In general, inside of a cell molecules move by a process called diffusion.

总的来说,细胞内的分子通过扩散作用来移动

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These details aren't important, it is described in your book, but the thing that is important to know is that diffusion occurs fast over short distances, but because of the physics of diffusion it occurs slowly over longer distances.

这些具体细节无关轻重,书上都有,重点是你要明白,距离越短扩散越快,由于扩散现象的物理特性,距离越长扩散越慢

耶鲁公开课 - 生物医学工程探索课程节选