what作为连词的用法

A is to B what C is to D,意思是说A之于B正如C之于D.what 作为连词,引导状语从句.
有这种用法.
又如：
Air is to us what water is to fish.
空气对我们 (人) 的关系犹如水对鱼的关系
What lungs are to the human,leaves are to the plant.
叶之于植物犹如肺之于人