What are the functions of membrane proteins?

Integral proteins serve as:

(i) channels that permit the passage of selected ions through the membrane;

(ii) carriers (or transporters) ‘that ferry substances across the membrane by binding to them;

(iii) pumps that are carriers which can split ATP and use the energy derived for membrane transport of substrates;

(iv) receptors (located on the outside) which bind to specific molecules and send a chemical signal to the cell interior, initiating intracellular reactions;

(v) enzymes, catalyzing reactions at the membrane surfaces – both outer and inner.