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The elephant is a creature of epic proportions,
大象是一种有着惊人比例的生物，
and yet it owes its enormity to more than 1,000 trillion microscopic cells,
它巨大的身体，比显微镜下的细胞大千万亿倍，
and on the epically small end of things, there are likely millions of unicellular species,
同时，在针尖一般大小的地方可能存在着约数百万的单细胞生物，
yet there are very few we can see with the naked eye. Why is that?
但是几乎没有是能用肉眼看到的。为什么呢？
Why don't we get unicellular elephants, or blue whales, or brown bears?
为什么不存在单细胞的大象，或是蓝鲸，亦或是棕熊呢？
To find out, we have to peer into a cell's guts.
为了探寻答案，我们得深入观察一个细胞的内部。
This is where most of the cell's functions occur,
这里是运作大部分细胞功能的地方，
enclosed by a cellular membrane that acts as the doorway into and out of the cell.
细胞膜把它们封闭在内，也作为细胞质进出的通道。
Any resources the cell needs to consume, or waste products it needs to expel, first have to pass through this membrane.
所有细胞消耗的来源或者是其需要排出的代谢废物，需要首先通过细胞膜。
But there's a biological quirk in this set up.
但这个系统中存在一种生物学的特性。
A cell's surface and volume increase at different rates.
细胞的表面积与体积以不同的速率增长。
Cells come in many shapes, but imagining them as cubes will make the math easy to calculate.
细胞有着多种形状，但是将它们想象成立方体在数学上会更容易计算。
A cube has six faces. These represent the cell membrane, and make up its surface area.
一个立方体有六个面。它们就是细胞膜，组成细胞的表面区域。
A cube measuring one micrometer on each side,
立方体的边长为1微米，
that's one millionth of a meter, would have a total surface area of six square micrometers.
也就是1米的百万分之一，所以表面积就是6平方微米。
And its volume would be one cubic micrometer.
体积是1立方微米。
This would give us six units of surface area for every single unit of volume, a six to one ratio.
因此每1个单位的体积对应6个单位的表面积，也就是比例为6:1。
But things change dramatically if we make the cube ten times bigger, measuring ten micrometers on each side.
但是如果将这个立方体放大十倍就大不同了，边长变为10微米。
This cell would have a surface area of 600 square micrometers
这个细胞的表面积会变为600平方微米，
and a volume of one thousand cubic micrometers, a ratio of only .6 to one.
体积变为1000立方微米，比例仅为0.6:1。
That's less than one unit of surface area to service each unit of volume.
每单位体积仅与不到一个单位的表面积相合作。
As the cube grows, its volume increases much faster than its surface area.
随着立方体增大，它的体积增加得比表面积快得多。
The interior would overtake the membrane, leaving too little surface area for things to quickly move in and out of the cell.
内部物质远多于细胞膜，仅留下很小的表面让细胞质快速进出。
A huge cell would back up with waste and eventually die and disintegrate.
巨大的细胞将被代谢废物所堵塞，最终死亡瓦解。
There's another plus to having multitudes of smaller cells, too.
拥有许多小的细胞还有一个好处。
It's hardly a tragedy if one gets punctured, infected, or destroyed.
即便一个细胞被刺穿、感染或杀死也不会太糟。
Now, there are some exceptionally large cells that have adapted to cheat the system, like the body's longest cell,
如今，也有一些例外的大型细胞适应并瞒过了系统，比如人体最长的细胞，
a neuron that stretches from the base of the spine to the foot.
从脊椎的底部延伸至脚底的神经细胞。
To compensate for its length, it's really thin, just a few micrometers in diameter.
为了弥补长度的影响，它非常细，直径仅为几微米。
Another example can be found in your small intestine, where structures called villi fold up into little fingers.
另一个例子在你的小肠里，那里有折叠成小拇指形状的结构，叫做绒毛。
Each villus is made of cells with highly folded membranes that have tiny bumps called microvilli to increase their surface area.
每根绒毛由细胞膜高度折叠的细胞组成，上面有细小的凸起，称为微绒毛，来增加表面积。
But what about single-celled organisms?
那单细胞生物呢？
Caulerpa taxifolia, a green algae that can reach 30 centimeters long,
杉叶蕨藻，一种绿色的约30厘米长的水藻，
is believed to be the largest single-celled organism in the world thanks to its unique biological hacks.
被认为是世界上最大的单细胞生物，多亏其独特的生物学结构。
Its surface area is enhanced with a frond-like structure.
像植物一样的结构增加了它的表面强度。
It uses photosynthesis to assemble its own food molecules and it's coenocytic.
它进行光合作用，组成食物分子。
That means it's a single cell with multiple nuclei, making it like a multicellular organism but without the divisions between cells.
而且，它在一个细胞内有多个细胞核，使得它像多细胞生物一样，只是细胞之间不分离。
Yet even the biggest unicellular organisms have limits, and none grows nearly as large as the elephant, whale, or bear.
即使是最大的单细胞生物也有极限，没有能长到像大象、蓝鲸和熊那样大小的。
But within every big creature are trillions of minuscule cells
但是在每一个巨大生物内部，是上万亿的小细胞
perfectly suited in all their tininess to keeping the Earth's giants lumbering along.
完美保持着它们的微型身材，推动地球上巨灵的进化生长。
