Permeation is enhanced by membrane proteins

The conformational change of the carrier protein moves the solute from one side of the membrane (a state designated S) to the opposite side (designated S ), from which it is released. If the rate of release of transported solute is rapid compared to the rate of conformational change ( 3 k2), the concentration of solute on the opposite side of the membrane is negligible (S 0), and binding of solute to carrier is at equilibrium. The flux of solute across the membrane is given by  

The concentration of the solute-carrier complex is assumed constant (i.e., the rate of formation of the complex is equal to the rate of dissociation), so that  

Both facilitated and simple diffusion depend on concentration gradients net solute transport always occurs from high to low concentration. Unlike diffusion, facilitated transport systems are specific, since they depend on binding of the solute to a site on the transport protein. For example, the D-glucose 

The neutral amino acid transporter is responsible for movement of neutral amino acids from the blood into the brain these compounds are not soluble in membranes and, therefore, would not diffuse into the brain in the absence of a transport system. This transporter is very efficient in brain capillaries, but it is found in other tissues as well (Table 5.5). The concentrations of amino acids in the blood are close to the values of for brain capillaries, suggesting that the transporter is near saturation under normal conditions. Because of this, and because the transporter is equally efficient with a number of amino acids, changes in the blood concentration of one neutral amino acid can influence the rate of transport of all the other neutral amino acids. 

Amino acid Intestinal epithelia Renal tubule Exocrine pancreas Red blood cells Liver Blood-brain barrier 

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