Transporters are the gatekeepers for all cells and organelles, controlling uptake and efflux of crucial compounds such as sugars, amino acids, nucleotides, inorganic ions, and drugs. Transporters can be divided into passive and active transporters. Passive transporters allow passage of solutes across membranes down their electric and/or concentration gradients. Active transporters, which typically move molecules against their concentration gradients, are classified as primary or secondary-active transporters according to the directness of coupling to cellular energy. Primary active transporters utilize ATP hydrolysis as the energy source for protein function (e.g. Na/K pump), while secondary active transporters, known as co-transport or coupled transporters, do not directly couple their function to ATP hydrolysis. The electrochemical potential created by pumping ions out of the cell is utilized to power the function of secondary active transporters.

A scientist prepares a genetically engineered cell culture to test the rate of transport of different classes of molecules across the cell membrane. In order to procure the best results, the scientist tests a series of different molecules with their respective molecule-specific transporters.

Table 1. Flux rates of various compounds using different transportation mechanisms

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