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

翻译人员: Liyu Liu 校对人员: Echo Yang

Cell membranes are structures of contradictions.

细胞膜是一种矛盾的组织。

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

这些油性膜比蜘蛛丝还要细百倍，

yet strong enough to protect the delicate contents of life:

但是足以保护细胞膜内 脆弱的生命内容物。

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

像水一般的细胞质， 遗传物质，细胞器

and all the molecules it needs to survive.

以及细胞生存所需要的所有分子。

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

细胞膜是怎样工作的， 细胞膜的强度又是从何而来呢？

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

首先，细胞膜容易让人联想到

like the tight skin of a balloon,

气球密闭的外皮，

but it's actually something much more complex.

但细胞膜其实是一种更为复杂的组织。

事实上，细胞膜一直 处于不断的变化之中，

In reality, it's constantly in flux,

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

来回交换各种成分 来帮助细胞摄入食物，

remove waste,

排除废物，

let specific molecules in and out,

让特殊的分子进出，

communicate with other cells,

和其他细胞交流，

gather information about the environment,

获取四周环境的信息，

以及自我修复。

and repair itself.

The cell membrane gets this resilience, flexibility, and functionality

细胞膜通过结合各种各样流动的成分

by combining a variety of floating components

得到这样的恢复力、适应性和功能性，

in what biologists call a fluid mosaic.

生物学家称之为流动镶嵌。

流动镶嵌的主要成分

The primary component of the fluid mosaic

is a simple molecule called a phospholipid.

是一种被称为磷脂的简单分子。

磷脂有一个极性的带电头部，

A phospholipid has a polar, electrically-charged head,

which attracts water,

具有亲水性，

and a non-polar tail, which repels it.

以及一个非极性的疏水尾部。

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

它们尾尾组合配对 形成一个双层的结构，

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

厚度仅有5到10纳米， 延展至整个细胞。

The heads point in towards the cytoplasm

磷脂的头部朝内指向细胞质，

and out towards the watery fluid external to the cell

朝外面对细胞外部的液体

with the lipid tails sandwiched in between.

夹杂在中间的是磷脂尾部。

在体温下，这种双分子层有着 像植物油一样的密度，

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

is studded with other types of molecules,

镶嵌了许多其他种类的分子，

包括蛋白质，

including proteins,

carbohydrates,

糖类，

and cholesterol.

和胆固醇。

胆固醇帮助保持细胞膜适当的流动性。

Cholesterol keeps the membrane at the right fluidity.

It also helps regulate communication between cells.

它也有助于调节细胞间的交流。

Sometimes, cells talk to each other

有时候，细胞通过

by releasing and capturing chemicals and proteins.

释放和捕捉化学物质和蛋白质交流。

The release of proteins is easy,

释放蛋白质很简单，

but the capture of them is more complicated.

捕获蛋白质则更加复杂。

That happens through a process called endocytosis

这一过程被称为细胞内吞作用，

in which sections of the membrane engulf substances

是指部分细胞膜吞没物质

and transport them into the cell as vesicles.

并以囊泡的形式 将这些物质运输至细胞内。

Once the contents have been released,

一旦囊泡内容物被释放出来，

the vesicles are recycled and returned to the cell membrane.

囊泡会被回收并返回细胞膜。

The most complex components of the fluid mosaic are proteins.

蛋白质是流动镶嵌中最为复杂的成分。

它们的主要工作之一

One of their key jobs is to make sure

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

是确保正确的分子进出细胞。

非极性的分子， 比如氧，

Non-polar molecules, like oxygen,

carbon dioxide,

二氧化碳，

and certain vitamins

和部分维生素

可以轻松穿过磷脂双分子层。

can cross the phospholipid bilayer easily.

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

但是极性的荷电分子则 无法通过脂质内层。

Transmembrane proteins stretch across the bilayer to create channels

跨膜蛋白会横跨双分子层创造通路，

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

让钠和钾离子之类的特定的分子通过。

Peripheral proteins floating in the inner face of the bilayer

漂浮在双分子层内侧的周边蛋白质

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

帮助将细胞膜固定到细胞内部支架。

细胞膜内的其他蛋白质可以帮助 融合两个不同的双分子层。

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

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

这可以为我们带来好处， 比如当精子使卵子受精。

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

但是这也会伤害我们， 比如病毒进入细胞的时候。

And some proteins move within the fluid mosaic,

同时，部分蛋白质 会在流动镶嵌中移动。

coming together to form complexes that carry out specific jobs.

这些共同组成了 可以承担特殊任务的复合体。

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

例如，一个复合体也许会 激活我们的免疫系统细胞，

then move apart when the job is done.

在工作完成时就会分离。

Cell membranes are also the site of an ongoing war

细胞膜也是人类和所有入侵物

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

持续战争的战场。

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

事实上，一些我们所知道的 最具毒性的物质

are membrane-breaching proteins made by infectious bacteria.

是由传染性的细菌制造的 能引起细胞膜破裂的蛋白质。

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

这些成孔毒素会在 细胞膜上穿出巨大的洞，

causing a cell's contents to leak out.

导致细胞内容物泄漏。

Scientists are working on developing ways to defend against them,

科学家正致力研究 抵御这些毒素的方法，

比如运用纳米海绵，

like using a nano-sponge that saves our cells

通过吸取破坏细胞膜的有毒物质 来保护我们的细胞。

by soaking up the membrane-damaging toxins.

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

流动镶嵌使得生命成为可能。

Without a cell membrane, there could be no cells,

没有细胞膜，细胞就无法形成，

and without cells, there would be no bacteria,

而没有细胞，就不会有细菌，

no parasites,

没有寄生虫，

no fungi,

没有霉菌，

no animals,

没有动物，

and no us.

也不会有我们的存在。