Four fundamental classes of transport system are present at all membranes. Pumps catalyze transport of ions or complex organic molecules against their thermodynamic gradients. At membranes other than the ATP-synthesizing membranes of mitochondria and chloroplasts, pumps are generally driven by ATP hydrolysis.
Carriers translocate a wide range of simple solutes, including ions, sugars, and amino acids.
Carriers are distinguished from pumps by not executing scalar reactions such as ATP hydrolysis. Ion channels are purely dissipative with respect to catalysis of transport and operate at very high turnover rates. Channels gate between open and closed states, and gating is frequently controlled either by membrane voltage or by a ligand, depending on the function of the channel. Less selective cation and anion channels also are present in both membranes, as are malate-selective channels in the tonoplast. Aquaporins facilitate rapid transport of water across the plasma membrane and tonoplast, bypassing an alternative pathway for water transport that involves permeation through the lipid bilayer.