[In a dramatic voice] Somewhere deep in the mountains sits a castle, protected by a great wall.

Atop the wall sits a vigilant gatekeeper. But what’s that on the horizon?

A rider approaches! “Who goes there, friend or foe!” See, the gatekeeper’s job is to keep out anyone that might harm the realm, while also allowing free passage to the castle’s loyal subjects. They probably should let in the baker and the blacksmith, since the castle’s gonna need food and armor, but that group of bandits has to stay out. “Back off our potato stockpiles!” [As Dr. Sammy] This might sound like a fairy tale, but a similar struggle is actually happening every day–inside of you.

Okay, so it’s not exactly a tussle over potatoes, but our cells are kind of like castles. And the cell membrane, forming the outer limits and acting as a barrier, is both the wall and the gatekeeper. The cell membrane, in other words, controls who gets in and who gets out.

And that’s an important job because our lives depend on the right stuff at the right times going into and out of our cellular castles. Hi! I'm Dr.

Sammy, your friendly neighborhood entomologist, and this is Crash Course Biology. Hey there, let down the drawbridge, we need to let this theme music in. [Music] Much like a castle without a gatekeeper, our cells would be in trouble without a membrane. Like, the cells lining our digestive system have to let in nutrients from the food that we eat so that we can grow and survive, but there. are also things with restricted access.

Likewise, when things need to get out, the membrane shows them the door. Well, it kind of is the door. This happens, for example, when our liver cells make urea, which needs to be excreted, first from the cells and then from us as the major component of our urine.

Trust me, that’s not something you want just hanging out in your body; if too much of it builds up it can be toxic. So it’s really important that the membrane opens the door both to let things in and to kick them out. Now, surrounding our cellular castle wall, we find the enchanted forest of the extracellular matrix.

Here, the tangled web of fibers holds neighboring cells together and helps them maintain their shape. Made of proteins, carbohydrate chains, and water, the matrix of each one of our organs is unique to its function. Like, our beating hearts need a matrix that supports the stress of repeated pulsations.

That comes in handy when you need to slay a dragon, or — even worse — when you are smitten by a charismatic squire across the room at a grand feast. Our brain also needs a matrix that protects nerve cells and helps them develop properly. But sadly, even the best nerve cells can’t make us good conversationalists when the sweaty palms and cute squire jitters hit.

Trust me, I’ve been there! Okay, so, you’ve been playing Dungeons & Dragons in your mind this whole time — picturing this magical village, and a castle with a high wall, etc. But remember, we’re talking about the architecture of cells.

Your body has many trillions of them. So there are really many trillions of magical villages that make you, well, you. So, each cell within the forest of the cell matrix has its very own cell membrane guarding the gates.

The membrane is made up of a phospholipid bilayer, which, I know, is a great band name. It’s a term that just rolls off the tongue? Phosopholipid bilayer.

Yeah…lemme break it down to ya though. So, there’s this small molecule of glycerol, which is a type of naturally occurring alcohol that helps the body run. And this little molecule attaches to one phosphate group and two fatty acids, or lipids.

These are very common types of molecules that are present throughout all kinds of organisms. And they make a good team because the phosphate section of the wall is a charged ion, kind of like table salt, that dissolves in water. And the fatty acids are lipids, with long carbon chains, so they’re like oils that don’t mix with water.

So these molecules arrange themselves based on how they react to water. Water-hating lipid tails have a problem: they’re surrounded by water, both inside and outside the cell. So the lipids will group together and use the water-loving heads as a shield.

Two of them layer together to make a bilayer. Basically, they’re like two lines of knights lining up back to back, the heads facing outward on both sides protecting the tails from their dreaded enemy, water. These days, we can use powerful microscopes to look at the cell membrane and be like: "yup, tails to tails." But before that, researchers had to get pretty clever to even begin to imagine what it might look like inside a cell.

So it’s thanks to modern technology and years and years of research unpacking the cell membrane, that we can now visualize its structure in such intricate detail. And we’ve learned that the cell membrane isn’t just phospholipids. For example, there are proteins that make up 50% of some membranes by weight.

Dotting the lipid membrane like the contrasting stones in a mosaic, these proteins move around in the fluid cell surface, playing a variety of different roles. In fact, scientists have built a whole model, called the fluid mosaic model, around this idea. It helps biologists to imagine the phospholipid bilayer as the dynamic structure it really is.

Because when I say dynamic, I’m talking. DYNAMIC. I mean, there are tons of different types of membrane proteins.

And all of them help the cell membrane carry out its work. These proteins bob along like ships on the wriggling sea of membranes. Let’s start a new campaign over in the Thought Bubble and meet the new party… Acting like the flag that identifies a ship are the carbohydrate-linked glycoproteins.

Another good band name. These friendly flags waving on the surface of our cells help our immune system distinguish our own cells from harmful invaders and help our cells communicate with each other. Communication is vital at sea, and among cells.

So, receptor proteins act like lamplight signals. When a chemical activates the part of the protein on the outside of the cell, it begins a chain reaction inside that can tell the cell to start or stop specific processes. Over in the Venetian canals of the membrane world are transport proteins, which open and close, helping the membrane to control what goes in and out at a given time.

Not today, Blackbeard! And sometimes we need a convoy of cells that line up closely, like the ones keeping all the harsh digestive stuff inside our stomachs. Junction proteins work like ropes to the neighboring ship, linking proteins on nearby cell membranes.

