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Cell membranes are structures 
of contradictions.

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These oily films are hundreds of times
thinner than a strand of spider silk,

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yet strong enough to protect
the delicate contents of life:

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the cell's watery cytoplasm, 
genetic material, organelles,

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and all the molecules it needs to survive.

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How does the membrane work,
and where does that strength come from?

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First of all, it's tempting to think of a
cell membrane

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like the tight skin of a balloon,

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but it's actually something 
much more complex.

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In reality, it's constantly in flux,

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shifting components back and forth
to help the cell take in food,

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remove waste,

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let specific molecules in and out,

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communicate with other cells,

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gather information about the environment,

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and repair itself.

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The cell membrane gets this resilience,
flexibility, and functionality

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by combining a variety 
of floating components

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in what biologists call a fluid mosaic.

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The primary component of the fluid mosaic

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is a simple molecule 
called a phospholipid.

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A phospholipid has a polar, 
electrically-charged head,

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which attracts water,

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and a non-polar tail, which repels it.

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They pair up tail-to-tail
in a two layer sheet

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just five to ten nanometers thick
that extends all around the cell.

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The heads point in towards the cytoplasm

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and out towards the watery fluid 
external to the cell

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with the lipid tails 
sandwiched in between.

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This bilayer, which at body temperature
has the consistency of vegetable oil,

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is studded with other types of molecules,

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including proteins,

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carbohydrates,

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and cholesterol.

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Cholesterol keeps the membrane 
at the right fluidity.

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It also helps regulate communication
between cells.

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Sometimes, cells talk to each other

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by releasing and capturing 
chemicals and proteins.

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The release of proteins is easy,

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but the capture of them 
is more complicated.

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That happens through a process called
endocytosis

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in which sections of the membrane
engulf substances

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and transport them into the cell
as vesicles.

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Once the contents have been released,

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the vesicles are recycled and returned
to the cell membrane.

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The most complex components
of the fluid mosaic are proteins.

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One of their key jobs is to make sure

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that the right molecules 
get in and out of the cell.

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Non-polar molecules, like oxygen,

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carbon dioxide,

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and certain vitamins

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can cross the phospholipid 
bilayer easily.

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But polar and charged molecules can't
make it through the fatty inner layer.

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Transmembrane proteins stretch
across the bilayer to create channels

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that allow specific molecules through,
like sodium and potassium ions.

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Peripheral proteins floating
in the inner face of the bilayer

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help anchor the membrane to the cell's
interior scaffolding.

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Other proteins in cell membranes
can help fuse two different bilayers.

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That can work to our benefit,
like when a sperm fertilizes an egg,

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but also harm us,
as it does when a virus enters a cell.

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And some proteins move within
the fluid mosaic,

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coming together to form complexes
that carry out specific jobs.

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For instance, one complex might
activate cells in our immune system,

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then move apart when the job is done.

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Cell membranes are also the site
of an ongoing war

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between us and all the things
that want to infect us.

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In fact, some of the most toxic
substances we know of

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are membrane-breaching proteins
made by infectious bacteria.

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These poor-forming toxins poke
giant holes in our cell membranes,

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causing a cell's contents to leak out.

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Scientists are working on developing
on ways to defend against them,

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like using a nano-sponge 
that saves our cells

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by soaking up 
the membrane-damaging toxins.

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The fluid mosaic is what makes 
all the functions of life possible.

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Without a cell membrane,
there could be no cells,

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and without cells,
there would be no bacteria,

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no parasites,

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no fungi,

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no animals,

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and no us.