Diffusion Through Ion Channels

Another type of transport involves membrane proteins known as ion channels. When ion channels transport ions from higher to lower concentrations they are a form of passive transport. Ion channels transport ions such as sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl~). These ions are important for a variety of cell functions. Because they are not soluble in lipids, however, ions cannot diffuse across the phospholipid bilayer without assistance. Ion channels provide small passageways across the cell membrane through which ions can diffuse. Each type of ion channel is usually specific for one type of ion. For example, sodium channels will allow Na+ ions to go through, but will not allow Ca2+ ions or Cl~ to enter the cell.

Some ion channels are always open. Others have "gates" that open to allow ions to pass or close to stop their passage. The gates may open or close in response to three kinds of stimuli: stretching of the cell membrane, electrical signals, or chemicals in the cytosol or external environment. These stimuli therefore control the ability of specific ions to cross the cell membrane.

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