  1. Active Transport: low concentration to high concentration. Requires energy.
    Passive transport: high concentration to low concentration. Requires no energy

  2. Hank, is it possible that there is an error about tonicity ? I learned that if the solute concentration is higher inside a cell than it is outside of the cell, then that solution is called hypotonic… what is the real answer ?

  3. whats the difference between whether something is charged, non polar and polar? how does this effect whether or not they can pass through the membrane with simple diffusion?

  4. I am a little confused here. So outside the cell(positive side of the cell) is packed of sodium, and by using the channel, it pumps three sodium from the inside of the cell to the outside, right. Now minutes later, Hank says that “those sodium ions waiting outside with this intense desire to get inside the cell.” Should it be the opposite?
    PLEASE HELP!!!!!!!!!!!!!

  5. Hypertonic means the amount of solute is higher outside the cell. That’s why water rushes out and the cell shrivels up? Unless you’re talking about the movement of solute and not water. Needs to be more specific

  6. Listen I know this is from SEVEN YEARS AGO but hank looks so sad 🙁 hope ur feelin' better Hank. Thanks for everything you do

  7. i heard you saying hypertonic is when the concentration of the solution is higer inside and its not it is out side

  8. Thanks to this video I got a 60% on my test. And that's a big plus for me cause I normally get a 40%. And I'm an A + student. This biology class is the first class I was failing. And I just found this video 4 am. So that's a big plus for me
    I know my next test I will score higher. Thanks so much. My teacher taught me nothing in a 3 hour class and I learned more in 10 minutes

  9. Thank you so much, you make learning so easy and fun. You guys have created something of extreme value, you should be very proud :))

  10. Isn't it the other way around? hypertonic is the shrivelled one and the hypotonic is the lysed full about to explode one because the hypo one is the one with a higher concentration on the inside…?

  11. Isn’t hypotonic when the cell is like about to explode because of too much solvent and then hypertonic is when the cell is shrinking ?

  12. i'm sorry but there is a factual inaccuracy in this video. David tennant is obviously the best doctor

  13. He just taught me more about what’s on my test tomorrow than my teacher has in the last two weeks…

  14. Brilliant, funny and absolutely educational. This is going to help with my understanding and upcoming nurses med exam. Well done Hank!! – Thank you ; )

  15. I'd love to read the 25,000 crabs book, but I cannot find it. Anyone catch the author's name?

  16. i think 3.8 m of people watch this cause they don’t understand the lecture of their teacher just like me :))()()

  17. Hank said that hypertonic solution has higher concentration inside the cell, while google says otherwise. I'm confused.

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