Facilitated diffusion definition

Facilitated diffusion is a kind of biological transport in which specific solutes or classes of solutes interact with certain structural elements of biological membranes to significantly speed up their membrane-crossing rates.

• It is a passively mediated transport in which, with the help of a transport protein, particles or chemicals travel from an area of higher concentration toward a zone of lower concentration through a biological membrane.
• No chemical energy or ATP is required, since the flow of substances follows the concentration gradient (from higher to lower).
• However, without assisted diffusion, the compounds that are carried through the membrane would not do so swiftly or readily.
• Similar to this, the membrane elements that enhance diffusion are referred to as transport mediators 

Principle (How does facilitated diffusion work?)

• Not all molecules can be transported across a plasma membrane’s lipid bilayer with equal ease.
• Considering that the membrane is hydrophobic, it prevents the transport of certain highly polar molecules as well as hydrophilic ones.
• Based on the concentration gradient, only a limited number of the hydrophilic molecules, combining smaller hydrophilic molecules, may migrate swiftly over the membrane.
• Larger non-polar molecules, on the other hand, need assistance from transport mediators such as membrane carriers and channels.
• One of the two pathways, one including carrier proteins and the other requiring channel proteins, can be used to transfer material across the membrane.
• When it comes to channel proteins, the transmembrane proteins that make up the membrane function as a pore or channel that lets molecules pass through.
• These channels penetrate the plasma membrane, allowing access to the cytosol from the outside world or to the biological membranes of various cellular organelles.
• Through the transmembrane channels that protein complexes create, molecules that are similarly charged ions are transported.
• In the case of carrier proteins, membrane-implanted transporters or carrier proteins are utilised.
• These proteins have a particular affinity for certain extracellular matrix components.
• The molecules attach to the carrier proteins, which causes certain conformational changes in the molecules and makes it easier for the molecules to pass through the membrane and enter the cytosol.
• This enhanced diffusion method is used for bigger molecules like enzymes.

Channel proteins and carrier proteins

What are Channel proteins?

• Channel proteins are essential proteins found in biological membranes that create a channel to facilitate the movement of molecules across the membrane.
• Since transmembrane proteins extend across membranes, they are sometimes known as channels, and the species that flow through them are virtually invariably ions.
• Numerous channels are quite selective, easily passing certain ions while being almost completely resistant to others.
• Based on structural models, channel diameters are predicted to be no larger than 4 to 5, which is comparable to the widths of typical biological ions.
• Selectivity and specificity of diffusion are enhanced by channels that readily transport cations but not anions, or that have markedly different permeabilities for two ions with equal charge.
• The extracellular and intracellular matrix are both accessible to the hydrophilic regions of these channels.
• They also feature a core made of hydrophilic material that allows water to pass through the membrane layers.
• The channel proteins known as aquaporins rapidly move water across the plasma membrane.
• The selectivity properties of channels are determined by the interactions among ions as well as the mouths or walls of pores.
• Ions are able to flow via these channels without entering the plasma membrane’s non-polar core layer.
• Typically, channel proteins are gated, allowing certain signals to cause the channels to open or close.
• These signals may either be electrical or just molecular binding.

What are Carrier proteins?

• Other classes of proteins in the membranes that contribute to enhanced diffusion are the carrier proteins.
• As their name implies, carrier proteins transport chemicals across membranes.
• Based upon the concentration gradient, these proteins transport the attached molecule to the inside of the cell after binding to certain areas in the molecules and triggering conformational changes.
• Since carrier proteins are large, it is improbable that they move a substance by diffusing across membrane faces.
• As a result, in the majority of models, the carrier completes its duty by changing its conformation.
• Although the exact process causing the conformational shift is not fully known, it is believed that the damaged hydrogen bonds cause a change in the molecule’s shape.
• Carrier binding sites are quite picky. Sugar transporters, for instance, discriminate between d-and I-sugars.
• The binding site’s structure, or the arrangement of charges there, should resemble a specific region of the intended substrate.
• The plasma membrane’s selectivity is increased by this selectivity.
• Other solutes in the system may have an impact on how quickly a certain substrate crosses the membrane.
• The maximal transport rates occur when all the carrier proteins have bonded to their ligands, when they reach their maximum capacity.
• In addition, carrier proteins are utilised in active transport to facilitate the movement of molecules at the expense of power.

 Factors affecting facilitated diffusion

• Since aided diffusion is a kind of passive transport, several environmental conditions influence it. Among them are:

1. Concentration Gradient

One crucial element that controls the diffusion process is the concentration gradient across the membrane.

Diffusion moves from a concentrated area to one with less concentration.

As the concentration difference widens, the gradient generates potential energy, which causes diffusion to speed up.

2. Temperature

Typically, the activation energy of the solvent viscosity, that regulates diffusion through channel proteins, is greater than the energy barrier connected with the conformational change of the carrier.

As the temperature rises, carrier transport rates rise more quickly.

The reaction between the ligand in the molecules and the carrier proteins happens more quickly as the temperature rises.

3. Saturation

As there are a finite number of carrier proteins in the membrane, once all the proteins are connected to molecules, they can not further bind other molecules.

At this point, the diffusion rate cannot be increased despite an increase in the concentration gradient.

4. Selectivity

The selectivity of the transport process and the transport rate often have a reciprocal connection.

This is true because binding sites that make distinctions between the different solutes that are accessible often achieve selectivity.

These focused interactions slow down transit in general.

Examples of Facilitated diffusion

1. Glucose and amino acid transport

• Facilitated diffusion is shown by the movement of amino acids and glucose into cells from the circulation.
• These molecules are ingested by active transport in the small intestine and subsequently released into the circulation.
• Due to their size, glucose and amino acids must be transported from the circulation into the cell by proteins known as glucose transporters or amino acid permeases, respectively.

2. Gas Transport

• Another example of enhanced diffusion is the movement of oxygen in the blood and muscles.
• The carrier protein in the blood is haemoglobin, while the carrier protein in muscles is myoglobin.
• Higher pressure on one side of the membrane and lower pressure on the other cause the blood to diffuse.
• Carbon dioxide and carbon monoxide are transported by a similar process.

3. Ion Transport

• Since ions are polar molecules, they are unable to traverse membranes that have the same charge.
• The transmembrane proteins known as ion channels are used to convey these ions.
• For certain ions like potassium, sodium, and calcium, these channels are only open.
• Due to their great specificity and lack of chemical energy, these channels enable rapid transport rates.

Applications/Importance of Facilitated Diffusion

• The balance between the inner and outer surroundings must be maintained, and facilitated diffusion is essential to this process.
• Similar to how various biological membranes are selected for through facilitated diffusion.
• Facilitated diffusion is used to carry out important cellular processes such the transfer of ions, nutrients, and oxygen that are necessary to maintain the cell’s ideal state of homeostasis.

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.