Facilitated diffusion gated channels

ABC superfamily 239 action potential 231 active transport 233 antiport 254 cotransport 254 depolarization 263 electrochemical gradient 232 facilitated diffusion 233 gated channel 233 hypertonic 258 hypotonic 258 isotonic 258... 

Membrane proteins carry out a wide range of critical functions in cells, and they include passive and active transporters, ion chamiels, many classes of receptors, cellular toxins, proteins involved in membrane trafficking, and the enzymes that facilitate electron transport and oxidative phosphorylation. For example, the voltage-gated ion channels that facilitate the passive diffusion of sodium and potassium across the axonal membrane are responsible for the formation of an action potential. Active transport proteins establish ion gradients and are necessary for the uptake of nutrients into cells. Soluble hormones bind to membrane receptors, which then regulate the internal biochemistry of the cell. 

Figure 5.12 Transport proteins in cell membranes, (a) Energy-dependent, ATP-powered ion pumps such as the Na /K exchange ATPase (b) channels, gated or non-gated, which permit diffusion through an aqueous pathway, such as voltage-gated Na channels (c) passive, facilitated transport systems, which can act in uniport, symport, or antiport modes, such as the glucose transporter. The filled arrow indicates gradient of molecules indicated by the filled symbol.

---