What is the biggest single-celled organism? - Murry Gans

譯者: Aaron Shoo 審譯者: Anny Chung

大象是體型非常龐大的生物，

The elephant is a creature of epic proportions,

and yet it owes its enormity to more than 1,000 trillion microscopic cells,

但這麼大的體型，

要一千兆顆以上的細胞才能組成。

and on the epically small end of things,

而在非常微小的世界裡，

there are likely millions of unicellular species,

有數百萬種的單細胞生物，

yet there are very few we can see with the naked eye.

但肉眼只能看見極少數。

Why is that?

為什麼呢？

Why don't we get unicellular elephants,

為什麼沒有單細胞版的大象？

or blue whales,

單細胞版的藍鯨？

or brown bears?

單細胞版的棕熊？

To find out, we have to peer into a cell's guts.

讓我們從細胞的核心找答案。

This is where most of the cell's functions occur,

這是細胞功能運作的主要場所，

enclosed by a cellular membrane

外層有細胞膜包覆，

that acts as the doorway into and out of the cell.

掌管物質進出的門戶。

Any resources the cell needs to consume,

任何細胞消耗的物質，

or waste products it needs to expel,

或是要排出的廢物，

first have to pass through this membrane.

首先都要通過細胞膜。

But there's a biological quirk in this set up.

但這組織結構有些矛盾。

A cell's surface and volume increase at different rates.

細胞表面積和內容量的增長率不一樣。

Cells come in many shapes,

細胞可以有各種形狀，

but imagining them as cubes will make the math easy to calculate.

但假設它們都是立方體比較好計算。

A cube has six faces.

立方體有六面。

These represent the cell membrane, and make up its surface area.

這代表細胞膜，組合成表面積。

A cube measuring one micrometer on each side,

一個長寬高一微米的立方體，

that's one millionth of a meter,

也就是百萬分之一公尺，

would have a total surface area of six square micrometers.

表面積是六平方微米。

And its volume would be one cubic micrometer.

而它的內容物是一立方微米。

This would give us six units of surface area

所以每一單位的內容物，

for every single unit of volume,

分到六單位的表面積，

a six to one ratio.

比例是六比一。

But things change dramatically if we make the cube ten times bigger,

但如果方塊大了十倍， 情況會完全不同。

measuring ten micrometers on each side.

假設長寬高都是十微米。

This cell would have a surface area of 600 square micrometers

細胞的表面積是六百平方微米，

and a volume of one thousand cubic micrometers,

而內容量是一千立方微米，

a ratio of only .6 to one.

比例只剩 0.6 比一。

That's less than one unit of surface area to service each unit of volume.

所以一單位的內容物 分不到一單位的表面積。

As the cube grows, its volume increases much faster than its surface area.

立方體成長時 內容物比表面積的增長快多了。

The interior would overtake the membrane,

內容物會超出細胞膜負荷，

leaving too little surface area for things to quickly move in and out of the cell.

細胞膜會來不及處理物質進出。

大顆的細胞會被廢物塞滿， 最後死掉、分解。

A huge cell would back up with waste and eventually die and disintegrate.

There's another plus to having multitudes of smaller cells, too.

所以擁有許多小細胞是有好處的，

It's hardly a tragedy if one gets punctured, infected, or destroyed.

如果某個細胞被刺破、 感染或是消毀也沒關係。

Now, there are some exceptionally large cells

但有些特別大的細胞，

that have adapted to cheat the system,

自有自己的一套辦法，

like the body's longest cell,

就像人體最長的細胞，

a neuron that stretches from the base of the spine to the foot.

神經細胞可以從尾椎延伸到腳底。

To compensate for its length, it's really thin,

為了彌補長度，它非常纖細，

just a few micrometers in diameter.

直徑只有幾微米。

Another example can be found in your small intestine,

另一個例子在你的小腸，

where structures called villi fold up into little fingers.

稱作絨毛的結構摺疊成小指針 。

Each villus is made of cells with highly folded membranes

每根絨毛細胞的細胞膜層層折疊，

that have tiny bumps called microvilli to increase their surface area.

上面有稱作微絨毛的突起增加表面積。

But what about single-celled organisms?

但單細胞生物呢？

Caulerpa taxifolia, a green algae that can reach 30 centimeters long,

杉葉蕨藻， 是可高達 30 公分長的綠藻，

is believed to be the largest single-celled organism in the world

被認為是世上最大的單細胞生物。

thanks to its unique biological hacks.

這歸功於它獨特的生物特徵。

Its surface area is enhanced with a frond-like structure.

表面長成葉狀的結構。

It uses photosynthesis to assemble its own food molecules

用光合作用收集所需的食物分子。

and it's coenocytic.

而且它是多核細胞。

That means it's a single cell with multiple nuclei,

也就是說雖然是單細胞生物， 卻有很多細胞核，

making it like a multicellular organism but without the divisions between cells.

所以它像多細胞生物一樣運作， 但卻沒有分裂。

Yet even the biggest unicellular organisms have limits,

儘管如此， 最大的單細胞生物也有限制，

and none grows nearly as large as the elephant, whale, or bear.

所以不會大得像大象、鯨魚或是熊。

But within every big creature are trillions of minuscule cells

但每種體型大的生物 也是幾兆個小細胞組成，

perfectly suited in all their tininess

各自默默地固守崗位

to keeping the Earth's giants lumbering along.

讓這些大怪物趴趴走。