Membrane conveyer

See also: Channel

The membrane conveying are intrinsic Protéine S with the lipidic membrane cellular which allow the passage of the Métabolite S. They have two principal properties:

  • they are selective.
  • they are controlled in a very fine way.

The membrane is impermeable with the absorbent molecules. This makes it possible to avoid with the cell losing its contents and makes it possible to separate the interior of outside. But the chemical reactions proceed for the majority in aqueous solution and the conveyers allow to cross the membrane to reach the place where they will be used. There are two main categories: channels and pumps.

Channels

See also the detailed articles ionic Channel and Biophysics of the ionic channels

The protein-channel ensure a passive transport of molecules through the membrane. The passage of the molecules through a channel follows the laws of the diffusion. However they can be more or less selective. They can also close and open according different to stimuli (electric, chemical, mechanical…). They play a big role in the definition of the Potentiel of membrane, the sensitivity of certain cells to certain external signals…

Pumps

The pumps are different from the channels by the fact that it is not any more the electrochemical Gradient molecules which ensures the ionic movement but the coupling of transport a exergonic enzymatic reaction, as the hydrolysis of ATP. The movement of the molecule thus becomes one-way and can even occur against the electrochemical gradient. The molecule thus concentrates or on the contrary is completely eliminated from the cell. Transport is active here and either liability as for the channels.

The most known conveyer is the pump Na+/K+ (or Na+/K+ ATPase) which expels three ions Sodium and makes enter two ions Potassium for each molecule of hydrolized ATP. This conveyer is very important in the cell because it ensures the creation and the maintenance of the membrane electrochemical gradient and is indirectly at the origin of the transmembrane majority of the movements. If this pump stops, the electrochemical gradient disappears and the ionic movements stop plasmic Membrane on both sides.

Other very important active conveyers in the cell are those which concentrate the Calcium Cytoplasme towards the endoplasmic Réticulum and thus maintain a concentration cytosolic free about the hundred nanomoles. This very low concentration will be exploited by the cell; calcium is used by many receiving like signal to prevent the cell of the presence of its molecule activatrice (Hormone in general) on its active site.

Co-transport

The transport of molecules against their electrochemical gradient inevitably does not require the hydrolysis of the ATP. There exist many cases where the energy is provided by a Ion or another Molécule which follows its electrochemical gradient. This phenomenon is called transport coupled or Co-transport, because it couples an ionic channel with a membrane pump and uses the energy of to activate the other. According to the direction of respective displacement of the two molecules one speaks about symport (the ion and the transported molecule crosses the membrane in the same direction) or about antiport (the two chemical species move in opposite direction). This coupled transport is very much used by the cell to recover the molecules necessary to its metabolism in the external medium.

Energy comes from the electrochemical gradient, maintained amongst other things by the Na+/K+ pump, and which one can consider that in fact well the ATP provided energy, in an indirect way, from where the primary education term of transport to indicate the ATPasiques pumps and of secondary transport for coupled transport.

See too

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