They move from regions of high concentration to regions of low concentration by this natural flow down a concentration gradient.

从高浓度区域,向低浓度区域流动,顺浓度梯度流动

耶鲁公开课 - 生物医学工程探索课程节选

So if we consider lithium, potassium, and sodium, they're all together in the same group on the periodic table, knowing what we do about biology we can immediately think of sodium and potassium, or even just knowing what you know about table salt, for example, that these are two elements that we find, and particularly in the ion form in very high concentrations in our body.

我们来考虑一下锂，钾和钠，它们都在周期表中的同一族里，想要知道我们将元素周期律,用到生物学会怎么样，我们首先想到的就是钠和钾，甚至只需要知道食盐，我们发现它们是以很高浓度的离子形式,存在于我们体内的两种元素。

麻省理工公开课 - 化学原理课程节选