 Dear students, in this module we shall discuss the facilitated diffusion. That is a type of diffusion that occurs with the help of transport pertains. The principles of diffusion that is passiveness and concentration gradient also apply to this type of diffusion. This type of diffusion facilitates the movement of polar substances and many ions across the membrane. The transport pertains that are involved in facilitated diffusion are very specific. They transport some substances but not all. There are two types of transport pertains involved in facilitated diffusion. The first one are the channel pertains and the second are carrier pertains. The channel pertains are the integral pertains of the membrane that extend across the two surfaces of the membrane. They are comparatively large. Their one end faces the extracellular end of the plasma membrane and the inner end faces the intracellular surface of the membrane. So, they extend across the membrane. Small pores are present in these pertains which are water-filled. And these pores are called membrane channels. These are membrane channels due to which ions or other molecules cross the membrane. These pores are not very large. They are very small and are often less than 1 nanometer in diameter. These membrane channels have the capacity to open or close. Their opening and closing is regulated. Polar substances can pass through these channels only when they are open. The channel pertains are of two major types. One are the water channel pertains and the second type include the ion channel pertains. In addition to these two, there are many specialized pertains which also function as membrane channels. First, we shall discuss the water channel pertains which are commonly known as aquaporins. Aquaporins facilitate the diffusion of water molecules and small ions, for example, sodium, potassium, calcium and chloride in their dissolved form. In water channel pertains may be hydrophilic channels or passageways whose water molecules or ions move quickly in dissolved form. The ion channel pertains have gated channels that open or close in response to an electrical or chemical stimulus. When their gates are open, they allow specific ions to move through them. The examples of ion channel pertains include voltage gated channels. Voltage gated channels include very active channels such as voltage gated sodium, channel voltage gated potassium channel and voltage gated calcium channel. In addition to this, there are ligand gated ion channels in channel pertains. Membrane has some specialized pertains which facilitate some small molecules or uncharged polar molecules which cannot be crossed with a simple diffusion. For example, carbon dioxide, nitric oxide or carbon monoxide. In addition to channel pertains, the facilitated diffusion is also carried out by special transport pertains called carrier pertains. Many substances that are polar and cannot be transported either by simple diffusion or by ion channels, they are transported through carrier pertains. Such molecules include many sugars, amino acids, nucleotides and certain metabolites. Carrier pertains are also very specific about the species of molecules they facilitate to diffuse. Carrier pertains are present in all types of membranes. There are two major functional forms of these carrier pertains. First are the unipoters and second are the coupled transporters. Coupled transporters are also of two types, simporters and antipporters. Unipoters are such pertains which transport single solute from one side of the membrane to the other. Whereas, coupled transporters are required to transfer two solutes simultaneously. Coupled transporters are of two types, simporters that transfer two solutes in the same direction and antipporters that transfer two solutes in opposite directions. The transport mechanism of these carrier pertains depends on their capacity to alternate between two conformations. They have a solute binding site that can sequentially translocate from one side of lipid bilayer to the other. This type of translocation is because of the change in the shape of the pertain. When the solute molecule binds with the pertain molecule, the shape of the pertain changes. If one end was on the outer surface at the same time, then the shape changes to the opposite end. This happens because of the binding and releasing of transported molecules.