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

Summary

This video script delves into the fascinating world of cell membranes and their selective permeability, akin to exclusive nightclubs. It explains the essential processes of substance transport across cell membranes, highlighting the critical roles of passive and active transport mechanisms. The script uses relatable analogies, such as a crowded club, to elucidate concepts like diffusion, osmosis, and the significance of maintaining isotonic conditions for cell health. It further explores the role of ATP in active transport, introduces the sodium-potassium pump, and describes vesicular transport through endocytosis and exocytosis, illustrating how cells efficiently regulate their internal environments.

Takeaways

• 🧬 Cells, like nightclubs, must selectively allow substances to pass through their membranes to maintain their function and health.
• 💧 Passive transport, such as diffusion, does not require energy and allows substances like oxygen and water to move into cells easily.
• 🌀 Diffusion is the process where particles move from areas of high concentration to areas of low concentration to achieve equilibrium.
• 🚰 Osmosis is a type of diffusion involving water moving across a membrane to balance concentrations on both sides.
• 🌊 Hypertonic, hypotonic, and isotonic solutions describe the concentration of solutes relative to the cell's interior, affecting water movement.
• 🩸 The human body, particularly the kidneys, must maintain isotonic conditions to prevent cells from exploding or shriveling.
• 🔋 Active transport requires energy, typically in the form of ATP, to move substances against their concentration gradient.
• 🔌 The sodium-potassium pump is a vital active transport protein that maintains the electrochemical gradient across cell membranes.
• 🦀 The discovery of the sodium-potassium pump by Jens Christian Skou involved studying crab nerves and earned him a Nobel Prize.
• 🛅 Vesicular transport, including exocytosis and endocytosis, is another form of active transport that involves the movement of substances in vesicles.
• 🦠 Phagocytosis, pinocytosis, and receptor-mediated endocytosis are methods of endocytosis that cells use to engulf and internalize substances.

Q & A

What is the concept of selective permeability in relation to cells and nightclubs as mentioned in the script?

Selective permeability refers to the ability of cells to allow certain substances to pass through their membranes while preventing others. The analogy to nightclubs is that, like cells, nightclubs only let in certain people and keep out others, such as those who are not of age or not on the guest list.

What are the two main categories of substance movement across cell membranes discussed in the script?

The two main categories are active transport and passive transport. Active transport requires energy to move substances against their concentration gradient, while passive transport allows substances to move along their concentration gradient without energy.

Can you explain the process of diffusion as it relates to the script's analogy of John Green at the show?

Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration until a uniform distribution is achieved. In the script, John Green's discomfort with crowds is likened to particles moving away from areas of high concentration (crowded spaces) to less crowded areas.

What is osmosis and how does it relate to the regulation of water content in cells?

Osmosis is a type of diffusion that specifically involves water molecules moving across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. It is crucial for regulating the water content in cells to maintain a balance and prevent cells from bursting or shriveling.

What are the terms hypertonic, hypotonic, and isotonic, and how do they describe the concentration gradients across cell membranes?

Hypertonic refers to a higher solute concentration inside the cell compared to the outside. Hypotonic is the opposite, with a lower solute concentration inside the cell. Isotonic describes a situation where the solute concentration is the same on both sides of the cell membrane, which is the ideal state for maintaining cell integrity.

How do aquaporins assist in the movement of water across the cell membrane?

Aquaporins are channel proteins that facilitate the passage of water molecules through the cell membrane. They have hydrophilic channels that allow water to pass through the hydrophobic core of the phospholipid bilayer, with each aquaporin capable of passing billions of water molecules per second.

What is active transport and why is ATP necessary for this process?

Active transport is the movement of substances against their concentration gradient, from an area of lower concentration to an area of higher concentration. ATP (adenosine tri-phosphate) is required because it provides the energy needed for this process, acting as a universal energy currency within cells.

Can you describe the function of the sodium-potassium pump and its significance in cells like muscle and brain cells?

The sodium-potassium pump is a transport protein that actively moves sodium ions out of the cell and potassium ions into the cell, against their respective concentration and electrochemical gradients. This is vital for cells that require high energy, such as muscle and brain cells, as it helps maintain the electrochemical gradient necessary for nerve impulses and muscle contractions.

Who discovered the sodium-potassium pump and how did he come to his discovery?

