The cell membrane, or plasma membrane, is made up of a lipid bilayer in which hydrophilic heads face outward and hydrophobic tails face inward. This arrangement within the cell membrane allows it to

This arrangement allows it to become partially permeable. It allows fat-soluble substances e. g vitamin D to pass directly through cell membrane and prevents water-soluble molecules e. g glucose, amino acids etc. from entering or leaving the cell except through transport proteins.