Definition of Facilitated diffusion
Facilitated diffusion is the method of biological transport where certain biomembrane structural components interact with specific solutes or classes of solutes significantly increasing the speed that they cross the membrane.
- It is a passively-mediated process where substances or particles are moved through a membrane of a living organism from an area with more concentration to an area with a lower concentration. This is made possible by a particular transport protein.
- Because the movement of the substances is according to the gradient of concentration (from greater to less) there is no chemicals or ATP is needed.
- However, the chemicals that are transported through the process of facilitated diffusion are not able to be able to move quickly or easily through the membrane.
- Similar to the membrane components that are responsible for facilitation of diffusion are referred to as transport mediators.
The word “facilitated” comes from Latin facilis which, in turn, originated from the Greek word facio which means “do” or “make” and -“ilis”. The word “diffusion” came directly from Latin diffusionem which translates to diffusio which means “a spilling out”. Synonyms: facilitated transportation or passively-mediated transport.
Principle of Facilitated diffusion/How does facilitated diffusion work?
The bilayer of lipids in a plasma membrane doesn’t permit the transportation of all molecules at equal ease. Because it is hydrophobic it isn’t able to allow mobility of the hydrophilic molecules as well as highly polar m molecule. Some of the molecules that are hydrophilic, together with smaller molecules that are hydrophilic, can swiftly move across the membrane , based by the concentration gradient.
However, larger non-polar molecules need help by transport mediators such as channels and membrane carriers. The motion across the membrane is achieved through one of two methods; one that involves carriers proteins and the second one that involves channel proteins. For channel proteins, transmembrane proteins in the membrane function as channels (pore) within the membrane that allows the transportation of molecules. The channels are able to extend over the plasma membrane connecting the external world to the cytosol or across the biological membranes of various cells and organelles.
Molecules that are similar to charged ions are transported through the transmembrane channels fashioned in protein complexes. For the case of transporter proteins transporters, carriers, or proteins that are embedded within the membrane of life are used. They have a particular affinity for certain molecules in the matrix extracellular. They bind with the molecules, causing some changes in the conformation of molecules, facilitating their passage of the membrane to the cell wall. This process for facilitating diffusion has been used to transport larger molecules, such as enzymes.
Transmembrane protein are the cells’ proteins that are found within the membrane that aid in the flow of certain cells across the membrane. There are specific protein channels and carriers that speed up the transport process.
What are Channel proteins?
Channel proteins are proteins that are present within the membrane of the living organism that permit the transfer of molecules through the membrane creating channels. The species that travel via channels, often referred to as transmembrane proteins because they traverse membranes are usually composed of ions. Some channels are extremely specific, passing certain Ions quickly, but being inaccessible to other ions.
Based on the structural models, the diameters of channels are thought to be not more than 4-5 A roughly equivalent to the dimensions of common biological Ions. In the same way, channels can be able to pass cations easily but not anions. They also show very different permeabilities to two ions with similar charges, thereby increasing the specificity and the selectivity for the diffusion. They have hydrophilic domains that are exposed in both an extracellular as well as intracellular matrix.
In addition, they have a hydrophilic center that creates an opening that is hydrated through the membrane layer. Aquaporins are proteins that form channels that carry liquid across the membrane of plasma an extremely rapid rate. The channels’ selectivity properties are due to interactions between ions and the mouth , or the walls of the pores. The movement of ions in these channels permits motion while avoiding an unpolar layer in the central plasma membrane.
Channel proteins are generally controlled by gates and permit the closing and/or opening of channels in response to certain signals. The signals could be electrical signals or simply the binding to molecules.
What are Carrier proteins?
Carrier proteins are a different group of proteins involved in facilitating diffusion in membranes. The Carrier protein, just as their name implies, transport substances across membranes. They attach to specific areas of the molecules, that cause conformational changes. They then transfer the bound molecule into the inside of the cell based on the degree of concentration.
Carrier proteins are hefty and bulky, so it is highly unlikely to transport a substance by moving from one side of the membrane other. In most of the cases the carrier completes its job through an alteration in conformation. The mechanism behind the change in conformation isn’t completely understood, but it is presumed that hydrogen bonds get affected that results in an alteration in the shape of the molecular.
Binding sites for carriers are highly specific. For instance sugar carriers distinguish between I- and d’sugars. The binding site or the charge distribution on the binding site must correspond to the configuration of a distinct area of the substrate that you wish to target. This enhances the ability that the plasma membrane has.
