Channels, carriers, pumps কী?

Channels are integral proteins with a water-filled pore through which a specific ion or small molecule diffuses down its gradient at up to $10^8$ per second; many are gated, opening in response to a voltage, a ligand or a mechanical force. Aquaporins are water channels. Carriers bind their solute on one face, change conformation, and release it on the other, a thousand times a second at most; the glucose transporter of the red cell is one. Channels and carriers working down a gradient perform facilitated diffusion, passive but selective and saturable. Pumps are carriers that couple the transport of a solute against its gradient to the hydrolysis of ATP: primary active transport. The sodium–potassium pump of animal cells exports three $\mathrm{Na^+}$ and imports two $\mathrm{K^+}$ per ATP; the proton pump of plant, fungal and bacterial membranes exports $\mathrm{H^+}$.