Facilitated diffusion

Not like simple diffusion the facilitated or carrier-mediated diffusion of a molecule across a biological membrane is dependent on specific integral membrane

                             Figure: Kinetics of (a) simple and (b) facilitated diffusion

proteins,  frequently  known as  uniporters.  The molecule binds to the protein on one side of the membrane and the protein then undergoes a conformational modified, transports the molecule across the membrane and then releases it on the other side. The Molecules transported across membranes in this way involve hydrophilic molecules like as glucose, other amino acids and sugars. The transport proteins are specific for one particular molecule or a set of structurally same molecules. The transport proteins are capable of being saturated to display Michaelis–Menten-type binding kinetics (K_m   and V_max), and  are infiuenced  through  temperature,   inhibitor  and  pH  molecules   in  a  same  manner  to enzymes.

An instance of facilitated diffusion is the uptake of glucose into erythrocytes through the glucose transporter.  Erythrocyte  glucose  transporter  is an integral membrane  protein  of  mass  45  kDa  which  is  asymmetrically   orientated  in  the plasma  membrane.  That  uniporter  protein  structure  traverses  the  membrane with  12  α-helices that  form  a central  pore  by that  the  glucose  molecule  is  passed  upon  conformational   modifies  in  the protein that is shown in the above figure. All the steps in the transport of glucose into the cell are freely reversible the way of movement of glucose being dictated through the relative concentrations of glucose on either side of the membrane.  In the order to maintain the concentration  gradient  across  the membrane  the glucose  is rapidly  phosphorylated  inside the cell through hexokinase  to glucose  6-phosphate that  is  no  longer  a  substrate  for  the  glucose  transporter. 

Erythrocyte glucose transporter is highly specific for D-glucose or K_m   1.5 mM, the nonbiological L-isomer being transported at a barely measurable rate. D- galactose and D-Mannose, that differ from D-glucose in the configuration at one carbon atom are transported  at intermediate  rates. Therefore the transporter has a higher affinity lower Km for glucose than for other sugars.Several  cells,  like  as  erythrocytes  and  those  in  the  kidney,  hold  water-channel  proteins,  aquaporins,  which  accelerate  the osmotic  fiow  of water.  Every aquaporin protein is a tetramer of identical 28 kDa subunits by each subunit containing six transmembrane α-helices. The water molecules move by a central pore in every subunit. Aquaporins allow cells to move large quantities of water rapidly across their plasma membrane.