the surface areas over which resources are exchanged with the environment 为什么要用over which

"Over which" is used to indicate the area or surface that is involved in the exchange of resources with the environment. It emphasizes the spatial extent or coverage of the exchange process. For example, the surface areas over which plants exchange gases with the atmosphere include the leaves, stems, and roots. Using "over which" clarifies that the exchange is happening across a certain area or surface rather than just at a single point.