Our cells might seem like they’re just sitting around, quietly doing their jobs, but they’re actually busy keeping our bodies in a state of constant physical and chemical balance called homeostasis.

And that requires a remarkable  amount of coordination. Conditions in an organism’s environment are constantly changing, both externally and internally.

So our cells need constant communication to stay on top of tasks and with–[phone rings] Well, that’s new. [phone rings] “New phone, who dis?” Oh, it’s Hank Green. [laughs] I should probably take this. One sec.

Sammy: Hey! No, we’re filming today. [Cartoon Hank murmurs] Yeah, right now. [More Cartoon Hank sounds] No, I haven’t grown a goatee yet. See, the way that my facial hair situation is set up, that really wouldn’t... [Cartoon Hank murmurs] Yeah, I know you had one during the original Biology series. [Cartoon Hank jabbers] Oh, okay, let me write that down. Huh.

Okay, you said ‘Miracle Gro-tee Cream’... [Cartoon Hank makes affirmative sounds] aha, overnight!? Oh, I’ll look into that, yeah. [Cartoon Hank chatters] Okay, I love you, bye. [Sammy laughs] Okay, so... Why did I say that?!

Ok, sorry. Where were we? Oh, yeah, communication.

It can be complicated — for people, and for cells. Our cells have to constantly communicate with each other to adjust to changes and keep us functioning properly. Cells speak a language of their own, and today we’re going to listen in.

Hi! I'm Dr. Sammy, your friendly neighborhood entomologist, and this is Crash Course Biology.

One sec... [Dial tone] Let's see, we got the... [Rotary phone dials] Just a moment... [Dial tone] [Dial tone] aaand... [Ringing sound] [Through phone: "Yeah?"] Callie, could you send up my theme music? [Callie through phone: "Yep!"] Thanks, babe. Did I say 'babe!?' [THEME MUSIC] So, imagine if you went out for a jog but your heart didn’t get the memo to speed up and pump more oxygen to your working muscles — [retching sound] you’d probably pass out, or worse! Thankfully, your body keeps itself regulated so that doesn’t happen.

But in order to achieve that regulation, your cells need to stay on point. Cell communication, or cell signaling, is how your body’s cells keep in touch. It’s a process mediated by a chatty bunch of molecules.

Like, melatonin shushing everything to control your sleep cycles, or cortisol, the stress-induced hormone that puts your body in fight-or-flight mode when it enters a dangerous situation. And it’s not just our cells that are busy communicating back and forth, these conversations are happening all around you. Like, imagine if a flower normally got watered every day, but then there was a drought for a few days.

The flower needs to regulate its water usage based on its availability. Which means its cells need  to be in constant contact, letting each other know when the water is abundant and when it’s not. All of these life-sustaining activities start with a signaling cell— the cell that’s sending the message.

No cell can be everywhere all at once. Sometimes they’ve got to send a message to a part of the body where they’re not present physically. That’s when they use signaling molecules, special molecules that carry messages from place to place.

And to read these messages, the receiving cell needs the correct receptor protein. Not all cells can, nor should they, respond to each and every molecular signal. My liver has no business responding to messages intended for my heart.

Thankfully, our bodies are set up like password-protected wifi. The signal may reach as far as the neighbor’s house, but if they don’t have the password to log in, they can’t do anything. Similarly, some cells have receptors that let them receive and respond to the signals, while others don’t.

The responding cell reacts to the signaling cell, following the instructions it receives. Sort of like me writing that note to myself, reminding me to get a subscription to Miracle Gro-tee Cream. The message might ask them to speed up a cellular process, change the proteins they’re making, or even divide and make copies of themselves.

Even though cell communication is happening on a submicroscopic level, we can sometimes feel its effects. When your stomach growls telling you it’s time to eat, or your heart races putting you on alert after someone blows through a red light, it’s because a molecular signal was released. While those reactions might seem instantaneous to you, there are a few things that have to happen before a cell can respond to a message.

In signal reception, molecules need to grab onto each other, in a process called binding. The signal molecule binds to the receptor protein and causes it to change shape in some way. This activates a pathway to trigger a responsive cellular function.

The pathway, known as signal transduction, involves multiple steps, and several more molecules along the way will change shape. Sometimes there’s more binding —or even unbinding— that causes these shifts along the path. Other times, chemical modifications to proteins or changes in ion concentrations contribute to signaling.

Once the pathway is open and the signal is interpreted, it’s time for the cell to respond. Just like when I get a text asking “Can you pick up Chipotle on the way home,” (my answer is always yes, by the way), the cell responds based on the instructions it receives. It might make a protein the cell needs under these conditions, or kick-start a reaction to make a hormone.

