0:00:06.956,0:00:11.405
Cell membranes are structures [br]of contradictions.

0:00:11.405,0:00:16.886
These oily films are hundreds of times[br]thinner than a strand of spider silk,

0:00:16.886,0:00:20.955
yet strong enough to protect[br]the delicate contents of life:

0:00:20.955,0:00:25.057
the cell's watery cytoplasm, [br]genetic material, organelles,

0:00:25.057,0:00:28.316
and all the molecules it needs to survive.

0:00:28.316,0:00:34.547
How does the membrane work,[br]and where does that strength come from?

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

0:00:37.206,0:00:39.637
like the tight skin of a balloon,

0:00:39.637,0:00:43.027
but it's actually something [br]much more complex.

0:00:43.027,0:00:45.758
In reality, it's constantly in flux,

0:00:45.758,0:00:49.217
shifting components back and forth[br]to help the cell take in food,

0:00:49.217,0:00:51.135
remove waste,

0:00:51.135,0:00:53.997
let specific molecules in and out,

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communicate with other cells,

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gather information about the environment,

0:00:58.057,0:01:00.528
and repair itself.

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The cell membrane gets this resilience,[br]flexibility, and functionality

0:01:04.887,0:01:08.488
by combining a variety [br]of floating components

0:01:08.488,0:01:13.117
in what biologists call a fluid mosaic.

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The primary component of the fluid mosaic

0:01:15.467,0:01:19.100
is a simple molecule [br]called a phospholipid.

0:01:19.100,0:01:22.748
A phospholipid has a polar, [br]electrically-charged head,

0:01:22.748,0:01:24.237
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[br]in a two layer sheet

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just five to ten nanometers thick[br]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 [br]external to the cell

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with the lipid tails [br]sandwiched in between.

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This bilayer, which at body temperature[br]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.

0:02:00.042,0:02:03.600
Cholesterol keeps the membrane [br]at the right fluidity.

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It also helps regulate communication[br]between cells.

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Sometimes, cells talk to each other

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by releasing and capturing [br]chemicals and proteins.

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The release of proteins is easy,

0:02:15.371,0:02:18.720
but the capture of them [br]is more complicated.

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That happens through a process called[br]endocytosis

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in which sections of the membrane[br]engulf substances

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and transport them into the cell[br]as vesicles.

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Once the contents have been released,

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the vesicles are recycled and returned[br]to the cell membrane.

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The most complex components[br]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 [br]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 [br]bilayer easily.

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

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Transmembrane proteins stretch[br]across the bilayer to create channels

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

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Peripheral proteins floating[br]in the inner face of the bilayer

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help anchor the membrane to the cell's[br]interior scaffolding.

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

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

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

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And some proteins move within[br]the fluid mosaic,

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coming together to form complexes[br]that carry out specific jobs.

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For instance, one complex might[br]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[br]of an ongoing war

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between us and all the things[br]that want to infect us.

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In fact, some of the most toxic[br]substances we know of

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are membrane-breaching proteins[br]made by infectious bacteria.

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

0:04:09.785,0:04:13.605
causing a cell's contents to leak out.

0:04:13.605,0:04:17.058
Scientists are working on developing[br]on ways to defend against them,

0:04:17.058,0:04:19.507
like using a nano-sponge [br]that saves our cells

0:04:19.507,0:04:24.486
by soaking up [br]the membrane-damaging toxins.

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

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Without a cell membrane,[br]there could be no cells,

0:04:32.248,0:04:34.826
and without cells,[br]there would be no bacteria,

0:04:34.826,0:04:36.136
no parasites,

0:04:36.136,0:04:37.406
no fungi,

0:04:37.406,0:04:38.746
no animals,

0:04:38.746,0:04:40.728
and no us.