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Facilitated Diffusion: Definition, Principle, Examples

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Definition of  Facilitated Diffusion

Facilitated diffusion is a type of biological transport in which certain structural components of biological membranes work with specific solutes or groups of solutes to assist them to flow across the membrane much more quickly.

  • It’s a type of passive-mediated transport in which a transport protein transports particles or molecules from a high-concentration to a low-concentration location across a biological membrane.
  • Chemical energy or ATP are not needed because compounds travel within the region of concentration gradient (i.e. from higher concentration to lower concentration).
  • The compounds that are carried via facilitated diffusion, on the other hand, would not be able to pass the membrane readily or quickly otherwise.
  • Similarly, membrane components that enhance diffusion are referred to as transport mediators.

What is the principle behind assisted diffusion?

Facilitated Diffusion: Definition, Principle, Examples

Figure: Facilitated Diffusion in cell membrane, Showing ion channels and carrier proteins             

Image Source: Wikipedia (Mariana Ruiz Villarreal).

  • A plasma membrane’s lipid bilayer does not allow all molecules to flow through with equal ease.
  • Hydrophilic in addition to a few highly polar m molecules cannot pass through the membrane because it is hydrophobic.
  • Only a few hydrophilic molecules, particularly lesser hydrophilic molecules, may move rapidly all over the cellular membrane due to the concentration gradient.
  • Bigger non-polar molecules, on the other hand, require the assistance of transport mediators such as membrane carriers and channels.
  • Carrier proteins and channel proteins are the two types of proteins that move across the membrane.
  • Channel proteins are Transmembrane proteins which operate as a pore inside the membrane, permitting molecules to pass from end to end.
  • These channels run through the plasma membrane, linking the cytosol to the outside world or crossing the biological membranes of several cellular organelles.
  • Molecules use transmembrane channels created by protein complexes to transfer ions with similar charges.
  • In the context of carrier proteins, transporters or carrier proteins inserted inside the cellular membrane are utilised.
  • Some chemicals in the extracellular matrix bind to these proteins with a specific affinity.
  • When the carrier proteins connect to the molecules, conformational changes occur inside the molecules, allowing them to pass through the membrane and into the cytosol.
  • Larger molecules, such as enzymes, use this facilitated diffusion process.

Channel Proteins and Carrier Proteins

What are Channel proteins, and what do they do?

  • Essential proteins found inside biological membranes which form a channel that allows molecules to move across the membrane are Channel proteins.
  • Ions are nearly always the species which flow all the way through channels, commonly known as transmembrane proteins since they transcend membranes.
  • Numerous channels are highly selective, allowing various ions to enter freely whereas blocking others.
  • Channel diameters should not be more than 4–5 mm, with reference to similar to the widths of general biological ions, according to structural models.
  • Channels can pass cations and not anions, or show drastically varied permeabilities for two ionic species with the same charge, which improves diffusion selectivity and specificity.
  • Both the exterior and intracellular matrix are accessible to the hydrophilic sections of these channels.
  • They also feature a hydrophilic core which allows moisture to pass all the way all the way through the membrane layers.
  • Aquaporins are a kind of channel protein which rapidly transports water across the plasma membrane.
  • The selectivity of channels is influenced by communications among ions and the mouth or walls of pores.
  • The transport of ions through these channels facilitates movement while avoiding the plasma membrane’s non-polar core layer.
  • Channel proteins are frequently gated, allowing them to close or open in response to certain signals.
  • These communications might be either electrical or chemical in nature.

Carrier proteins

  • A different type of protein found within membranes that aid in facilitated diffusion are Carrier proteins.
  • Carrier proteins transport chemicals across membranes, as their name implies.
  • These proteins selectively bind areas of molecules, inducing conformational changes, and subsequently transport the associated molecule down a concentration gradient into the cell’s interior.
  • Carrier proteins are huge, thus they are improbable to transfer solute via diffusing from one membrane face to the other.
  • As a result, in most models, the carrier completes its duty by changing conformation.
  • Although the process of the conformational change is unknown, it is thought that the hydrogen bonds are disrupted, causing the molecule’s shape to alter.
  • Carrier binding sites are quite selective. Sugar transporters can recognise the difference between d- and I-sugars, for example.
  • The conformation, or charge distribution, of the binding site should resemble a specific region of the targeted substrate.
  • Such selectivity is added to the plasma membrane’s selectivity.
  • Other solutes in the system can alter the rate at which a substrate passes through the membrane.
  • The transport rates reach their maximum when all of the carrier proteins are linked to their ligands.
  • Active transport uses carrier proteins to transfer molecules at a high energy cost.

Factors that influence facilitated diffusion

  • As aided diffusion is a passive form of transportation, it is influenced by a variety of circumstances. A few instances are given below:
  1. Concentration Gradient
  • The concentration gradient across the membrane is an important element in the regulation of the diffusion process.
  • Diffusion takes place from a high-concentration area in the direction of a low-concentration area in every case.
  • As the concentration difference grows, the gradient generates more potential energy, resulting in quicker diffusion.
  1. Temperature
  • Channel protein diffusion is impeded because the energy barrier related to the carrier’s conformational shift is often greater than the activation energy of the solvent viscosity.
  • As the temperature rises, carrier transport rates accelerate.
  • As the temperature rises, the reaction rate among the carrier proteins and the ligand in the molecules becomes faster.
  1. Saturation
  • Because carrier proteins in the membrane are limited in number, they are unable to bind new molecules. once those that are already there have been bound.
  • Even if the concentration gradient is raised, the rate of diffusion cannot be enhanced at this stage.
  1. Selectivity
  • In general, the pace of transport and the transport process’ selectivity are inversely proportional.
  • This is due to the fact that selectivity is usually achieved by using binding sites that distinguish amongst the available solutes.
  • Transport is slowed by these selective and strong contacts.

Facilitated diffusion examples

  1. Transport of Glucose and amino acid
  • The transport of glucose and amino acids from the circulation further into cell is aided by diffusion.
  • Active transport in the small intestine allows these substances to enter the body and then be discharged into the circulatory system(bloodstream).
  • Because glucose and amino acids are bigger molecules, glucose transporters and amino acid permeases must carry them from the bloodstream to the cell.
  1. Gas Transportation
  • The transfer of oxygen through the muscles and blood also shows increased diffusion.
  • Hemoglobin is a blood-transporting protein, whereas myoglobin is a muscle-transporting protein.
  • Increased pressure solely on a membrane’s single side and lesser pressure on the other enhance blood diffusion.
  • Carbon dioxide and carbon monoxide are transported in an alike manner.
  1. Transport of Ions
  • Ions are polar molecules that can’t penetrate membranes with similar charges since they’re polar.
  • These ions are carried through ion channels, which are transmembrane proteins.
  • Specific ions, including as calcium, potassium, and sodium, are channelled by these channels.
  • These channels are extremely specialised and permit for rapid transmission devoid of the use of chemical energy.

Facilitated diffusion Uses and Importance

  • Facilitated diffusion is important for preserving the balance among the external as well as inside environments.
  • Facilitated diffusion, likewise, makes sure the selectivity of various biological membranes.
  • Facilitated diffusion is used to carry out important physiological operations such as oxygen, nutrient, and ion transport, all of which are necessary for the cell to maintain optimal homeostasis.



Facilitated Diffusion Citations 

  1. Wittenberg, J. B. (January 1966). “The molecular mechanism of hemoglobin-facilitated oxygen diffusion”. J. Biol. Chem. 241 (1): 104–14.

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