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

這麼碩大的體積，是由超過一千兆個微小的細胞所構成

and on the epically small end of things,

而在微觀世界中

there are likely millions of unicellular species,

有幾百萬種單細胞生物

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

但憑我們肉眼能看到的，極其少數

Why is that?

這是為什麼阿？

Why don't we get unicellular elephants,

為什麼沒有單細胞的大象、

or blue whales,

藍鯨

or brown bears?

或棕熊呢？

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

我們得仔細端詳細胞內部，才能一探究竟

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

這裡是大部分細胞功能運作的地方

enclosed by a cellular membrane

外層由細胞膜將其包覆

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

並作為細胞的出入口

Any resources the cell needs to consume,

細胞所需的任何養分

or waste products it needs to expel,

或要排除的廢物

first have to pass through this membrane.

都得先通過這層膜

But there's a biological quirk in this setup.

但這樣的結構，在生物上有個奇怪現象

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

細胞的表面積和體積，是以不同比率增加

Cells come in many shapes,

細胞有很多種形狀

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

把它們想像成立方體，會比較容易計算

A cube has six faces.

一個立方有六個面

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

這些面代表細胞膜，組成細胞表面積

A cube measuring one micrometer on each side,

一個每邊長一微米的立方體

that's one millionth of a meter,

一微米就是百萬分之一公尺

would have a total surface area of six square micrometers.

這樣表面積總共是六平方微米

And its volume would be one cubic micrometer.

而它的體積則是一立方微米

This would give us six units of surface area for every single unit of volume,

所以每單位體積會有六單位的表面積

a six to one ratio.

六比一的比例

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

但若把立方變成十倍大，情況就會截然不同

measuring ten micrometers on each side.

立方每邊長為十微米

This cell would have a surface area of 600 square micrometers

細胞表面積就會是六百平方微米

and a volume of one thousand cubic micrometers,

體積則是一千立方微米

a ratio of only .6 to one.

比例變成只有 0.6 比 1

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

每單位體積分不到一單位的表面積

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

當立方變大時，體積會比表面積增加得更快

The interior would overtake the membrane,

內部體積很快就會讓細胞膜無法負荷

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

使表面積過小，物質就不能迅速進出細胞

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

巨型的細胞會屯滿廢物，最後死亡、崩解

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

擁有許多小細胞，有其好處

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

就算其中一個細胞被刺穿、感染或破壞也無傷大雅

Now, there are some exceptionally large cells that have adapted to cheat the system,

有些超大的細胞，找到方法躲過前述生物機制

like the body's longest cell,

像是人體最長的細胞

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

是從脊髓基部延伸到腳的神經元

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

作為其長度的代償，細胞非常細

just a few micrometers in diameter.

直徑只有幾微米

Another example can be found in your small intestine,

另一個例子則在小腸裡

where structures called villi fold up into little fingers.

絨毛會摺疊成像小手指的結構

Each villus is made of cells with highly folded membranes

每個絨毛是由表面高度折疊的細胞所組成

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

而細胞上面的突起，叫微絨毛，它能增加表面積

But what about single-celled organisms?

那單細胞生物又是如何呢？

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

杉葉蕨藻，一種可長達三十公分的藻類

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

被認為是世界上最大的單細胞生物

thanks to its unique biological hacks.

因為它有獨特的生物結構

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

蕨類葉片般的結構提升它的表面積

It uses photosynthesis to assemble its own food molecules

它利用光合作用，自行合成所需養分

and it's coenocytic.

而且它是多核生物

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

意思是，它是擁有多細胞核的單細胞生物

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

它就像多細胞生物，只是沒有細胞分裂

Yet even the biggest unicellular organisms have limits,

就算是最大的單細胞生物也有極限

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

所以沒有單細胞生物可以長到跟大象、鯨魚或熊一樣大

But within every big creature are trillions of minuscule cells

但每個大型生物，都是由數兆個微小的細胞組成

perfectly suited in all their tininess

每個細胞都乖乖地固守崗位

to keeping the Earth's giants lumbering along.

好讓地球上的這些龐然大物能遊走四方