The relationship of leaves to plants is the same as that of lungs to animals.

Literal

Leaves [possessive-の] plants [against-に対する] relationship [topic-は] lungs [possessive-の] animals [against-に対する] relationship [with-と] same is-[である].

A biology-textbook-style analogy. ~に対する marks 'toward / with respect to.' The whole structure frames a proportional comparison: 'X:Y :: A:B.' である is the formal copula. Uses precise analytical structure.