What defining characteristic makes a protein a ""transmembrane protein""?

Transmembrane proteins are the integral proteins that exhibit a characteristic feature of spanning the whole membrane at least one time. These are the proteins, which cross the membrane and helps in the conduction of ions and molecules through it. Some of the transmembrane proteins that span the cell membrane many times are known as polytopic transmembrane proteins.  

Some of these proteins act as receptors, while some produce channels. The movement of ion, which does not require any form of energy is termed as passive transport, while in certain cases energy in the form of ATP is used for the transportation of ions across the membrane, which is termed as active transport.  

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