ABSTRACT
The plasma membrane is a selectively permeable barrier. Some molecules can pass directly through the bilayer unaided, whereas other molecules require the presence of integral membrane transport proteins. The molecule moves from a high concentration to a lower concentration. There are two types of passive transport: simple diffusion and facilitated diffusion. Only relatively small uncharged or hydrophobic molecules cross the lipid bilayer by simple diffusion. Passive transport by simple diffusion does not require the presence of integral membrane proteins. Passive transport by facilitated diffusion requires the presence of specific integral membrane proteins to facilitate the movement of the molecule across the membrane. The active transport of molecules requires an input of metabolic energy. This can be derived either from direct coupling to the hydrolysis of adenosine triphosphate (ATP) or by coupling to the movement of an ion down its concentration gradient.
