Carrier-mediated transport exchange diffusion

The term exchange diffusion has another usage in the literature—namely, to describe the carrier-mediated movement of some solute in one direction across a membrane in exchangefor a different solute being transported in the opposite direction. Again, the Ussing-Teorell equation is not obeyed. 

Wodzki R. Exchange diffusion transport of ions through liquid membranes. Part II. Stationary process mediated by a multisided carrier. PolJChem, 1991 65(9-10) 1715-1729. 

At physiological pH, 7.2, 99% of anunonia is in the ionic form. The lui-ionized form diffuses across cell membranes. It was previously thought to simply pass via the phospholipid bilayer but it is now known to use specific transporters in human erythrocytes it is one of the rhesus-antigen proteins (rhesus associated glycoprotein) that mediates the exchange. The NH4+ ion is transported much more slowly via another carrier-mediated process. 

Many substrates cross cell membranes by processes other than passive diffusion. When the transport is carrier-mediated, e.g., facilitated diffusion, active transport, and exchange diffusion, the carrier modifies the conductance of the membrane and may either increase or decrease the flux of the substrate across the membrane. A common characteristic of all carrier-... 

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