Cell membranes are way more complicated than you think - Nazzy Pakpour

譯者: Ann Chen 審譯者: Helen Chang

Cell membranes are structures of contradictions.

細胞膜是種充滿矛盾的結構

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

這些油膜比一縷蜘蛛絲薄上數百倍

yet strong enough to protect the delicate contents of life:

然而強韌至足以保護 生命精緻的內含物質：

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

質地似水的細胞質、 遺傳物質、細胞器

and all the molecules it needs to survive.

以及賴以生存的所有分子

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

細胞膜是如何運作 以及從哪裡獲得力量？

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

首先，我們很容易把細胞膜

like the tight skin of a balloon,

想成氣球繃緊的外膜

but it's actually something much more complex.

但它確實比這複雜多了

In reality, it's constantly in flux,

事實上，它不斷地流動

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

來回移動各種組成 以幫助細胞攝食

remove waste,

移除廢物

let specific molecules in and out,

讓特定分子進出細胞

communicate with other cells,

與其他細胞溝通

gather information about the environment,

收集有關環境的訊息

and repair itself.

以及自我修復

The cell membrane gets this resilience, flexibility, and functionality

細胞膜具有回彈性 柔靭性及功能性

by combining a variety of floating components

是因為它結合各種流動的成分

in what biologists call a fluid mosaic.

這就是生物學家 所謂的「流動鑲嵌模型」

The primary component of the fluid mosaic

流動鑲嵌模型的主要組成

is a simple molecule called a phospholipid.

是一種簡單的分子稱為「磷脂」

A phospholipid has a polar, electrically-charged head,

磷脂具有極性、帶電的頭端

which attracts water,

能吸引水

and a non-polar tail, which repels it.

而不具極性的尾端則排斥水

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

它們尾端對尾端配對 形成兩層的薄面

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

只有 5-10 nm 的厚度 環繞整個細胞

The heads point in towards the cytoplasm

頭端朝內指向細胞質

and out towards the watery fluid external to the cell

及朝外指向細胞外面的液體

with the lipid tails sandwiched in between.

而脂質的尾端則夾在其間

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

這脂質雙層在體溫下 具有植物油的黏稠度

is studded with other types of molecules,

且鑲嵌其他多種分子

including proteins,

包括蛋白質

carbohydrates,

碳水化合物

and cholesterol.

和膽固醇

Cholesterol keeps the membrane at the right fluidity.

膽固醇讓膜保持適度的流動性

It also helps regulate communication between cells.

也幫助調節細胞間的溝通

Sometimes, cells talk to each other

有時，細胞能相互交談

by releasing and capturing chemicals and proteins.

是藉著化學物質 與蛋白質的釋放與捕捉

The release of proteins is easy,

釋放蛋白質很容易

but the capture of them is more complicated.

但捕捉它們則比較複雜

That happens through a process called endocytosis

那是經由「胞吞」的方式進行

in which sections of the membrane engulf substances

即部分細胞膜包吞物質

and transport them into the cell as vesicles.

然後形成小囊泡 將它們送入細胞內

Once the contents have been released,

一旦內含物被釋出

the vesicles are recycled and returned to the cell membrane.

小囊泡則被回收 重新回到細胞膜

The most complex components of the fluid mosaic are proteins.

流動鑲嵌模型最複雜的成分是蛋白質

One of their key jobs is to make sure

它們其中一個關鍵功用

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

是確保正確的分子進出細胞

Non-polar molecules, like oxygen,

非極性分子，例如氧

carbon dioxide,

二氧化碳

and certain vitamins

和某些維生素

can cross the phospholipid bilayer easily.

能輕易地通過磷脂雙層

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

但極性、帶電分子 則無法通過脂質內層

Transmembrane proteins stretch across the bilayer to create channels

「跨膜蛋白」跨過 整個脂質雙層而形成通道

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

能讓特定的分子通過 如鈉和鉀離子

Peripheral proteins floating in the inner face of the bilayer

浮在脂質雙層內側的「膜周邊蛋白」

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

會協助細胞膜 固定於細胞的內支架

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

細胞膜上的其他蛋白質 能協助融合兩個不同的雙層

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

這能有利於我們 如精子使卵子受精

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

但也能傷害我們 例如病毒侵入細胞時

And some proteins move within the fluid mosaic,

有些蛋白質 在流動鑲嵌模型的裡面流動

coming together to form complexes that carry out specific jobs.

聚集形成執行特殊任務的複合物

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

例如一種複合物 能激活免疫系統的細胞

then move apart when the job is done.

當任務結束後就分開

Cell membranes are also the site of an ongoing war

細胞膜也是一個我們 和所有感染物之間的戰場

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

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

事實上，我們知道一些最毒的物質

are membrane-breaching proteins made by infectious bacteria.

是由感染細菌製造的破膜蛋白質

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

這些「形成微孔毒素」 在細胞膜上戳出大洞

causing a cell's contents to leak out.

導致細胞內含物漏出去

Scientists are working on developing ways to defend against them,

科學家們正在研發對抗它們的方法

like using a nano-sponge that saves our cells

如使用一種奈米海綿 來解救細胞

by soaking up the membrane-damaging toxins.

它能吸取破壞細胞膜的毒素

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

流動鑲嵌模型使得 所有生命的功能變得可能

Without a cell membrane, there could be no cells,

缺了細胞膜，細胞將不存在

and without cells, there would be no bacteria,

缺了細胞，就沒有細菌

no parasites,

沒有寄生蟲、黴菌及動物

no fungi,

no animals,

and no us.

也就沒有我們了