And very rarely does it bring home the wrong order, cuz babe, I could’ve sworn you said no beans, no guac! Let’s take a look at how cells send and receive signals to help our favorite duck-billed mammal, the platypus, bring puggles into the world. Yep, let’s go to the Thought Bubble… Our pal the platypus is ready to have puggles, or baby platypuses.

So, the signaling cells in the ovaries of the platypus relay the message. They produce signaling molecules, which act as the message itself, in the form of the hormone estrogen, and its cousin, progesterone. These lipid hormones can slip through the membrane of the responding cell.

Once inside the cell, they’ll seek out receptor proteins, called intracellular receptors to receive their message. After estrogen binds to its receptor, it activates the specific genes in the responding cells that make proteins. These changes in protein production are one reason that organisms experience dramatic changes during puberty.

In this case, there’s less voice cracking; and more egg-laying. Now, there’s not a whole lot of research on platypus egg production, which seems like a missed opportunity since it’s one of only two mammals that lays eggs. Shoutout to the echidna.

But if the process is anything like a chicken, more chemical signals get released by the pituitary gland to help the ovary make the egg. Some of these are proteins that can’t just head straight through the cell membrane like estrogen and progesterone did. So instead, they deliver their message to receptors on the cell’s surface.

This triggers a pathway inside the cell, and egg production begins. Gosh, I love springtime. Thanks, Thought Bubble!

Functions like reproduction require a lot of communication to happen inside the body. And just like in your own life, that communication happens across different distances. The shortest distance, of course, is from you to you — like when you write yourself a reminder note to stick on your bathroom mirror.

Autocrine signaling occurs when signals are relayed to the same cell, and it has lots of important functions, like helping organisms develop by reminding  stem cells to produce specific  cell types and organs. Autocrine signaling also gives us clues to how cancer works. You see, cancer happens when cells make copies of themselves in an out-of-control way.

The cells repeat the same message over and over and over. And in the cells’ case, this leads to uncontrolled duplication. One step removed, and there’s direct signaling.

That’s when cells have to physically touch in order to communicate. This type of communication does all kinds of important work, like helping neighboring cells in your heart muscles beat in sync with each other. When cells communicate with nearby cells that they’re not in direct contact with, it’s called paracrine signaling.

And this allows nerves all throughout your body to function by sending signals to keep you breathing, walking, or just being FIERCE, baby, my cells are at WERK. Meanwhile, the text messages of the molecular world happen through endocrine signaling, where the sender doesn’t have to be close to the receiver at all. Molecules use the bloodstream to send chemical messages to the receiving cells.

And sometimes these are group texts, with the signal being picked up by lots of receiving cells. For example, if you’ve ever gone a while without drinking water, you can thank endocrine signaling for keeping your body hydrated by sending water from the soon-to-be urine in your kidneys back to your bloodstream. That’s right!

Your body can de-pee you so that you don’t dry up like a raisin! That way you don’t have to try to get that water back after the fact, Bear Grylls style. It’s always good to have a backup plan, but stay hydrated, friends!

Sometimes, a receiving cell only gets a few molecules of the signaling molecule, but it still needs to respond in a big way. This can happen through a process called amplification, where each step in the signal transduction pathway sets off a cascade of even more signals. It’s like when you share something on Twitter with your 10 followers, and they each retweet it and then their followers retweet it, and so on.

That way, the message gets amplified, like a viral post about my Bee in a wig named Bee-hanna singing her hit song, “Bees Better Have My Honey.” Be on the lookout for Bad Girl Bee-Bee! She blowin’ up! [Laughs] Or think about the rush you feel when something scary happens, like when you’re swimming in the ocean and something touches your foot. The hormone adrenaline binds to the cells in your liver, which triggers a chain reaction that leads to glucose being released into your bloodstream and deployed throughout your muscles, so you can swim frantically away… from what was probably just seaweed.

So, even though your body might seem like it’s on autopilot most of the time, it’s crucial that your cells are in constant communication with each other. They’re sending out all kinds of messages to keep your heart pumping, your lungs working, and your hands away from the stove – seriously, don’t touch that. Every living multicellular thing from the smallest bug to the tallest giraffe has a whole production happening inside of them – all the time.

I mean, just look at me. I know, I know – I make it look easy. But putting together an episode of Crash Course takes a whole lot of communication between writers, editors, producers, and – [phone rings] Hank.

I’ll call him back. And inside all of us, there  are trillions of cells at work making sure that we’re able to keep bringing you these episodes. And speaking of next time, we’re going to learn more about cellular energy.

I’ll see you then! 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.