Jens Christian Skou, a Danish medical doctor, discovered the sodium-potassium pump in the 1950s. He studied the nerves of crabs, which are larger and easier to dissect than human nerves, and noticed a protein that could pump sodium out of cells. His discovery was published in 1957 and later earned him the Nobel Prize in Chemistry.

What is vesicular transport and how does it differ from other forms of active transport?

Vesicular transport, also known as cytosis, involves the use of vesicles—tiny sacs made of phospholipids—to transport substances across the cell membrane. It differs from other forms of active transport as it can move large particles or even whole cells, and it can transport substances both into and out of the cell through processes like exocytosis and endocytosis.

Can you explain the process of endocytosis and its different types as mentioned in the script?

Endocytosis is the process by which cells transport substances into the cell by engulfing them with the cell membrane to form vesicles. The script mentions three types: phagocytosis, where the cell engulfs large particles or bacteria; pinocytosis, where the cell absorbs dissolved substances in fluid; and receptor-mediated endocytosis, where specialized receptor proteins on the cell membrane form vesicles to transport specific molecules.

Outlines

00:00

🌡️ Cell Membrane Permeability and Transport Mechanisms

This paragraph introduces the concept of cell membrane permeability, comparing it to the selective entry policy of a nightclub. It explains that cells, like clubs, allow only necessary substances to pass through their membranes. The narrator uses humor to relate to the audience by discussing the challenges of getting into a club in cold weather. The main topic of substances moving through cell membranes is introduced, highlighting its importance for life, as it is not only about acquiring necessary substances and eliminating waste but also about cell communication. The paragraph outlines two primary transport methods: active transport, which requires energy, and passive transport, which does not. It also introduces the concept of diffusion and uses the analogy of a person moving away from a crowd to explain how substances spread out from areas of high concentration to areas of low concentration. The paragraph concludes with an introduction to osmosis, the specific type of diffusion involving water across a membrane, and how cells regulate their water content through isotonic, hypertonic, and hypotonic conditions.

05:09

🔋 Active Transport and the Role of ATP

The second paragraph delves into the process of active transport, which requires energy to move substances against their concentration gradient. It uses the analogy of navigating through a crowded bar to describe the energy expenditure involved in this process. The importance of ATP (adenosine tri-phosphate) is emphasized, likening it to credit cards in the economy of the body. The paragraph introduces the sodium-potassium pump, a crucial transport protein for cells with high energy demands, and tells the story of its discoverer, Jens Christian Skou. The explanation includes how the pump works against both concentration and electrochemical gradients, utilizing the energy from ATP to maintain the necessary balance of ions across the cell membrane. The paragraph also touches on vesicular transport, or cytosis, which includes exocytosis for releasing substances outside the cell and endocytosis for internalizing materials, setting the stage for a deeper exploration in the following paragraph.

10:11

🦠 Endocytosis: Phagocytosis, Pinocytosis, and Receptor-Mediated Endocytosis

The final paragraph focuses on the different types of endocytosis, a form of active transport where substances are brought into the cell within vesicles. Phagocytosis is described as 'devouring cell action,' where a cell engulfs and destroys invaders like bacteria. Pinocytosis, or 'drinking action,' is highlighted as the process by which cells absorb dissolved nutrients. Lastly, receptor-mediated endocytosis is explained as a specialized mechanism for absorbing substances present in low concentrations, using cholesterol receptors as an example. The paragraph wraps up with a review invitation, encouraging viewers to revisit certain sections of the video for clarification and to engage with the content creators on social media platforms.

Keywords

💡Selective Permeability

Selective permeability refers to the ability of a cell membrane to allow certain substances to pass through while blocking others. This is crucial for maintaining the cell's internal environment and is a central theme in the video. For example, the video likens a nightclub's selective entry to a cell's selective permeability, emphasizing the importance of letting in necessary substances while keeping out unwanted ones.

💡Cell Membrane

The cell membrane, also known as the plasma membrane, is a biological barrier that separates the cell's interior from its external environment and controls the passage of substances in and out of the cell. In the script, the cell membrane is the focus for understanding how substances move into and out of cells, with various mechanisms described to illustrate this process.

💡Passive Transport

Passive transport is the movement of substances across the cell membrane without the use of cellular energy, typically from an area of higher concentration to an area of lower concentration. The video script uses the analogy of a person moving away from a crowded area to explain diffusion, a type of passive transport, where substances like oxygen and water move into cells without energy expenditure.

