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Facilitated Diffusion Definition
Facilitated diffusion (also known as facilitated transport or passive-mediated transport) is the process of spontaneous passive transport (as opposed to active transport) of molecules or ions across a biological membrane via specific transmembrane integral proteins.
What is Facilitated Diffusion?
Facilitated Diffusion is a passive transport where the movement of molecules is mediated through the plasma membrane with the help of transporter protein such as carrier is called as facilitated diffusion.
The transfer of molecules takes place from a higher concentration area to a lower concentration in facilitated diffusion as well, like simple diffusion. The solute concentration variation through the membrane is the driving force responsible for facilitated diffusion.
Although majority of facilitated diffusion does not suffice the need of ATP, however in few cases it does require ATP. Facilitated diffusion take place due to the exactitude between the carriers and the solute.
In facilitated diffusion molecules can progress in both the direction i.e., towards or against the concentration gradient. Kinetic energy along with concentration gradient helps to carry out facilitated diffusion.
The molecules which can pass through are water soluble huge molecules through the plasma membrane in facilitated diffusion.
Facilitated Diffusion Principle
Plasma membrane’s lipid bilayer is hydrophobic, thus water-soluble molecules cannot pass through; however tiny water molecules due to their concentration gradient can pass through the membrane. Although hydrophobic huge molecule requires either carriers or channel protein to pass through the membrane.
When molecules passes with the help of the channel protein, there are pores present in the transmembrane of the membrane, resulting in the flow of molecules and are spread throughout the cytosol and outer surrounding thus, extending to the other organelles.
Through the transmembrane channel, charged molecules will move and these transporters protein are incorporated to the membrane, having attraction towards the matrix.
However, in other cases, molecule will attach to the carrier protein, which will change the shape of the molecule leading to the molecule move inside the cytosol, and this process is seen in enzymes, which are huge molecules.
Facilitated Diffusion and Channel Proteins
These proteins aid in the movement of molecules, by channel formation through the membrane and in this membrane lies the transmembrane proteins which are ions.
These channels have a diameter of 4-5 Armstrong and can be choosy and allow only a particular ion to move through such as positive ion and will have differences for various ions, thus being picky towards a particular ion. In the extra and intracellular matrix, hydrophilic domains and core are possessed by these channel, thus cracking the layers wide open.
The water movement through the membrane is mediated by the aquaporins at a very faster pace. On the receival of the electrical signal or attachment of molecule, the doors gets closed and open.
Facilitated Diffusion and Carrier Proteins
Carrier proteins perform facilitated diffusion, by transporting the molecules through the membrane. They do so by attaching to the molecule and getting changes in its shape as they are heavy and then it can move within the cell on the basis of the concentration.
The change in shape also has an impact on the hydrogen bonds. Carrier protein are quite specific in terms of their binding sites, such as they recognize between the D and L sugar type, which makes the plasma membrane as well specific.
Saturation occurs when carrier protein attaches to the substrate, thus allowing movement to take place at a faster pace. They play role in active transport which requires energy for the transportation of molecule.
Factors Affecting Facilitated Diffusion
Environmental factors are the one which has an impact on facilitated diffusion:
a) Concentration Gradient: The diffusion through the membrane occurs due to concentration gradient, which happens from a high concentrated region to a lower concentrated one. However quick diffusion happens due to the concentration diffusion.
b) Temperature: For the shape to change, the amount of energy required is quite high than the activation energy. As the temperature increases the carrier transportation also increases, thus elevating the reaction rate between the carrier protein and the substrate in the molecule.
c) Saturation: On the membrane, carrier proteins are present in specific amount and once these sites are occupied, no more proteins can bind. Thus, even if concentration gradient rate is elevated, rate of diffusion cannot be elevated.
d) Selectivity: Selectivity and transportation rate are reciprocal to each other, as selectivity is achieved from the binding sites which do not treat all the solutes equally, thus ceasing the movement.
Facilitated Diffusion Examples
1) Glucose and amino acid transport: Example is the movement of glucose and amino acid from blood to the cell is the facilitated diffusion example. Through active transport they reach the intestine and are left into the blood. These molecules are moved to cell from the blood through carriers such as amino acid permease and glucose transporters, as they are quite huge.
2) Gas transport: A different example is when to the muscle and the blood, oxygen is transported. The carrier protein is hemoglobin blood and myoglobin in muscle, which results in diffusion due to increase in pressure and thus gets moved to the other side with low pressure. The same process is involved in carbon monoxide and dioxide.
3) Ion transport: Ions possess the same charge like the membrane and are polar thus, cannot move through and thus have transmembrane protein also known as ion channel which are choosy for ions like Na, K and Ca and the transport has to be quick as no energy is used.
Biological Importance of Facilitated Diffusion
The homeostasis between the outer and inner environment is regulated by facilitated diffusion. It also makes the biological membranes specific. Various functions are regulated by facilitated diffusion such as ion transport, oxygen transport and transportation of sugar molecules.
Facilitated Diffusion Citations
- Facilitated diffusion in chromatin lattices: mechanistic diversity and regulatory potential. Mol Microbiol . 2005 Aug;57(4):889-99.
- Facilitated Diffusion Mechanisms in DNA Base Excision Repair and Transcriptional Activation. Chem Rev . 2018 Dec 12;118(23):11298-11323.
- Facilitated diffusion of Argonaute-mediated target search. RNA Biol . 2019 Sep;16(9):1093-1107.