Final answer:
The cell membrane is a pliable structure composed of phospholipids. It is selectively permeable, allowing only specific substances to pass through the lipid bilayer.
Step-by-step explanation:
The cell membrane is an extremely pliable structure composed primarily of back-to-back phospholipids (a "bilayer"). Cholesterol is also present, which contributes to the fluidity of the membrane, and there are various proteins embedded within the membrane that have a variety of functions.
The membrane's lipid bilayer structure provides the first level of control. The phospholipids are tightly packed together, and the membrane has a hydrophobic interior. This structure causes the membrane to be selectively permeable. A membrane that has selective permeability allows only substances meeting certain criteria to pass through it unaided.
In the case of the cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer. Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol.