<P> Specialized transmembrane proteins recognize the substance and allow it to move across the membrane when it otherwise would not , either because the phospholipid bilayer of the membrane is impermeable to the substance moved or because the substance is moved against the direction of its concentration gradient . There are two forms of active transport , primary active transport and secondary active transport . In primary active transport , the proteins involved are pumps that normally use the chemical energy in the form of ATP . Secondary active transport , however , makes use of potential energy , which is usually derived through exploitation of an electrochemical gradient . The energy created from one ion moving down its electrochemical gradient is used to power the transport of another ion moving against its electrochemical gradient . This involves pore - forming proteins that form channels across the cell membrane . The difference between passive transport and active transport is that the active transport requires energy , and moves substances against their respective concentration gradient , whereas passive transport requires no energy and moves substances in the direction of their respective concentration gradient . </P>
when is active transport used in the body
Specialized transmembrane proteins recognize the substance and allow it to move across the membrane when it otherwise would not