Question

What is the feature of the cell membrane that causes it to be able to "choose" what materials enter and exit?

Answer 1

Cell membranes are semi-permeable. That is why they can allow only some substances to enter the cell. It basically depends on the size of the molecules. Smaller atoms like water can easily pass through. However, macromolecules such as sugars, oils and proteins cannot easily pass because they contain a long chain of monomers.