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What is Facilitated Diffusion? – Definition, Characteristics, Types, Importance

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  • Last Updated : 23 Jun, 2022

The term “facilitated diffusion” is used to describe a case in which the diffusion process will not happen without one or more conditions. These conditions can be anything from higher temperatures to more nutrients. In some cases, the added condition will stop the original condition from happening. However, in other cases, it will cause a shift in reaction direction so that diffusion operates as though there were two different concentrations (e.g., two different maximums).

Facilitated Diffusion refers to where the terms “facilitation” and diffusion. It’s part of a larger topic called boundary layer processes, which have to do with how substances like air and water move through physical boundaries such as walls or floors. In facilitated diffusion, it’s the boundary layer of air that is being considered.

Facilitated diffusion is a phenomenon in which diffusion occurs through a surface barrier (e.g., a wall or floor) under various conditions.

Facilitated Diffusion occurs when two substances with different vapor pressures are located well away from one another and moving in opposite directions. In this case, the more volatile substance will flow along the lower permeable area, while the less volatile substance will flow along the upper permeable area. One reason for facilitated diffusion may be to aid in temperature control by allowing gases to move from higher to lower elevations.

An example of facilitated diffusion is how humans move up the evolutionary ladder when they gradually turn into apes. However, it can also be divided into two endpoints: intracompartmental and interfacial. Intracompartmental facilitated diffusion occurs when individuals in an inaccessible habitat move to an area that is closer to where they need resources, leading to a decrease in resource use because it allows for easier access by these individuals. Interfacial facilitated diffusion happens through contact between individuals who are separated physically, and relies on their proximity to each other as a medium for transmission from one person to another.

Intracompartmental facilitated diffusion occurs when a group travels from an area that is difficult to reach to an area that is much easier to access and move around in. This change in geographic location is done so that individuals have easier access to their necessary resources, which would include food, shelter, and protection. It has been observed in a variety of animal species including rodents, fish, birds, sloths, and even humans.

Rodents have been observed to use intracompartmental facilitated diffusion many times for different reasons. One example with sloths shows that there are two distinct groups within the same species on either side of a river.

The two distinct groups are the ones that are nearest to the river and the ones that are the farthest away. The groups that are closer to the river have young first, but they have a higher mortality rate. They do this so that their offspring can be more protected from predators close to them. However, those farther away have a lower survival rate than those closest to the river because there is less food in order for them to survive.

When these animals migrated from being far apart from each other into much closer proximity, there was an increase in both behaviors and mortality rates for those closer to each other. They did this because there was now a much more readily available food source for both groups. Also, there were no longer any predators around to threaten their survival, and they were able to reproduce more frequently.

Interfacial facilitated diffusion is when two individuals in different geographic locations are physically separated but are able to reach each other through the substance that they produce or circulate through their body and spread it throughout a physical medium. An example of this would be how ants would travel from one place to another to spread out their pheromone.

 Characteristics of facilitated diffusion

Facilitated Diffusion


Facilitated diffusion is a special case of a solute diffusing across a semi-permeable membrane. When the solution is added to the semi-permeable membrane, it will spread evenly over the surface and then stop. Moreover, in this model, diffusion will happen while water passes through pores of size equal to that of the molecules being transported by facilitated diffusion.

This theory can be explained by considering how solutes are transported across membranes that are permeable to small ions but not large ones like sodium or potassium ions. The charge on a molecule is distributed in such a way that allows it to diffuse in the membrane. These small charged particles will jump across the membrane and travel through pores of equal size.

For example, consider aqueous solutions containing molecular ions Na+ and K+ with charges of +0.3 and +1 respectively. The diffusion of Na+ relative to K+ through a semipermeable membrane is facilitated due to the higher value of K+. For example, the diffusing Na+ easily passes through pores with diameters less than 0.21Å, while the diffusion coefficient for free Na+ is much lower at 0.0043m2/s when compared to that of free K+.

Factors affecting Facilitated diffusion

Facilitated diffusion is a type of passive transport that occurs when the movement of solvents or solutes through a membrane is helped by an intermediate molecule or transport protein. As the name suggests, facilitated diffusion relies on some outside force to help move solvents and solutes through a membrane. This outside force could be chemical potential gradients, electrical gradients, concentration gradients, etc. 

