The plasma membrane contains channels that favor the transport of materials from both sides, i.e., from the intracellular medium to the extracellular environment. Membrane channels are a class of proteins that enable the passive transport of ions and chemical compounds by using the electrochemical gradient in both sides of the membrane. For example, channel proteins in the cell membrane known as aquaporins enable the transport of water across the cell membrane much more rapidly than by diffusion across this barrier.
