At the boundary of every cell, the functions as a selective barrier that allows sufficient passage of oxygen, nutrients, and wastes to service the entire volume of the cell.

The cell membrane functions as a selective barrier. We call these kind of membranes as differentially permeable or semi permeable membrane. It can decide what substances can pass in and out. Therefore it can surround the cytoplasm (which is the inside of a cell) to separate the cell's content from the surroundings.