The speed of a specific substrate is transported across the membrane could be affected by other liquids within the system. Once all of the protein carriers are attached to their ligands they are saturated and their transport rates reach the highest. Carrier proteins also play a role in active transport to aid the movement of molecules at the use of energy.
Different Factors affecting on facilitated diffusion
Facilitated diffusion is a form of passive transportation that is influenced by a variety of environmental factors. One of them is:
- Concentration Gradient: The concentration gradients across the membrane are the main factor that controls the process of diffusion. The diffusion is always from an area with high concentration to another region with less concentration. The gradient generates potential energy that increases when the difference in concentration increases which leads to rapid diffusion.
- Temperature: The energy barrier due to the change in conformation of the carrier is usually higher than the activation energies of viscosity in solvents, which affects the diffusion through channel proteins. The rate of transport increases for carriers quicker when temperatures increase. The temperature rises, which increases the speed of the reaction between carriers proteins and the ligand within the molecules.
- Saturation: As the amount of proteins that carry the membrane can be limited when all proteins have been bound, they will no longer be able to bind additional molecules. at moment, the speed of diffusion is not increased despite the rise within the concentration gradient.
- Selectivity: There is generally speaking an interrelationship between the rate of transport and the quality of the process. This is because selectivity is typically achieved through binding sites that distinguish between the different solutes available. These intense and selective interactions are known to slow down the transport.
Examples of Facilitated diffusion
1. Glucose and amino acid Transport
- The transfer of amino acid and glucose from bloodstreams into the cells can be described as enhanced diffusion.
- The small intestinal tract the molecules are taken in through active transport, and release into the bloodstream.
- Since amino acids and glucose are bigger molecules, they require transport proteins, referred to as glucose transporters as well as amino acid permeases respectively, for their transportation from the bloodstream to the cell.
2. Gas Transport
- The oxygen transport process in muscles and blood is another example of facilited diffusion.
- In blood hemoglobin is the main carrier protein. In muscles, it’s the protein responsible for carrying the load is myoglobin.
- The blood’s circulation occurs because of the increased blood pressures on one end of the membrane, and an equal pressure on the other.
- Similar mechanisms are involved in the transportation of carbon monoxide and carbon dioxide.
3. Ion Transport
- Ions are polar molecules , and therefore cannot cross membranes with the same charges.
- Ions are carried through transmembrane proteins, which are known as the ion channel.
- They are specifically designed to specific ions, such as sodium, potassium and calcium.
- These channels are extremely specific and enable fast speedy transport without the use of anything chemical.
Applications/Importance of Facilitated diffusion
- Facilitated diffusion plays an essential part in maintaining the equilibrium between the outside and internal environments.
- In the same way, facilitated diffusion enhances the selectivity of various biological membranes.
- Critical cellular functions like the transportation for oxygen, nutrients and ions, which are crucial to ensure optimal homeostasis within cells are carried out by diffusion that is facilitated.
Facilitated diffusion vs. active transport
Both active and facilitated transport require a gradient of concentration for the process to take place. Both can transport sugars, ions and salts. They’re similar in the sense that they employ Membrane proteins to transport. Permeases are a prime illustration of membrane proteins utilized in facilitated diffusion , whereas membrane pump proteins (e.g. sodium-potassium pump) are utilized in active transport. They differ however in the direction of transportation. When active transportation is used substance is transported from an area with a lower concentration, to an area with a high concentration. This upward movement of materials in active transport needs and uses chemicals in the form ATP. While the process of facilitated diffusion doesn’t require or makes use of ATP. In fact, the natural or kinetic chemical entropy drives the process.
Facilitated diffusion vs simple diffusion
Simple as well as facilitated diffusion are two types that are passive transportation. They transport substances from a region of higher concentration, to an environment with lower concentration. But the former differs from the latter with regard to the manner in which molecules are moved through the membrane. Facilitated diffusion involves membrane proteins in order to transport biological molecules. Simple diffusion is not supported with membrane protein. Since membrane proteins are essential to carry out transport during facilitated diffusion, the effects of temperature are often more evident when compared to simple diffusion. The speed of the process is also influenced through saturation limit. (1) In addition it depends on the capacity to bind that the protein in membranes that is involved. Simple diffusion has a speed is much more simple. For more details on the differences and similarities between facilitated and simple diffusion, see the table below.