What Is Facilitated Diffusion & Their Importance, Experiments, and Examples?

In living organisms, the diffusion of substances is mediated by the cell membrane. By definition, facilitated diffusion is a type of passive transport which utilizes “agents” known as channel proteins and carrier proteins to speed up the transport process. While there are some proteins found in the cell membrane, only the said types of proteins play a significant role in facilitated diffusion.

• Channel proteins facilitate the entry and exit of substances in the cell. Two types of channel proteins exist–open and gated channel proteins. On the one hand, open channels are transmembrane proteins that create a pore in the cell membrane, thus permitting charged molecules to pass through. On the other hand, gated channel proteins work like channels that either closed or open to regulate the entry of substances. After the process, the proteins revert to their previous positions.
• For instance, carrier proteins, which are embedded in the cell membrane, change the conformation of the substances to be transported and then release them to the other side of the membrane. Carriers with high rates of associations and dissociations are very efficient in executing the process. Carrier proteins are affected by various external factors like temperature and saturation. There are a limited number of such transporter agents which sometimes “max-out” at a rate of transport. Aside from that, for a transport process to occur, a configuration change is highly significant.
• Unlike free diffusion of molecules, facilitated diffusion strongly relies on the binding of substances with channel and carrier proteins. Also, the rate of saturation during free diffusion is relatively slower than that of facilitated diffusion, which slows down when a “saturation point” is reached. Temperature also matters since facilitated transport occurs when it is mild. Its dependence on temperature is because of the presence of an activated binding.
• However, not all substances can freely pass through the cell membrane. Facilitated diffusion is selective because the agents above can reversibly work with high permeability and selectivity.
• In order to be selective, the membrane allows certain substances like ions and molecules to pass through it, while it prevents other substances like the carrier molecule itself.
• Chemical conditions like electric charge and pH make it possible for the diffusion across membranes.

Interestingly, several attempts have been made by scientists and engineers to imitate the process of facilitated transport in non-biological membranes. Such efforts were primarily made as a mechanistic basis of industrial-scale liquid and gas separations.

However, at present, there has been no reported successful attempt yet, mostly associated with the poor dissociation and overall rigidity of the carrier proteins used.

According to the calculations and experiments were done by Cussler and his colleagues (1988), facilitated diffusion can happen in polymer membranes as long as they have “chained” carriers bound to them.

Their results show that membranes with the said carriers will display many similarities to that of the membranes with bound mobile carriers. Such membranes will have varying solute fluxes with decreasing solute concentration and have asymptotical solute fluxes at relatively increasing solute concentration.

Also, solute fluxes will have to increase with increasing concentration of carriers (probably higher than that of the first power). With that, solute fluxes tend to display high selectivity; thus displaying the chemical reactions that occur between the solutes and the carriers.

The process of facilitated diffusion is particularly crucial in living systems as it is one of the governing mechanisms of the numerous physiological and biochemical processes required for survival. As alluded to earlier, facilitated diffusion is a kind of transport that permits the crossing of substances and molecules across membranes.

One of the most commonly transported substances in the cell is glucose, a molecule of sugar that is essential for the energy of cells.

• Glucose is continuously supplied to the cells via the bloodstream, while at the same time, being consumed during cellular metabolism.
• The amount of glucose inside the cell is always lower than that of the outside; however, because of its very large molecule, glucose alone cannot pass through the membrane.
• As a result, glucose molecules enter via facilitated diffusion with the help of glucose-specific carriers that bind to them.

In the bloodstream, the oxygen is known to have a very high affinity with hemoglobin molecules on the surfaces of red blood cells.

• In the facilitated diffusion of oxygen, the protein hemoglobin act as the carrier that brings it inside the red blood cells.
• During this process, the rate of oxygen diffusion is increased by hemoglobin, and after that, the newly-formed oxyhemoglobin will be displaced.

Aside from glucose and oxygen, amino acids and nucleic acids are transported into the cell via facilitated diffusion.

• Because of their bulky structures and charged molecules, these alone cannot enter and exit the cell through simple diffusion.
• Moreover, just like what happens with glucose molecules, amino acids and nucleic acids are assisted by a variety of carrier proteins in the process.

Sodium ions diffuse across membranes through the help of channels present in the cell membrane’s hydrophobic barrier.

• In this type of transport, the energy that facilitates the movement of ions comes from the difference in ion gradient, and nothing comes from the transport system.
• Moreover, just like what was mentioned earlier, the channels that facilitate the movement of sodium ions exhibit substrate specificity.

• Both passive transport and active transport are processes that involve the movement of substances like ions, salts, and sugars across membranes.
• Aside from that, both processes make use of proteins as a means to transport these substances across the cell membrane. Moreover, as mentioned earlier, these processes require the change in the conformation of the proteins involved.

• Regarding differences, one of the most foolproof ways to distinguish the two looks at the energy requirements of both processes. On the one hand, passive transport mechanisms like facilitated diffusion happen spontaneously because substances are merely going down the concentration gradient. However, because the substances are huge to be unable to pass through the membrane, proteins are needed.
• On the other hand, energy in the form of adenosine triphosphate (ATP) is needed in active transport in order to enable the alteration of the shape of the proteins. Unlike facilitated diffusion, substances are going against the concentration gradient. Moreover, as such, requires energy utilization.