inside of a cell molecules move by a process called diffusion.

细胞内的分子通过扩散作用来移动
They move from regions of high concentration

从高浓度区域
to regions of low concentration

向低浓度区域流动
by this natural flow down a concentration gradient.

顺浓度梯度流动
Movement down a concentration gradient occurs fairly rapidly

顺浓度梯度扩散在一微米左右的短距离内
over short distances like a micron or so,

相当迅速
like the size of a bacterium.

一微米相当于一个细菌的大小
Diffusion mixes things very well,

扩散作用使物质充分混合
but as the size gets bigger,

但随着细胞变大后
diffusion becomes very slow.

扩散作用就捉襟见肘了
You're familiar with this,

举个常见的例子
if I had a jar of acetone or some kind of a volatile liquid,

假如你有一瓶丙酮或者其它挥发性液体