The plasma membrane is an outer boundary that separates the interior of a cell from its external environment and is found in all cells. The molecules that make up the plasma membrane have a unique quality that helps to ensure the integrity of the cell. Considering the structure of the plasma membrane, which of the statements best explains why human cells do not dissolve in water? The fatty acid tail regions of the phospholipids of the plasma membrane repel water. The tough cell wall of human cells creates a biological barrier that keeps water out. Pores of the skin push water back out as soon as water comes into contact with skin cells. Cholesterol in the plasma membrane repels water because lipids are hydrophobic.