💡Diffusion

Diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration until equilibrium is reached. The video script describes diffusion as the natural spreading out of individuals in a crowded club, analogous to how oxygen and water move into cells through the cell membrane.

💡Osmosis

Osmosis is a specific type of diffusion that involves the movement of water molecules across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. The video script uses the example of water moving across a membrane to explain how cells regulate their water content and the concept of isotonic, hypertonic, and hypotonic conditions.

💡Isotonic

An isotonic solution has the same solute concentration on both sides of a membrane, preventing net movement of water across the membrane. The video script explains that cells strive to maintain an isotonic state to prevent damage from water influx or efflux, which could cause cells to explode or shrivel.

💡Aquaporins

Aquaporins are channel proteins that facilitate the movement of water molecules across the cell membrane. The script highlights aquaporins as essential for water transport, noting that each aquaporin can pass billions of water molecules per second, illustrating their importance in cellular processes.

💡Active Transport

Active transport is the movement of substances across the cell membrane against their concentration gradient, from an area of lower concentration to an area of higher concentration, requiring energy. The video script uses the analogy of a person pushing through a crowded bar to explain the concept of active transport, which is essential for moving substances against their natural tendency to diffuse.

💡ATP (Adenosine Triphosphate)

ATP, or adenosine triphosphate, is the primary energy currency of the cell, used to power various cellular processes, including active transport. The video script likens ATP to credit cards, emphasizing its importance as a form of energy storage and transfer within the cell.

💡Sodium-Potassium Pump

The sodium-potassium pump is a vital transport protein that maintains the electrochemical gradient across the cell membrane by pumping sodium ions out of the cell and potassium ions into the cell, using energy from ATP. The video script discusses the discovery of this pump and its role in creating the conditions necessary for nerve cells to function.

💡Vesicular Transport

Vesicular transport, also known as cytosis, involves the movement of substances within the cell using vesicles, small sacs made of phospholipids. The video script describes two types of vesicular transport: exocytosis, where substances are expelled from the cell, and endocytosis, where substances are taken into the cell, including phagocytosis and pinocytosis.

💡Phagocytosis

Phagocytosis is a type of endocytosis where the cell engulfs large particles, such as bacteria, forming a vesicle around it for digestion. The video script uses the example of a white blood cell engulfing a bacterium to illustrate how phagocytosis helps the immune system.

Highlights

Cells, like nightclubs, are selectively permeable, only letting in what they need and kicking out what they don't.

Water and oxygen can pass easily into cells through passive transport, a process that requires no energy.

Diffusion is when molecules move from an area of high concentration to low concentration, similar to how people spread out in a crowded room.

Osmosis is a type of diffusion where water moves across a membrane to balance concentrations.

Hypertonic, hypotonic, and isotonic solutions describe different concentration gradients across a cell membrane.

The sodium-potassium pump is essential for nerve cells, moving ions against concentration and electrochemical gradients using ATP.

Vesicular transport, or cytosis, moves large materials across the cell membrane using vesicles.

Phagocytosis is a form of cytosis where cells engulf and destroy harmful particles, like bacteria.

Pinocytosis is a similar process, but it involves the ingestion of fluids rather than large particles.

Receptor-mediated endocytosis allows cells to absorb specific molecules, like cholesterol, using specialized receptors.

Channel proteins, like aquaporins, facilitate the passive transport of water through the cell membrane.

Active transport requires energy in the form of ATP to move substances against their concentration gradient.

The phospholipid bilayer of cell membranes is hydrophilic on the outside and hydrophobic on the inside, creating a barrier for water.

Sodium-potassium pumps were discovered by Jens Christian Skou, who studied nerve cells in crabs.

The balance of sodium and potassium ions across a cell membrane is crucial for nerve function and communication.

Transcripts

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, you know, 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 and without a lot of help, like water or oxygen.

But a lot of other things that they need, like sugar, other nutrients, or 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.

[Theme Music]

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 solutions of salt water, or solutions of sugar water, or booze, which is just a solution of ethanol in 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 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 he 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 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, 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, 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 the non-polar, 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 channelling 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 Doctor 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 its 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, (uh I ended up going to the wrong direction but it 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.

[ Bio-lography Music]

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 the 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,

so when ATP connects with the protein pump, an enzyme breaks the covalent bond on 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 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 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, the two bilayers rearrange so that they fuse.

And 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 kind of 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 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.

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 uh 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.