Finally, some membrane proteins help chemical reactions along. Like the combustion reaction of a ship’s cannon fire, an enzyme turns one chemical into another. And membranes can line up a bunch of enzymes close together, pulling off a whole series of quick chemical reactions.

And all of these proteins, swabbing the old poop deck together, make up the fluid mosaic model, helping us appreciate all the hands that are on deck in a cell membrane. Thanks, Thought Bubble! You might think moving stuff in and out of a cell takes a lot of work.

And sometimes it does, but not always! Some things move by passive transport – which means they just move, thanks to physics. Like, sitting a ball down on a hill and letting gravity do its thing.

The transported material needs to be able to mix with whatever it’s passing through. So nonpolar, water-hating substances might go directly through the membrane, but water-loving things need a protein transporter. And when we say water-loving, we mean they dissolve in water.

Those dissolved substances usually try to spread out as much as possible. Like how the smell of baking bread seems to float down the hall from the kitchen to find you in your chambers. The scent of the bread comes from delicious-smelling airborne molecules filling up the kitchen, and then diffusing through the fluid medium that is the air into the rest of the castle.

So, diffusion is what happens when a chemical moves from where it’s more concentrated to where it’s less concentrated. And diffusion happens in our cells, too. This is a common example of passive transport - no energy is needed to get diffusion to happen.

For example, oxygen diffuses out of your blood cells where the oxygen concentration is high, and into the cells of your tissues, where the oxygen concentration is low. Your tissues can’t store up oxygen, so they need a constant supply delivered to them in this way in order to function. Dissolved liquids and solids can diffuse too.

You’ve seen this before: if you toss a sugar cube into tea, even if you don’t stir it, eventually it will dissolve. If we wait long enough, and don’t spill the tea, the sugar will evenly distribute itself throughout the liquid. And then, when you drink that sweetened tea, your body converts it into glucose that your cells use to produce energy.

If you haven’t eaten in a while, the glucose concentration in your cells will be low. It’s kind of like the transport protein creates a tunnel that the glucose can go through. So, at first, there was a traffic jam of glucose outside of the cell and not as much inside the cell.

This relationship is called a concentration gradient. But as more glucose passes through that tunnel, we start to see a more balanced concentration between the glucose that’s inside the cell and the glucose outside the cell. But because the sugar in your tea loves water, and the cell's membrane lipid bilayer hates water, the glucose can't pass straight through the membrane.

It needs a transport protein to separate it from the cell membrane and escort it into the cell. This is called facilitated diffusion. And even though it's making use of that transport protein, it's still a form of passive transport.

If it’s water that’s diffusing through a membrane, it gets its own special name: osmosis. And this movement of water happens inside and outside of all sorts of life forms. Think about the single-celled paramecium, for example, a tiny little unit floating along in a pond.

Our pal Parry has a higher salt concentration inside of it than the pond water does. So water diffuses into the paramecium naturally and keeps things balanced. If a freshwater paramecium suddenly found itself in a really salty environment, the water would rush out of the paramecium, toward the salty surroundings, leaving poor Parry all withered up.

This shows us just how important the correct water balance is in cells. Another example: plants use osmosis to pull water in through their roots. But thankfully, our cells aren’t only at the mercy of physics.

The cell also has the ability to change up the transport proteins in its membrane. So it can use a special energy molecule called ATP to get stuff moving in the needed direction, just like a gatekeeper can actively choose to raise or lower the castle’s drawbridge. And that is called active transport.

So, diffusion and osmosis are our passive transport strategies. And when you add in the energy-consuming technique of active transport, you’ve got a whole transportation system — almost. See, the cell doesn’t exactly have a protein transporter for larger, protein-sized things.

Take the hormone insulin, for example, which gets made in the cells of the pancreas. But since it needs to get out of the pancreas cells and move throughout the whole body, it sends the message that the blood sugar levels are high, a signal that cells should start feeding themselves. So, like a Trojan Horse, the water-loving insulin inside the pancreas cell disguises itself by hiding inside a container with its own phospholipid bilayer called a vesicle.

As it approaches the cell’s exterior, the vesicle becomes part of the cell membrane, dumping insulin out into the extracellular matrix in a process called exocytosis. And when a cell needs to take in something big, the membrane dents inward, creating a lipid bubble around the new addition and pulling it inside via a vesicle made from the cell membrane itself, in a similar process called endocytosis. And none of this would function properly without our cell membranes standing guard, ever vigilant, opening the gates for the helpful molecules and keeping out the harmful ones.

Whether letting insulin out or bringing nutrients in, the movement of stuff across the cell membrane is critical to an organism’s survival. Cell membranes are universal across life’s many forms and functions. They are another example of the evolutionary interconnectedness of all life on Earth.

From the simplest bacteria to all the cells that make up the wondrous organism of you, membranes are everywhere. Ever watchful, keeping the borders of our cellular castles safe. In our next episode, we’ll stick with our microscopic companions a little while longer as we learn how cells communicate.

I’ll see you then! Deuces! This series was produced in collaboration with HHMI BioInteractive.

If you’re an educator, visit BioInteractive.org/CrashCourse for classroom resources and professional development related to the topics covered in this course. Thanks for watching this episode of Crash Course Biology, which was filmed at our studio in Indianapolis, Indiana, and was made with the help of all these nice people. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.