Biomedical Admissions Practice Test

Answer: A, The cell membrane is comprised of phospholipids. The cell membrane is actually made up of two sheets of phospholipids in an arrangement called a phospholipid bilayer.

Key Takeaway: The phospholipid bilayer is designed so that the hydrophilic heads face the watery regions inside and outside the tail and the hydrophobic tails face each other in a water-free environment. The bilayer forms spontaneously and it is responsible for the selective permeability of the cell membrane. Small and nonpolar hydrophobic molecules and freely pass across the membrane, but charged ions and larger molecules can only move across with processes that require energy.