An egg is protected by its outer shell. however, the shell also allows moisture and air to pass through. which cell structure performs a similar function?

A cell membrane would most likely be close to an egg structure. A cell membrane is a selective permeable to ions and organic molecules and controls the movement of substances in and out of the cell. 

The basic function of the cell membrane is to protect the cell from its surroundings. It consists of the lipid bilayer with embedded proteins.

Interacting group of various species in a common location. for example, a forest of trees and undergrowth plants, inhabited by animals and rooted in soil containing bacteria and fungi, constitutes a biological community.