The illustration shows means of transport through the cell membrane for small molecules, ions in this case. Transport proteins (carriers) transport substances out of and into the cell, either passively along with the concentration gradient or against it while using up energy actively or passively (active transport). Former ideas described mobile carriers in biological membranes (valinomycin-modell), but nowadays, the term permease is used in the context of eukaryotic cells. It describes reugalted transmembrane proteins.
