is separated from those around it

用最外面的一層

by its outermost layer,

和外界分隔，

its membrane.

這層叫做細胞膜。

A cell membrane must be both sturdy and flexible.

細胞膜必須要又穩固又有彈性。

Imagine a membrane made of metal -

想像金屬製的細胞膜

great at keeping the cell's guts inside,

──可以把細胞內部保存得很好，

but horrible at letting materials flow in and out.

可是很難讓物質進進出出。

But a membrane made of fishnet stocking

但漁網做的細胞膜

would go too far in the opposite direction -

會恰恰相反

leaky, but easily torn.

──方便進去，但是容易破。

So, the ideal membrane falls somewhere in the middle.

所以，理想的細胞膜 應該介於中間。

Over the past few centuries,

經過幾個世紀以後，

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

我們在細胞膜的運作方式這方面 學到了很多。

The tale starts in the late 1800's

這個故事要從 1800 末期講起，

when, according to legend,

據說，

a German woman named Agnes Pockels was doing dishes.

一位叫做 Agnes Pockels 的德國女性 在做菜時發現的。

Her observation, that not all detergents

她觀察到，不是所有的清潔劑

dissolve grease in the same way,

都用同樣的方式 溶解油汙，

piqued her curiosity,

這激起了她的好奇心，

so she made careful measurements

所以她仔細地測量

of the size of soapy films

裝水的金屬盤

that formed on the surface

表面所形成的

of a metal tray filled with water.

肥皂膜的厚度。

Later, in the 1920's, GE scientists

後來，在 1920 年代， 電子學家

Irving Langmuir and Katharine Blodgett

Irving Langmuir 和 Katharine Blodgett

reexamined the problem with a more elaborate contraption

用更精細的發明 再次檢驗這個問題，

and found that those tiny slicks

然後發現這薄薄的一層

were in fact a single layer of oil molecules.

實際上就是 單一一層的油分子。

Each oil molecule has one side

每個油分子有一側

that loves water and floats on the surface,

是喜歡水的，所以飄浮在水的表面；

and one side that loathes water

另一側則討厭水，

and protrudes into the air.

因此朝向空氣。

So what does it have to do with cell membranes?

所以這個細胞膜有什麼關係？

Well, at the turn of the 20th century,

嗯，在 20 世紀交替的時候，

chemists Charles Overton and Hans Meyer

化學家 Charles Overton 和 Hans Meyer

demonstrated that the cell membrane

證實了細胞膜

is composed of substances that,

是由類似油的物質形成，

like oil,

是由類似油的物質形成，

have a water-loving part

有一側親水端

and a water-loathing part.

還有一側厭水端。 （譯註：或稱疏水端。）

We now call these substances lipids.

我們現在把這個物質 叫做脂質。

In 1925, two scientists,

在 1925 年，兩位科學家

Evert Gorter and Francois Grendel,

Evert Gorter 和 Francois Grendel

pushed our understanding further.

把我們的認知更往前推。

They designed an experiment meant to test

他們設計了一個實驗，

whether cell membranes

來測試細胞膜

are made of only one layer of lipids,

是由一層脂質組成，

a monolayer,

也就是單層膜，

or two layers stacked on top of one another,

或是兩層疊在一起 所組成，

called a bilayer.

也就是雙層膜。

Gorter and Grendel drew blood

Gorter 和 Grendel 從

from a dog,

狗、

a sheep,

綿羊、

a rabbit,

兔子、

a goat,

山羊、

a guinea pig,

天竺鼠、

and human volunteers.

還有人類自願者 身上抽血。

From each of these samples,

從這些樣本中，

they extracted all the lipids

他們把所有脂質

from all the red blood cells

都從紅血球中 萃取出來

and placed a few drops of this extract

然後滴一滴 萃取液

on a tray of water.

在水盤之上。

True to form, the lipids, like oil,

正如預期，就像油一樣，

spread out into a monolayer,

脂質散開形成單層膜，

whose size Gorter and Grendel could measure.

而 Gorter 和 Grendel 可以測量它的大小。

If they compared the surface area of that monolayer

如果他們把這單層膜的表面積

to the surface area to the intact red blood cells,

和完整的紅血球表面積相比，

they'd be able to tell

他們就有辦法判定

whether the red blood cell membrane

紅血球的細胞膜

is one or two layers thick.

是一層還兩層。

To understand the design of their experiment,

要了解這個實驗的設計，

imagine looking down at a sandwich.

想想由上往下看著三明治。

If you measure the surface area of what you see,

如果你估測你看到的面積，

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

你會覺得它是一層而已，

even though there are two slices,

儘管它有兩層，

one stacked perfectly atop the other.

第一層完全把第二層蓋住了。

But, if you open the sandwich

但是，如果你把三明治打開

and place the two slices side by side,

然後把兩片放在一起，

you get twice the surface area.

你就會得到兩倍的表面積。

The Gorter and Grendel experiment

而 Gorter 和 Grendel 的實驗

is basically the same idea.

基本上就是這個想法。

The open sandwich is the monolayer formed

打開的三明治相當於單層膜，

by extracted cellular lipids spreading out into a sheet.

它是由萃取的細胞脂質 擴散成一個平面。

The closed sandwich is the intact red blood cell membrane.

而蓋好的三明治就相當於 完整的紅色球細胞膜。

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

驚奇的是，他們發現了 2 比 1 的比例，

proving beyond the shadow of a doubt

確實證明

that a cell membrane is a bilayer,

細胞膜是雙層膜，

which when unstacked,

因為當它們沒疊在一起時，

yields a monolayer twice its size.

產生的單層膜有兩倍的面積。

So almost 30 years before the double-helix structure

所以幾乎是 DNA 雙螺旋結構

of DNA was elucidated,

被發現的 30 年前，

a single experiment

就靠一個實驗，

involving fancy versions of household materials

它包含了對家務事的新潮想法，

enabled deep insight

讓對於細胞基本結構

into the basic architecture of the cell.

的更深層認識變為可能。