WHAT IS FACILITATED DISSEMINATION? MAIN FEATURES - BIOLOGY - 2025

The facilitated diffusion is a type of passive transport cell in which to move molecules within the cell solution, we need the intervention of some energy source.

Diffusion is one of the ways of transporting substances between cells. For any type of diffusion to occur, there must be a concentration gradient, or what is the same, there must be an uneven distribution of molecules in the cell space.

Diffusion allows processes such as gas exchange in gills and lungs. The movement of a molecule of any substance from the aqueous solution towards the hydrophobic part of the phospholipid bilayer of the cell is the initiation of diffusion.

The speed of diffusion is expressed in molecules per second. Diffusion involves a measure that indicates the tendency of a molecule to "run away" from water: K.

The higher the K value, the greater the possibility that a molecule will dissolve in the lipid bilayer.

K is also a measure proportional to the permeability coefficient of the cell membrane (P), and to the difference in concentrations on both sides of the membrane (C1aq-C2aq).

There are two types of diffusion: simple diffusion and facilitated diffusion.

Characteristics of facilitated diffusion

Facilitated diffusion is a type of passive diffusion. It is carried out through long polypeptide chains: channel proteins and unitransporter proteins.

When permeases are involved in this diffusion, the transported substance binds to the protein on one side of the membrane, while when channel proteins are involved, they do not bind to the substance.

Channel proteins undergo opening and closing movements. These movements are regulated in different ways:

Regulation by ligand binding

It occurs when hormones, neurotransmitters, ions, or nucleotides are transported.

Regulation by voltage changes

It occurs when the polarity is changed at a point on the cell membrane, which changes the structure of the channel and opens it.

Mechanical regulation

It occurs thanks to direct stimulation on the membrane.

Among the main transporters that are presented in the facilitated diffusion are:

-The omnipresent glucose transporters (GLUT).

-Amino acid transporters.

-Facilitating urea transporters and others.

The facilitating transporter facilitates a translocation pathway across the membrane.

That is to say that the facilitating transporters constitute several families of genes in the human genome.

Thanks to facilitated diffusion, small ions such as K +, Na +, Cl-, monosaccharides, and amino acids can cross the cell membrane.

This process allows certain substances to pass through the membrane without having direct contact with hydrophobic lipids (which move away from the water).

The movement of the molecules that are transported by facilitated diffusion occurs in favor of the concentration gradient. This means that it only works from high to low concentration.

The speed of cellular transport by facilitated diffusion is greater than that of simple diffusion, but it depends on the amount of channels available in the membrane and its saturation point arrives if all the transporters go to their maximum capacity.

Through facilitated diffusion, a specific type of molecule or a group of closely related molecules is transported.

According to the evidence collected so far, there are few glucose transporting proteins, but this is a substance that enters most cells by facilitated diffusion.

Transport cycle in facilitated diffusion

The transport cycle that occurs in the facilitated diffusion process responds to the alternative access model, according to which the binding site of a transport protein is alternately exposed to one or the other side of the cell membrane.

Thus, there is no open and unrestricted permeation pathway connecting the fluid compartments separating the membrane.

Then the binding of the substrate to its binding site induces the conformation of an occluded state, after which new changes occur that expose the substrate to the other side.

After this the binding site is reoriented to the original membrane surface. This cycle is repeated as many times as required so that the substance no longer needs to be transported.

If the concentration gradient of the substance is reversed for physiological or experimental reasons, the direction of the transporter will also be reversed.

On the other hand, when the substrate concentration is the same on both sides of the membrane, the facilitating transporter is in equilibrium and its transporting action is not necessary.

Example of facilitated dissemination

One of these transporter proteins is insulin, which facilitates the diffusion of glucose, reducing its concentration in the blood.

Charged ions dissolved in water can only be transported thanks to the intervention of proteins that form transmembrane channels.

Larger molecules also cross the cell membrane thanks to transmembrane carrier proteins, such as permeases.

References

1. Biology on line (s / f). Facilitated diffusion. Recovered from: biology-online.org
2. Herrera Shirley and others (2011). Type of membrane bearings. Recovered from: membranascelulares.blogspot.com
3. Khanacademy (2011). What is facilitated diffusion? Recovered from: khanacademy.org
4. Physiology web (2016). Facilitated diffusion. Recovered from: physiologyweb.com
5. Zúñiga Blanco (2009). World Biology 10th and 11th. Second edition. Editorial Eduvisión. San Jose Costa Rica. Pages 43 and 44.