Every cell in your body is separated from those around it by its outermost layer, its membrane.
你身体里的每个细胞，都是通过最外面的一层膜将它与外界环境隔离开的，这就是细胞膜。
A cell membrane must be both sturdy and flexible.
细胞膜必须兼具坚固性和柔韧性。
Imagine a membrane made of metal -- great at keeping the cell's guts inside, but horrible at letting materials flow in and out.
想象一层由金属做成的细胞膜，它可以很好地保护细胞内部，却不能让物质轻松进出细胞。
But a membrane made of fishnet stocking would go too far in the opposite direction -- leaky, but easily torn.
但是如果细胞膜由网眼丝袜做成，将可能会有完全相反的性质--容易渗漏，但也易被损坏。
So, the ideal membrane falls somewhere in the middle.
所以理想的细胞膜介于这两种之间。
Over the past few centuries, we've learned a lot about the way membranes work.
在过去的几个世纪中，我们了解了很多关于细胞膜运作的方式。
The tale starts in the late 1800's when, according to legend, a German woman named Agnes Pockels was doing dishes.
故事要从19世纪晚期说起。根据传说，一个叫Agnes Pockels的德国女人正在洗盘子。
Her observation, that not all detergents dissolve grease in the same way, piqued her curiosity,
她发现，并不是所有的去污剂都通过同一种方式溶解油脂，这种发现激起了她的好奇心，
so she made careful measurements of the size of soapy films that formed on the surface of a metal tray filled with water.
所以她仔细地测量了装水的金属盘表面所形成的肥皂膜的厚度。
Later, in the 1920s, GE scientists Irving Langmuir and Katharine Blodgett reexamined the problem with a more elaborate contraption
之后，在20世纪20年代，两位通用电气科学家Irving Langmuir和 Katharine Blodgett，通过更精良的设备重新检视了这个问题，
and found that those tiny slicks were in fact a single layer of oil molecules.
他们发现那一层浮层实际上是一个单层的油分子层。
Each oil molecule has one side that loves water and floats on the surface,
每个油分子有一侧是亲水的，飄浮在水的表面，
and one side that loathes water and protrudes into the air.
而另一侧则疏水，因此朝向空气。
So what does it have to do with cell membranes?
那么这跟细胞膜有什么关系呢？
Well, at the turn of the 20th century, chemists Charles Overton and Hans Meyer
在20世纪初的时候，化学家Charles Overton和Hans Meyer
demonstrated that the cell membrane is composed of substances that, like oil, have a water-loving part and a water-loathing part.
证明了细胞膜是由某种像油分子结构的物质组成的，有亲水的部分和疏水的部分。
We now call these substances lipids.
我们现在把这个物质称为脂质。

In 1925, two scientists, Evert Gorter and Francois Grendel, pushed our understanding further.
1925年，有两位科学家，Evert Gorter和Francois Grendel，帮我们更近一步理解了这个问题。
They designed an experiment meant to test whether cell membranes are made of only one layer of lipids, a monolayer,
他们设计了一个实验来测定细胞膜是由一层脂质组成的，即单层膜，
or two layers stacked on top of one another, called a bilayer.
还是双层--就是一层叠加在另一层上面，叫做双层膜。
Gorter and Grendel drew blood from a dog, a sheep, a rabbit, a goat, a guinea pig, and human volunteers.
Gorter和Grendel抽取了各种来源的血液，包括狗、羊、兔子、山羊、豚鼠、还有人类志愿者。
From each of these samples, they extracted all the lipids from all the red blood cells
从这些样本中，他们把所有脂质都从红细胞中萃取出来
and placed a few drops of this extract on a tray of water.
然后滴了几滴萃取液到盛着水的盘子里。
True to form, the lipids, like oil, spread out into a monolayer, whose size Gorter and Grendel could measure.
不出所料，脂质，就像油分子一样，分散开来，形成一个单层膜。Gorter和Grendel能够测定它的面积。
If they compared the surface area of that monolayer to the surface area to the intact red blood cells,
如果他们把单层膜的表面积和红细胞表面积比较，
they'd be able to tell whether the red blood cell membrane is one or two layers thick.
他们就能得出红细胞的细胞膜是一层还是两层。
To understand the design of their experiment, imagine looking down at a sandwich.
为了理解他们实验的设计，想象你正俯视着一个三明治。
If you measure the surface area of what you see, you'll get the dimensions of a single slice of bread
如果测量你看到部分的表面积，你将会得到一片面包的大小，
even though there are two slices, one stacked perfectly atop the other.
即使三明治是由两片面包组成的，上面的一片完全遮住在下面一片。
But, if you open the sandwich and place the two slices side by side, you get twice the surface area.
但是如果你分开三明治，把这两片面包并排放后测量，你得到的面积是刚才的两倍。
The Gorter and Grendel experiment is basically the same idea.
Gorter和Grendel的实验基本上是同一道理。
The open sandwich is the monolayer formed by extracted cellular lipids spreading out into a sheet.
打开的三明治相当于单层膜，它是由萃取的细胞脂质扩散成一个平面。
The closed sandwich is the intact red blood cell membrane.
未打开的三明治则相当于完整的红细胞的细胞膜。
Low and behold, they observed a two--to--one ratio,
他们惊讶地发现了2:1的比例，
proving beyond the shadow of a doubt that a cell membrane is a bilayer,
证明了细胞膜是双层膜，
which when unstacked, yields a monolayer twice its size.
当打开它时，产生的单层膜有两倍的面积。
So almost 30 years before the double--helix structure of DNA was elucidated,
所以几乎是在DNA双螺旋结构被发现的30年前，
a single experiment involving fancy versions of household materials enabled deep insight into the basic architecture of the cell.
就靠一个包含了对家务事的新潮想法的实验，使得对于细胞基本结构的更深层认识变为可能。