Cell membranes are structures of contradictions.
These oily films are hundreds of times thinner than a strand of spider silk,
yet strong enough to protect the delicate contents of life:
the cell’s watery cytoplasm, genetic material, organelles,
and all the molecules it needs to survive.
How does a cell membrane work
How does the membrane work, and where does that strength come from?
First of all, it’s tempting to think of a cell membrane
like the tight skin of a balloon,
but it’s actually something much more complex.
In reality, it’s constantly in flux,
shifting components back and forth to help the cell take in food,
remove waste,
let specific molecules in and out,
communicate with other cells,
gather information about the environment,
and repair itself.
The cell membrane gets this resilience, flexibility, and functionality
by combining a variety of floating components
in what biologists call a fluid mosaic.
Fluid mosaic
The primary component of the fluid mosaic
is a simple molecule called a phospholipid.
A phospholipid has a polar, electrically-charged head,
which attracts water,
and a non-polar tail, which repels it.
They pair up tail-to-tail in a two layer sheet
just five to ten nanometers thick that extends all around the cell.
The heads point in towards the cytoplasm
and out towards the watery fluid external to the cell
with the lipid tails sandwiched in between.
Endocytosis
This bilayer, which at body temperature has the consistency of vegetable oil,
is studded with other types of molecules,
including proteins,
carbohydrates,
and cholesterol.
Cholesterol keeps the membrane at the right fluidity.
It also helps regulate communication between cells.
Sometimes, cells talk to each other
by releasing and capturing chemicals and proteins.
The release of proteins is easy,
but the capture of them is more complicated.
That happens through a process called endocytosis
in which sections of the membrane engulf substances
and transport them into the cell as vesicles.
Once the contents have been released,
the vesicles are recycled and returned to the cell membrane.
Proteins
The most complex components of the fluid mosaic are proteins.
One of their key jobs is to make sure
that the right molecules get in and out of the cell.
Non-polar molecules, like oxygen,
carbon dioxide,
and certain vitamins
can cross the phospholipid bilayer easily.
But polar and charged molecules can’t make it through the fatty inner layer.
Transmembrane proteins stretch across the bilayer to create channels
that allow specific molecules through, like sodium and potassium ions.
Peripheral proteins floating in the inner face of the bilayer
help anchor the membrane to the cell’s interior scaffolding.
Other proteins in cell membranes can help fuse two different bilayers.
That can work to our benefit, like when a sperm fertilizes an egg,
but also harm us, as it does when a virus enters a cell.
And some proteins move within the fluid mosaic,
coming together to form complexes that carry out specific jobs.
For instance, one complex might activate cells in our immune system,
then move apart when the job is done.
Cell membranes are also the site of an ongoing war
between us and all the things that want to infect us.
In fact, some of the most toxic substances we know of
are membrane-breaching proteins made by infectious bacteria.
These pore-forming toxins poke giant holes in our cell membranes,
causing a cell’s contents to leak out.
Scientists are working on developing ways to defend against them,
like using a nano-sponge that saves our cells
by soaking up the membrane-damaging toxins.
The fluid mosaic is what makes all the functions of life possible.
Without a cell membrane, there could be no cells,
and without cells, there would be no bacteria,
no parasites,
no fungi,
no animals,
and no us.
