An example of a selectively permeable entity is the phospholipid bilayer cell membrane, which surrounds the cells of the human body. Selectively permeable cell membranes allow the passage of molecules through active or passive transport.
Active transport uses cellular energy to move molecules, while passive transport does not require cellular energy. Active or passive transport can only be used for molecules that are small enough to pass through the plasma membrane. Large molecules pass through cells by surrounding themselves with fluid sacks called vesicles. Proper passage of molecules to cell membranes is essential for cell functions such as aerobic respiration and protein production.