# Cellular communication | Cells | MCAT | Khan Academy

November 8, 2019

– [Voiceover] To help us understand how cell to cell communication works, I want you to think back to your good old elementary school days. In particular, I want you
to think about the things you’d have to do in order to secretly talk to your friends during class. Here you are sitting in your classroom and you desperately want to
tell your friend here something so you write a little note
and you pass it to them.>From your hand directly to their hand. They see the note but no one else sees it, it’s just for them. But let’s say you want to tell your friend across the room something. I don’t know about you but
when I was in elementary school I used to write notes on pieces of paper, fold it up into a paper airplane, and I had pretty good aim back then so I’d throw my paper
airplane over to my friend across some distance and they’d get my message. And again, they’d be the
only one who got the message, no one else got my little
paper airplane message. If I wanted to tell a
few friends something, I might call a little huddle
and actually say a few things just to this group of friends here. My voice would cross this
small distance between us. Now, let’s kick things up a notch. I was, kind of, a rebel
in elementary school and sometimes I’d go
to the secretary’s desk and take over the intercom to say funny things to my friends, maybe to tell them to
meet me at the flagpole or on the playground
at recess or something. This intercom message that I’d send out, that would go to everyone, it would be broadcasted
to the whole school. Those who wanted to come
meet me would do that and those who wouldn’t, wouldn’t. We can think of cells as little people that do really similar things because you might not
always think about it but it’s really, really important that cells are able to talk to each other. Evolutionarily, cells being able to communicate with each other are a major reason why
we’re as complex as we are, as human beings. I’ll give you examples of when cells might talk to each other
as we go along here. How do cells pass notes to each? How do they directly
communicate with each other? One way is by, actual, direct contact. Cells have lots of protein stuck into their plasma membranes here that serve a lot of functions. The most important one
is for communication. Let’s look at a macrophage here, this is a type of white blood cell that’s a part of your immune system. When these macrophages
see a foreign invader, maybe a little bacteria or a virus, they can ingest it. They can ingest it then they break it down and then they display a little piece of it which is now called an antigen, on their surface. They show it off on their
surface with one of these cell surface proteins here. This one in particular is
called an MHC II protein. Now, this little antigen here has become the note that they want to pass on, this antigen is the message. Another white blood cell, maybe a helper T cell might come along and then grab hold of this antigen here with one of its cell membrane proteins, in this case a T cell receptor. Just by doing this, the
macrophage here managed to pass a message onto
the helper T cell here. Now, based on which antigen this is, the T cell can decide whether to start a full-blown immune response. Maybe it’ll go off and
ring more alarm bells by activating other antibody
cells which are called B cells or not, maybe they’ll just do nothing. It just depends on what
type of note this is. When cells directly touch to communicate, sort of, unsurprisingly this is called direct cell-cell communication
or just direct binding. Now, what about our other
methods of communication? Cells can also communicate
over short distances. This is our paper airplane here. For example, let’s look at two neurons. They’re in close approximation but one end of a neuron doesn’t quite touch the start
of the next neuron here, there’s a little gap there
called the synaptic cleft. What neurons do is they release little signals called neurotransmitters to communicate with each other. Neurotransmitters get released
from the end of this neuron and they’ll diffuse across
this little distance here until they bind onto one of the dendrites of this next neuron. That effectively passes the message on from this neuron to this next neuron. The paper airplane is thrown from here to here. This is called neural communication. Over here, just like calling a huddle, one cell can talk to a small
group of cells locally as well. For example, just underneath our skin, let’s say the skin inside our nose, we have these immune
cells called mast cells. They’re really important in
mediating allergic reactions, that’s why I purposely
picked the nose here. Let’s say you’re allergic
to certain pollens and one day in the
spring you find yourself walking through the park. Inevitably, you’d breathe in some pollen and then the pollen would go on to attach to these antibodies
stuck to our mast cells. What happens as a response to this is that the mast cells release
little chemical signals called histamine. Histamine acts as our short-range message. It travels around to cells in the area then let them know that
an allergen has been found and lets them know to start preparing for an allergic reaction to take place. This type of communication is
called paracrine signaling, paracrine, meaning nearby. Finally, the intercom take over. How do our cells talk to
huge groups of cells at once? They do that by endocrine signaling. For example, cells in our
Pituitary gland in our brain make a lot of the important
hormones in our bodies. Let’s say that they’re
making Growth hormone, GH, to send around to all
the cells in the body. Sending his hormone is their
form of communication here. They’ll create the Growth hormone inside their cell bodies and they’ll release them
into the bloodstream. Now, our Growth hormone can
travel through the bloodstream and get to, literally,
any place in the body. Every cell of your body
has the opportunity to get this Growth hormone message. It doesn’t mean that every cell will, necessarily, respond to the message. Some cells just don’t
have the right proteins to bind certain hormones, but either way, you can see that this is a long distance form of communication.