Membrane transport

The membrane transport is the passage of a Molécule or a Ion through a plasmic membrane. It implies a displacement between two compartments separated by a membrane, whose properties and composition will influence this transport. There exist several types of membrane transport.

Passive transport

Passive transport is a transport which is done without consumption of energy, it is thus made along the electrochemical Gradient (or Gradient of concentration). It allows, when two solutions of different concentrations are separated by a membrane, that molecules cross it so that to balance, concentrations become identical (isotonic solutions). There are 2 types: the diffusion (" diffusion simple" or " diffusion libre" and " diffusion facilitée") and osmosis.

Diffusion

The diffusion is not possible that if the solutions are separated by a permeable membrane.

Simple or free diffusion

The simple diffusion is the diffusion in the plasmic membrane (in the direction of the strong concentrations towards the weak concentrations, until balance of the concentrations on both sides of the membrane). The free diffusion is a passive physical phenomenon. This type of passage is not possible that if the molecule is “soluble” in the phospholipidic membrane, i.e. it can cross directly the double-layered one of Phospholipide S. the molecule must thus be Hydrophobe (Apolaire) or, if it is Hydrophile (Polaire), being sufficiently small (in practice: ethanol).

The characteristics of this transport are:

an absence of saturation, the speed of diffusion depends only on the difference in concentration (gradient of concentration, or electrochemical for ions);
an absence of specificity (it is not controlled);
and a certain slowness: the molecules must dissolve in the phospholipide double-layer before passing on other side.

This mechanism is slow compared to the facilitated diffusion.

Facilitated diffusion

There exists in biology a transport known as of facilitated diffusion. Like the simple diffusion, the difference in concentration is the engine of transport. However, the molecule directly does not cross the membrane, it must use a transmembrane protein of transport:

the proteins of channel (ionic channels): they should not change form to allow the passage.
Ce transport by proteins of channel is:
* very specific: they let pass only one or some kinds of molecules and not of others;
* extremely fast;
* and controlled, the proteins of channel have the capacity to be closed.
conveyers: they change form to move molecules from one side to another of a membrane. This transport is similar to that of the proteins channels, if it is not that it is generally slower and that it can also transport molecules or ions against their electrochemical gradient (one speaks then about active and nonpassive transport and pump in the place of conveyer).

This type of transport is also saturable, and speed is limited by accessibility to the conveyer. The throughput speed is governed by the equation of Michaelis Menten. The Perméase S are of this type, but also the conveyers of Glucose like much of others. It does not depend either on the hydrophobicity of the moved substances.

Osmosis

Osmosis is a passive physical phenomenon which takes place only if the solutions are separated by a semipermeable membrane. Only the water molecules cross the membrane of the hypotonic solution (the most diluted) towards the hypertonic solution (the most concentrated solution) until the solutions are isotonic (of the same concentrations). One meets osmosis as well for the alive cell as for the dead cell. If the two mediums are of the same concentrations, no water movement is perceptible: the cell is in osmotic balance.

Active transport

Active transport implies the transfer of a molecule against the gradient of concentration (i.e. compartment less concentrated = hypotonic solution towards the compartment more concentrated = hypertonic solution). It thus required there to provide energy because this transport is not spontaneous. There exist two types of active transport according to the energy source used but in the 2 cases a protein of transport is necessary.

Primary credit transport

In this type of transport, the conveyer directly uses the energy provided by a exoenergetic reaction (generally the hydrolysis of ATP). It is the case for example of the Na+/K+ ATPase.

Secondary active transport

In this type of transport, displacement against the gradient of concentration of the molecule is carried out by the dissipation of another gradient, even built to him by a primary credit transport. It is the case for example transport of chloride in certain epitheliums which secrete NaCl.

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