Let's talk specifically about this one where we have transmembrane proteins,
homotypic interaction creating the connection between cells.
This leads us to a really important concept.
So, with the exception of the cells that are in the bloodstream,
red cells and platelets and white cells,
every other cell has an up and a down.
There is -- it faces one way and it knows -- it knows that that's up
and they know that that's down.
There's an orientation or a polarity and cell polarity is a really important concept.
It turns out that even white cells develop a polarity
when they crawl out of the bloodstream and are chasing after bacteria in the tissue.
They will have a leading end and a tail end.
So, they will even have a polarity but in every other cell in the body not in the bloodstream,
there's an up and a down.
The up is called the apical surface and the bottom is called the basolateral surface.
Okay, and basically, that's the two sides of a cell and it makes a difference
because let's say this is GI epithelium.
These are enterocytes that are gonna be responsible for absorbing nutrition
from the gut lumen and transporting it to the vasculature on the other side.
I wanna make sure that I put the right receptors
and the right membrane transporters in the apical side to suck those nutrients across.
I wanna transport them in one direct and deliver them out the base into the vasculature.
I don't wanna do it the other way around.
So, up and down is really, really important.
Up and down is in part determined by cells sitting on a basement membrane.
That's the blue line, horizontal blue line at the bottom.
We'll come back to that but you also have the demarcation
between apical and basolateral at a tight junction.
And a tight junction is created by the homotypic interaction
of a variety of proteins that sit at that space.
That tight junction limits the movement of things between cells.
So, only a little bit of water can go between cells.
A little bit of -- a little tiny bit of certain small molecular
weight things can go between cells and that tight junction
also says everything above me is apical and everything below me is basolateral,
so, I'm gonna put specific transporters, I know where they need to go.
And with that, we have covered kind of the way that the membrane interacts,
interfaces with the outside world.