Facilitated Diffusion: Definition, Importance and Examples

Facilitated diffusion is defined as the transport of substances across a biological membrane from an area of higher concentration to lower concentration with the help of a transport molecule. The term facilitated came from the Latin facilis meaning “do” or “make”. The term diffusion came from the Latin diffusionem meaning “a pouring forth”. Many biochemical processes are regulated by diffusion and facilitated diffusion is one such form of diffusion. It plays an important role in several metabolic processes. 

Key Terms: Diffusion, Proteins, Active Transport, Passive Transport, Simple Diffusion, Membrane, Ions, Glucose

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What is Facilitated Diffusion?

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The passive movement of substances, such as biological molecules or ions,through a plasma membrane by means of a transport protein located in the plasma membrane, is known as facilitated diffusion. However, the membrane allows only selective molecules and ions to pass through it and prevents others. Since, substances themselves move from higher concentration to lower concentration, i.e., concentration gradient, there is no requirement of chemical energy neither it is used.

Facilitated Diffusion

Facilitated diffusion is the primary mechanism behind the binding of Transcription Factors (TFs) to designated target sites on the DNA molecule. It is a important process as it regulates what goes in and out of a cell. 

Facilitated Diffusion Vs Active Transport

Facilitated diffusion and active transport are same in ways such as, both need concentration gradient to occur and use membrane proteins as transport vehicle but they differ in the direction of transport. In a mode of active transport, substances are transported from an area of low concentration to an area of high concentration while in facilitated diffusion it is the opposite.

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Importance of Facilitated Diffusion

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Cellular transport is the unequal distribution of substances between the intracellular fluid and the extracellular fluid including facilitated diffusion. The movement between these two regions is an attempt to establish equilibrium.

Only a very few molecules are allowed to move freely across the cell membranes and they must be small in size, and non-polar. While this allows molecules like water, oxygen and carbon dioxide to diffuse across membranes, practically every biopolymer, most nutrients and many important small molecules are prevented. Facilitated diffusion becomes important in these situtations because of its integral membrane proteins to maintain the homeostatic optimal levels of molecules and ions inside the cell.

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Transmembrane Proteins

Proteins that are present in cell membrane and facilitate the movement of molecules across the membrane are known as transmembrane proteins. There are two types of transport proteins that help in facilitated diffusion and they are described as below:

• Channel Proteins: Entry and exist of substances in cell is maintained by channel proteins. There are two types of channel proteins, open channel proteins, which create a pore in the cell membrane and allow the charged molecules to pass through and gated channel proteins that are either closed or open and regulate the entry and exit of substances. 
• Carrier Proteins: These proteins are present on the cell membrane. They carry molecules, change the confirmation of molecules and release them to the other side. They are affected by temperature and saturation. 

Facilitated Diffusion by Proteins

Also Read: Nuclear Membrane

Factors affecting Facilitated Diffusion

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Driving force behind the diffusion of fluids is the Brownian Motion, including the facilitated diffusion. The factors affecting the facilitated diffusion are listed below:

• Size of Molecules: The lighter the moleculer the faster it diffuses and since small particles are lighter than the large ones, they diffuse faster.
• Temperature: As the temperature increases, energy of molecules increases and they diffuse faster. 
• Concentration: Molecules will always move from the region of higher concentration to the region of lower concentration.
• Distance: For small distances diffusion rate is faster than for the large distances.

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Facilitated Diffusion Vs Simple Diffusion

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Simple diffusion occurs unassisted by membrane proteins. Since membrane proteins are needed for transport in facilitated diffusion, the effect of temperature is more pronounced in it than in simple diffusion. Both of them are types of passive transport. They move substances from an area of high concentration to an area of low concentration. However, they differ in the way they transport the molecules across the membrane.

Below is a brief explanation of similarities and differences between the two:

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Examples of Facilitated Diffusion

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The various examples of facilitated diffusion are given below.

Glucose and Amino Acid Transport

Since glucose is a large polar molecule, it cannot pass through the lipid bilayer of the membrane and needs carriers called glucose transporters to pass through. After the digestion of carbohydrates, molecules of glucose are released into the bloodstream via facilitated diffusion. Glucose transporters take glucose from the bloodstream into the cell. Similarly, amino acids are transported from the bloodstream into the cell by facilitated diffusion through the amino acid permeases.

Also read: Structure of Glucose and Fructose

Gas Transport

The haemoglobin in the red blood cells and the myoglobin in the red skeletal muscle cells have an affinity for oxygen. Oxygen diffuses as a result of greater saturation pressure on one side of the membrane and less pressure on the other side, i.e., facilitated diffusion. A similar mechanism occurs with carbon monoxide and carbon dioxide.

Ion Transport

Ions, even though small molecules, because of the charge they carry, are unable to diffuse through the lipid layer of biological membrane. And so, they are transported in their concentration gradient by facilitated diffusion. Potassium ions, sodium ions, and calcium ions need membrane proteins, reffered to as channels, that can provide a passageway. These channels can allow the passage of ions down their concentration gradient at a very fast rate, without using chemical energy.

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Things to Remember

• The passive movement of substances, such as biological molecules or ions,through a plasma membrane by means of a transport protein located in the plasma membrane, is known as facilitated diffusion.
• In a mode of active transport, substances are transported from an area of low concentration to an area of high concentration while in facilitated diffusion it is the opposite.
• In facilitated diffusion, transport proteins are required while in simple diffusion they aren’t.
• As the temperature increases, energy of molecules increases and they diffuse faster.
• In facilitated diffusion, substances move from region of higher concentration to region of lower concentration.