Cells are incapable of producing all the molecules they need for their growth, sustenance and reproduction. Also, they need to excrete some molecules into the outside environment. Cells are enclosed by cell membrane, a phospholipid bilayer, which is selectively permeable. What this means is that only few selective molecules can enter the cell, while the others may need a mechanism for entry into the cell. Transport proteins in the cell membrane provide that assistance for molecular transport. There are three types of transport proteins: ATP powered pumps, transporters and channel proteins. These proteins help in both the active and passive transport of molecules, with and without expense of energy, respectively.
If a drug inhibits these transport proteins, the cells will not be able to obtain vital molecules, like water, glucose, amino acids, etc. and hence, will not be able to carry out their life processes.
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