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Facilitated Diffusion across Cell Membrane

Facilitated diffusion across cell membrane ? A blog post that explains Facilitated Diffusion mechanism, Factors affecting it, examples, and Importance of Facilitated Diffusion mechanism.

Facilitated diffusion mechanism is a form of diffusion that sees molecules travel from the region with a higher concentration to a lower concentration, assisted by a vehicle or carrier.

Facilitated Diffusion Mechanism

Facilitated diffusion mechanism refers to the movement of passive molecules through the gradient of concentration. It is a selective process, i.e., the membrane permits only specific molecules and ions through it, and however, it blocks other molecules from going through the membrane. The pH and charge and the electric charge assist in the diffusion of the membrane.

In living organisms, the living systems lipid-based membrane forms compartments that permit the movement of a particular concentration of water-soluble compounds. Small molecules, ions, proteins, and other substances have different concentrations on the membranes. Polar, charged, or hydrophilic molecules are unable to cross the membrane.

What is Facilitated Diffusion in Biology

What is Facilitated Diffusion mechanism in Biology; Facilitated Diffusion mechanism in cell membrane showing ion channels and carrier proteins. Image Source: Wikipedia

How Does it Work?

The bilayer of lipids that forms the plasma membrane doesn’t permit the transportation of all molecules at the same comfort. Because it is hydrophobic, it can’t permit the move of hydrophilic molecules and highly polar molecular structures.

Some of the hydrophilic compounds and smaller hydrophilic molecules can easily cross the membrane depending upon the gradient of concentration. However, larger non-polar molecules need transport mediators such as membrane channels and transporters.

The movement of the membrane is accomplished via one of two methods, one of which involves protein carriers and the other involving channel proteins.
In channel proteins, the transmembrane protein present within the membrane function as a channel (pore) within the membrane, which facilitates the transportation of molecules.

The channels are located over the plasma membrane connecting the external world to the cytosol or across the biological membranes of various cells and organelles. Molecules similar to charged ions are transported through the transmembrane channels created through protein-protein complexes.

When it comes to carriers, proteins transporters, carriers, or proteins embedded within the biological membrane are used. These proteins possess a distinct affinity to specific molecules in the extracellular matrix.

The carrier proteins attach to molecules, resulting in some changes in the conformation of the molecules, which facilitates the passage of the membrane into the cytosol. This method of diffusion facilitated is utilized for larger molecules, such as enzymes.

Channel Proteins and Carrier Proteins Molecules

Channel Proteins

Channel Proteins is a protein for transport that opens the “gate,” allowing molecules to cross the membrane. They have a specific binding location to the molecule or ion being targeted. The stimulus triggers the “gate” to close or open. The stimulus can include electrical or chemical signals such as temperature, chemical signals, or mechanical force based on a gated channel. 

For instance, the sodium gated channels in nerve cells are affected by a chemical stimulus, which triggers them to open and let sodium ions enter the cell. The glucose molecules are too massive to pass across the plasma membrane easily, which is why they move across the membrane using gates in the channels. This is how glucose can be absorbed very quickly through the cell’s membrane, which is vital since many cells depend on glucose to fuel their cells.

Carrier proteins

A carrier protein is a protein used for transport specifically designed to bind an ion, molecule, or maybe a group of substances. Carrier proteins “carry” the molecules or ions across the membrane, and they do this by altering form after binding the molecule or the ion. Carrier proteins play a role in active and passive transport. An example of channel proteins and the carrier protein is illustrated in the following figure.

What is Facilitated Diffusion in Biology

What is Facilitated Diffusion mechanism in Biology; Facilitated Diffusion mechanism in cell membrane showing ion channels and carrier proteins. Image Source: bio.libretexts.org

Factors Affecting Rate of Facilitated Diffusion

Facilitated diffusion mechanism is a form of passive transportation controlled by various environmental variables. The most prominent of these are:

1. Concentration

The concentration gradient over the membrane is a crucial aspect that controls the diffusion process. The diffusion will always occur in a concentration region to another area with the lowest concentration. The gradient produces potential energy, which grows when the concentration differential increases, is resulting in rapid diffusion.

2. Temperature

The energy barrier resulting from the conformational changes of the carrier is usually more significant than the activation energy of solvent’s viscosity that determines the rate of diffusion of channel proteins. Carrier rates rise more quickly as temperatures rise, increasing the reaction rate between the carrier’s proteins and the ligand molecules.

3. Saturation

Because the number of carrier proteins found in this membrane can be limited when all proteins have been bound, they are not attaching any additional molecules. In this case, the diffusion speed can’t be increased despite the rise in the concentration gradient.

4. Selectivity

In general and 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 that are available. These intense and selective interactions can slow down the flow of information.

Examples of Facilitated Diffusion Mechanism

Glucose Transportation

The glucose transporter that enables the movement of glucose is a carrier protein with two main conformational structures. Although the exact 3-dimensional structure is unknown, glucose -binding results in a change in conformation, which causes the binding site to be positioned towards the inside of the cell. The transporter reverts to its original configuration as the glucose enters the cell.

Facilitated Diffusion Mechanism

Ion Channels

Transmembrane proteins permit the specific transportation of ions and solutions across plasma membranes. Ionic pumps regulate the amount of extracellular fluid that is different from the cells’ cytosol. If an excessive amount of sodium ions is found in the extracellular area and the excess potassium ions are found inside the cells, a resting potential is created. The sodium ion channels open by a slight voltage change, and sodium ions swiftly move inside the cells.

Aquaporins

As with other transmembrane proteins, aquaporins aren’t entirely identified. However, it is well-known that there are numerous channels for the quick passage of water molecules within almost every cell. These highly conserved proteins can be found in bacteria, plants, fungi, and even animals.

Importance

1. There are a few molecules that can traverse cell membranes. The molecules should be tiny and non-polar to pass through the membrane. For example. Glucose is a giant molecule that cannot move through the membrane of cells. 

2. Ions such as sodium, potassium, and calcium are charged and are dispelled from cells’ membranes. Nucleic acids and amino acids are polar and big to pass through a cell’s membrane. Additionally, the movement of water through the membrane is sometimes tricky.

3. To facilitate the transport of the substances over the membrane, integral membrane proteins, also known as transmembrane proteins, are needed. Channel proteins as well as transport proteins.

Further Readings