The cell membrane surrounds the entire cell and separates its components from the outer environment. The cell membrane is a double layer made up of phospholipids (called the phospholipid bilayer). Phospholipids are molecules with a phosphate group head attached to glycerol and two fatty acid tails. They spontaneously form double membranes in water due to the hydrophilic properties of the head and hydrophobic properties of the tails.
The cell membrane is selectively permeable, meaning it only allows certain molecules to enter and exit. Oxygen and carbon dioxide pass through easily, while larger or charged molecules must go through special channels, bind to receptors, or be engulfed.