TRANSPORT MECHANISMS

Not only do different transporters reside in the membrane with different threedimensional

structures, but they also 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 membrane 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 a process is entropically unfavorable

and therefore needs coupling of the hydrolysis of adenosine triphosphate (ATP) as an

energy source. This coupling can be either primary or secondary. In primary active

transport, transporters that move molecules against their electrical or chemical gradient,

hydrolyzeATP. 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.