Integral proteins are integrated completely into the membrane structure, and their hydrophobic membrane-spanning regions interact with the hydrophobic region of the phospholipid bilayer. Proteins make up the second major component of plasma membranes. Therefore, phospholipids form an excellent lipid bilayer cell membrane that separates fluid within the cell from the fluid outside of the cell. In contrast, the middle of the cell membrane is hydrophobic and will not interact with water. Thus, the membrane surfaces that face the interior and exterior of the cell are hydrophilic.

The hydrophilic regions of the phospholipids tend to form hydrogen bonds with water and other polar molecules on both the exterior and interior of the cell.

When placed in water, hydrophobic molecules tend to form a ball or cluster. The tail is comprised of two non-polar lipid tails which are hydrophobic or water-hating molecules. The hydrophilic head contains a charged phosphate group which comes into contact with the aqueous fluid both inside and outside the cell. The main fabric of the membrane is composed of an amphiphilic phospholipid molecule. The plasma membrane separates the interior of a cell from its outside environment and is composed of phospholipids, proteins, cholesterol and carbohydrates.

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