Facilitated Diffusion- Definition, principle, factors, examples

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Facilitated diffusion definition

Facilitated diffusion is the process of biological transport in which specific structural components of biological membranes interact with particular solutes or classes of solutes, markedly increasing the rates at which they can cross the membrane.

It is a passive-mediated transport in which particles or substances are transported across a biological membrane from a region with a higher concentration of an area of lower concentration facilitated by some transport protein.
As the movement of substances occurs in the direction of the concentration gradient (from higher to lower), no chemical energy or ATP is required.
However, the substances which are transported via facilitated diffusion would not otherwise move easily or quickly across the membrane.
Similarly, the membrane components responsible for facilitated diffusion are called transport mediators.

Principle (How does facilitated diffusion work?)

Image Source: Wikipedia (Mariana Ruiz Villarreal).

The lipid bilayer of a plasma membrane does not allow the transport of all molecules with the same ease.
Since the membrane is hydrophobic, it doesn’t allow the movement of hydrophilic as well as some highly polar m molecules.
Few of the hydrophilic molecules, along with smaller hydrophilic molecules, can quickly move across the membrane based on the concentration gradient.
However, the larger non-polar molecules require aid from transport mediators like the membrane carriers and channels.
The movement across the membrane can be made through one of the two mechanisms; one involving the carrier proteins and the other involving the channel proteins.
In the case of channel proteins, the transmembrane proteins present in the membrane act like a channel (pore) in the membrane, which allows the transport of the molecules.
These channels extend across the plasma membrane, connecting the external environment to the cytosol or reaching across the biological membranes of different cellular organelles.
Molecules like-charged ions are transported via the transmembrane channels formed by protein complexes.
In the case of carrier proteins, transporters, or carrier proteins embedded in the biological membrane are utilized.
These proteins have a specific affinity towards some molecules on the extracellular matrix.
The carrier proteins bind to the molecules which results in some conformational changes in the molecules, facilitating the movement across the membrane into the cytosol.
This mechanism of facilitated diffusion is employed for larger molecules like enzymes.

Channel proteins and carrier proteins

What are Channel proteins?

Channel proteins are integral proteins present in the biological membrane that allow the transport of molecules across the membrane by forming a channel.
The species that pass through channels, also called the transmembrane proteins as they span across membranes, are almost always ions.
Many channels are very selective, passing some ions readily while being substantially impermeable to others.
Based on structural models, channel diameters are thought to be no more than 4 – 5 Å, similar to the widths of the common biological ions.
Similarly, channels may pass cations readily but not anions or may exhibit very different permeabilities to two ionic species having the same charge, thus increasing the selectivity and specificity of the diffusion.
These channels have hydrophilic domains exposed to both the extracellular and intracellular matrix.
Additionally, they have a hydrophilic core that provides a hydrated opening through the membrane layers.
Aquaporins are the channels proteins that transport water across the plasma membrane at a very high rate.
The selectivity properties of channels arise from interactions between the ions and the mouth or walls of the pores.
The passage of ions through these channels allows the movement while avoiding the non-polar central layer of the plasma membrane.
Channel proteins are usually gated and allow the closing or opening of the channels based on some signals.
These signals can either be an electrical signal or simply binding of a molecule.

What are Carrier proteins?

Carrier proteins are another group of proteins involved in facilitated diffusion present in the membranes.
Carrier proteins, as the name suggests, carry molecules across the membrane.
These proteins bind to specific regions in the molecules causing conformational changes and then move the bound molecule to the interior of the cell depending on the concentration gradient.
Carrier proteins are bulky, and it is unlikely that they transport solute by diffusing from one face of the membrane to the other.
Therefore, in most of the models, the carrier accomplishes its task by a conformational change.
The mechanism of the conformational change is not entirely understood, but it is assumed that the hydrogen bonds are affected, which results in a change in the shape of the molecule.
Binding sites on carriers are very selective. For example, sugar carriers distinguish between d- and I-sugars.
The configuration of the binding site, or the charge distribution at the site, should match that of a distinctive portion of the desired substrate.
This selectivity adds to the selectivity of the plasma membrane.
The rate at which a particular substrate is carried across the membrane can be influenced by other solutes in the system.
At a point where all the carrier proteins are bound to their ligands, they become saturated, and the transport rates become the maximum.
Carrier proteins are also used in active transport for the movement of molecules with the expense of energy.

Factors affecting facilitated diffusion

Since facilitated diffusion is a mode of passive transport, it is mediated by several environmental factors. Some of which are:

1. Concentration Gradient

Concentration gradient across the membrane is an essential factor that regulates the diffusion process.
The diffusion always occurs from a region with concentration to a region with lower concentration.
The gradient creates potential energy which increases as the concentration difference increases, which results in faster diffusion.

2. Temperature

The energy barrier associated with the conformational change of the carrier is generally higher than the activation energy of solvent viscosity, which determines the diffusion via channel proteins.
Carrier transport rates increase more rapidly with temperature.
The increase in temperature increases the rate of reaction between the carrier proteins and the ligand in the molecules.

3. Saturation

Since the number of carrier proteins in the membrane is limited, once all the proteins are bound, they can no longer bind more molecules,
At this point, the rate of the diffusion cannot be increased even with the increase in the concentration gradient.

4. Selectivity

There is, in general, a reciprocal relationship between the transport rate and the selectivity of the transport process.
This is so because selectivity is often accomplished by binding sites that discriminate among the available solutes.
These selective and intense interactions tend to retard transport.

Examples of Facilitated diffusion

1. Glucose and amino acid Transport

The transport of glucose and amino acid from the bloodstream into the cell is an example of facilitated diffusion.
In the small intestine, these molecules are taken in via active transport and then are released into the bloodstream.
Because glucose and amino acid are larger molecules, they require carrier proteins called glucose transporters or amino acid permeases, respectively for their transport from the bloodstream into the cell.

2. Gas Transport

The transport of oxygen in the blood and muscles is another example of facilitated diffusion.
In blood, hemoglobin is the carrier protein whereas in muscles, the carrier protein in the myoglobin.
The diffusion of blood occurs as a result of higher pressure on one side of the membrane and a lower one on the other side.
A similar mechanism is involved in the transport of carbon dioxide and carbon monoxide.

3. Ion Transport

Ions are polar molecules and thus cannot move across the membranes with similar charges.
These ions are transported via transmembrane proteins which are called the ion channels.
These channels are specific for specific ions like potassium, sodium, and calcium.
These channels are highly specific and allow fast transport rates without using any chemical energy.

Applications/Importance of Facilitated diffusion

Facilitated diffusion plays a vital role in maintaining equilibrium between the exterior and interior environments.
Similarly, facilitated diffusion ensures the selectivity of different biological membranes.
Important cellular mechanisms like transport of oxygen, nutrients, and ions which are essential to maintain optimal homeostasis in the cell are performed through facilitated diffusion.

References

Friedman, M. (2008). Principles and models of biological transport. Springer.
Wittenberg, J. B. (January 1966). “The molecular mechanism of hemoglobin-facilitated oxygen diffusion”. J. Biol. Chem. 241 (1): 104–14.
https://openoregon.pressbooks.pub/mhccmajorsbio/chapter/passive-transport-facilitated-transport/

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