For these purposes think about a channel, it only allows sodium to go through for example.

基于此目的,我们来讲离子通道,拿仅允许钠离子通过的通道作为例子

耶鲁公开课 - 生物医学工程探索课程节选

and the last quarter is a seminar in which you only go once a week for three hours

最后一个季度是研讨会，每周只需要参加三个小时，

群集课程 - SpeakingMax英语口语达人

I can only go grocery shopping for a couple of days.

我只能一次买几天所需的东西。

与众不同的纽约 - SpeakingMax英语口语达人