Transport defect: Within the body, many molecules are able to pass across the membranes that surround cells. These molecules can accomplish this feat due to specific transport systems. These systems include special receptors on the membrane of the cell and special carrier proteins. The receptor recognizes the molecule and receives it on the cell membrane. Then the molecule hitches a ride through the cell membrane on the back of a carrier protein.
With such remarkable specificity, it is little wonder that sometimes there are defects in transport systems. Several dozen different diseases are now known to be due to transport defects.
An example of a transport disease is cystinuria, the most common defect known in the transport of an amino acid (namely, cystine) and a significant cause of kidney stones. Like cystinuria, all transport defects are genetic (inherited).