The three factors that affect facilitated diffusion are: 

  1. Size and shape of molecules
  2. Permeability characteristics of membranes
  3. Concentration gradient for solvent between two areas separated by a membrane barrier

How facilitated diffusion is different from diffusion and active transport?

To synchronously allow a chemical to move through cells, ions are shared between the different membranes that make up the cell. The movement of ions is facilitated by diffusion and is aided by diffusion pumps. Movement in cells via active transport requires adenosine triphosphate (ATP). Active transport includes enzymes, ATP synthase, and an energy source.

In facilitated diffusion, lipids serve as carriers of the ionic species across lipid bilayer membranes. Diffusion is a passive process because the movement of substances required to facilitate diffusion is provided by external forces such as osmosis or assisted by membranes themselves. In facilitated diffusion, the movement of ions is passive due to osmosis and other external agents. The movement of ions is passive because of the way that the lipid bilayer membranes in cells are designed to allow different parts of the cell to exchange and transport ions. In facilitated diffusion, ion transport occurs from one side of a membrane to another through the permeation of pore proteins. Permeability involves the movement of substances from one side of a membrane to another and involves structural changes in lipid bilayer membranes. A second force for active transport is provided by pumps, which allow ionic substances or metabolites through cell membranes by pumping them against electrochemical gradients.

What are the different types of transporters used by facilitated diffusion?

Facilitated diffusion is the process by which substances move through a membrane from an area of lower concentration to one of a higher concentration, as long as there is a difference in chemical potential. Diffusion requires water and doesn’t have to include living cells. There are different transporters used for facilitated diffusions such as simple diffusion, coupled transport, and carrier-mediated transport.

In simple diffusion, the solute moves from an area of high concentration to low concentration until the concentrations are equalized. This is what happens when you pour salt into the water because the particles separate based on charge attraction and repulsive force between like-charged ions. 

Coupled transport is when there is more than one transporter present, such as NHE-PDC and P2X 7.

Carrier-mediated transport occurs when a receptor protein binds to an external transporter to increase the solute concentration. This happens with amino acids in the blood, glucose in the blood, and protein ligands in cell membranes.

Conditioning: Protein Ligands bind to carrier proteins, causing them to form large complexes with receptors on the outside of the cells that are attached to other carrier proteins. The carriers then will activate transporters and induce facilitated diffusion of substances. This is how amino acids in the blood move out of the vascular system into cells that need them.

Conceptual Questions

Question 1. What is the definition of facilitated diffusion? 


Facilitated diffusion happens when a molecule needs assistance in crossing through a membrane. This assistance comes in the form of a protein known as a carrier. The carrier will speed up the process so that it takes less time to cross the membrane. This process allows the molecule to be transferred more quickly across the membrane.

Question 2: Is facilitated diffusion passive or active?


Facilitated diffusion is passive. A carrier will be attracted to molecules by their electrical charges, and this attraction along with random collisions will pull them through.

Question 3: What is a carrier, and how does it work? 


Carriers are proteins or lipids within membranes and provide a pathway for other molecules to cross that membrane. They are also known as “carrier proteins. “A carrier will be capable of binding to the surface of other molecules and then assist with the formation of a pore or channel in the membrane. This pore will allow smaller molecules to pass through quicker, thus facilitating diffusion.

Question 4: How do we know how this happens? 


We can investigate this by looking at diffusing particles through a membrane. If we put two identical particles into a test tube containing two different solutions and only one solution is kept in place, one particle will enter the channel and move freely in one direction until it leaves the other solution. What happened? The agent that was not kept in place was an inhibitor.

Question 5: How can we use this to study diffusion?


We look at a most common example of facilitated diffusion. What happens when you make a solution containing only X and Y (for instance) and then add an inhibitor. If we keep the inhibitor in place and split the solution into two equal portions, one containing X and one containing Y, both solutions should be able to diffuse freely.

Question 6: Why does facilitated diffusion work like this?


The agent which inhibits barrier transport will remain stationary in the membrane unless it is attracted by another molecule that has a stronger affinity for it (in other words, the agent stays on the surface of molecules until there is another molecule that has a stronger affinity for it). In other words, the inhibitor will stay in place. The same thing happens to the other solution (the X and Y solution), it also remains stationary in the membrane until there is another molecule that has a stronger affinity for it.

Question 7: How can you prove that this happens? 


If we take a container (for instance) and put two solutions of different molecules into two separate ports, the molecules will be able to diffuse freely — because there is no barrier to stop them from moving from one part of the container to another part.

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