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Not only different transporters reside in the membrane with different three‐dimensional structures but also they transport their substrates through different transport mechanisms. According to their transport mechanisms, transporters can be divided into passive and active transporters: passive transporters, also called facilitated transporters, allow molecules to move across cell membranes down their electrochemical gradients. Such a spontaneous process decreases free energy and increases entropy in a system, and therefore does not consume any chemical energy. In contrast to facilitated transporters, active transporters typically move molecules against their electrochemical gradients; such process is entropically unfavorable and therefore needs the coupling of the hydrolysis of ATP as an energy source. This coupling can be either primary or secondary. In primary active transport, transporters that move molecules against their electrical/chemical gradient hydrolyze ATP. In the secondary active transport, transporters utilize ion gradients, such as sodium or proton gradients, across the membrane produced by the primary active transporters and transport substrates against an electrochemical difference.