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  • Every cell in your body

  • is separated from those around it

  • by its outermost layer,

  • its membrane.

  • A cell membrane must be both sturdy and flexible.

  • Imagine a membrane made of metal -

  • great at keeping the cell's guts inside,

  • but horrible at letting materials flow in and out.

  • But a membrane made of fishnet stocking

  • would go too far in the opposite direction -

  • leaky, but easily torn.

  • So, the ideal membrane falls somewhere in the middle.

  • Over the past few centuries,

  • we've learned a lot about the way membranes work.

  • The tale starts in the late 1800's

  • when, according to legend,

  • a German woman named Agnes Pockels was doing dishes.

  • Her observation, that not all detergents

  • dissolve grease in the same way,

  • piqued her curiosity,

  • so she made careful measurements

  • of the size of soapy films

  • that formed on the surface

  • of a metal tray filled with water.

  • Later, in the 1920's, GE scientists

  • Irving Langmuir and Katharine Blodgett

  • reexamined the problem with a more elaborate contraption

  • and found that those tiny slicks

  • were in fact a single layer of oil molecules.

  • Each oil molecule has one side

  • that loves water and floats on the surface,

  • and one side that loathes water

  • and protrudes into the air.

  • So what does it have to do with cell membranes?

  • Well, at the turn of the 20th century,

  • chemists Charles Overton and Hans Meyer

  • demonstrated that the cell membrane

  • is composed of substances that,

  • like oil,

  • have a water-loving part

  • and a water-loathing part.

  • We now call these substances lipids.

  • In 1925, two scientists,

  • Evert Gorter and Francois Grendel,

  • pushed our understanding further.

  • They designed an experiment meant to test

  • whether cell membranes

  • are made of only one layer of lipids,

  • a monolayer,

  • or two layers stacked on top of one another,

  • called a bilayer.

  • Gorter and Grendel drew blood

  • from a dog,

  • a sheep,

  • a rabbit,

  • a goat,

  • a guinea pig,

  • and human volunteers.

  • From each of these samples,

  • they extracted all the lipids

  • from all the red blood cells

  • and placed a few drops of this extract

  • on a tray of water.

  • True to form, the lipids, like oil,

  • spread out into a monolayer,

  • whose size Gorter and Grendel could measure.

  • If they compared the surface area of that monolayer

  • to the surface area to the intact red blood cells,

  • they'd be able to tell

  • whether the red blood cell membrane

  • is one or two layers thick.

  • To understand the design of their experiment,

  • imagine looking down at a sandwich.

  • If you measure the surface area of what you see,

  • you'll get the dimensions of a single slice of bread

  • even though there are two slices,

  • one stacked perfectly atop the other.

  • But, if you open the sandwich

  • and place the two slices side by side,

  • you get twice the surface area.

  • The Gorter and Grendel experiment

  • is basically the same idea.

  • The open sandwich is the monolayer formed

  • by extracted cellular lipids spreading out into a sheet.

  • The closed sandwich is the intact red blood cell membrane.

  • Low and behold, they observed a two-to-one ratio,

  • proving beyond the shadow of a doubt

  • that a cell membrane is a bilayer,

  • which when unstacked,

  • yields a monolayer twice its size.

  • So almost 30 years before the double-helix structure

  • of DNA was elucidated,

  • a single experiment

  • involving fancy versions of household materials

  • enabled deep insight

  • into the basic architecture of the cell.

Every cell in your body

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B2 TED-Ed membrane cell membrane red blood surface area sandwich

【TED-Ed】Insights into cell membranes via dish detergent - Ethan Perlstein

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    wikiHuang posted on 2014/01/02
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