Carrier Protein in Facilitated Diffusion: Complete Guide

In this article, we will know the importance of carrier protein in facilitated diffusion. How the carrier protein helps in diffusion process of various solute through the plasma membrane of the cell.

Carrier protein is the membrane bound protein that facilitates the specific substance via intercellular compartments into extracellular fluids. Carrier protein are located in the structure which usually have lipid bilayer like cell membrane, chloroplast and mitochondria.

Depend on the which kind of energy is utilized by the carrier protein, it is classified in three main classes: ATP-driven, electrochemical-driven and light-driven.

What is facilitated diffusion transport?

Facilitated diffusion is also called as facilitated transport or passive-mediated transport. It is the process of continuously passive transport of molecules, ions or solutes through the biological lipid bilayer membrane with the help of transmembrane integral proteins.

Due to passive kind of diffusion, the process does not require directly chemical energy drive from the ATP hydrolysis during transport itself. In this process, molecules move down as per the concentration gradient i.e. from high solute concentration to the low concentration gradient.

Due to involvement of transmembrane during transportation, movement of molecule is depending on the interaction between membrane emended protein or carrier protein and cargo. The facilitated diffusion rate depends on the concentration gradient of molecule across the membrane.

Facilitated diffusion transport mechanism

Facilitated diffusion carried by carrier protein utilize the transporter that are present in the bilayer of biological membrane. Carrier proteins are highly specific for particular molecules.

One particular carrier protein only binds to its specific molecule and then allow passage via membrane.If one specific molecule has high concentration let us say in the cell exterior, then the molecule binds to carrier protein and undergo conformation changes to ease the passage of molecule to the other side of the membrane.

Larger molecules like the permease are transported through the membrane by binding to carrier proteins that further change its conformation. Carrier proteins are known to be involved in passive diffusion as well active diffusion.

Facilitated diffusion process from Wikipedia

Does facilitated diffusion require carrier proteins?

To carry out the process of facilitated diffusion, different kind of proteins requires like channel proteins, gated proteins and carrier proteins. Each type of protein plays its specific role during transportation of molecule across biological membrane.

Carrier proteins is a type of transport protein that is particularly specific to an ion, molecule or solutes. When the target ion or molecule attached or bind to carrier protein, the carrier protein undergoes conformational changes and transport the ion or molecule across the membrane.

The site at which the targeted ion attached is called as active site of carrier protein. After delivery of targeted ion or molecule to the required site, carrier protein regains its original shape and get ready to take up next ion or molecule.

Why are carrier protein required in facilitated diffusion?

In the process of facilitated diffusion, carrier proteins provide the hydrophilic molecules with a move downhill as per concentration gradient. To transport the ions or molecules across the biological membrane, the transport proteins are required.

Carrier protein carry out the transport of sugars, amino acids and nucleosides. Glucose, the primary source of metabolic energy is uptake by the carrier protein and transported into the cell. the glucose transport protein is usually identified as a55-kd protein in the human RBCs (red blood corpuscles).

Types of carrier proteins in facilitated diffusion

O the basis of source of energy used by carrier protein it is divided into three types: ATP-driven, light-driven and electrochemical potential-driven.

In the ATP-driven carrier protein, the protein requires the ATP for the process of transport of molecule. In the electrochemical potential-driven carrier protein, protein gets its energy for transportation by the electrochemical potential. The light-driven carrier protein, molecule transport is carried out by the photons.

Role of carrier protein in the facilitated diffusion

Carrier proteins are required to carry out the transport of certain molecules or ion which are not transported across the membrane due to their large size. Smaller molecules or ion simply diffused as downhill of concentration gradient but for larger molecule it is difficult as biological membrane id made up of phospholipid which allow through it only specific size of molecule through it.

The carrier protein as name indicates the protein which carry the molecule from one side of biological membrane to the other side. They are present across the biological membrane and carry out transport of selective molecules like sugar, amino acid etc.

How do carrier proteins work in facilitated diffusion?

The carrier proteins bind to the specific solute or molecule through its active site. Then the active site undergoes conformational changes sequentially on one side and then on the other side.

Do carrier proteins change shapes in facilitated diffusion?

Yes, the carrier protein undergoes conformational changes after attached to the selected molecule or solute and after successful transport of that selected molecule, regain its original shape.

How does carrier protein change its shape?

There is the active site present on the exposed surface of the carrier protein. When the targeted molecule came and attached to active site, the active site undergoes to change shape of its active site so that no other molecule or overcrowding of molecules happen.

Valinomycin is the passive carrier protein which binds to the potassium and carries it across the cell membrane down to its concentration. Valinomycin had high degree of selectivity of potassium.

carrier proteins in facilitated diffusion
Facilitated diffusion carrier proteins from Wikipedia

Can facilitated diffusion by carrier proteins be saturated?

The facilitated diffusion has an important characteristic of saturation. The saturation point of the carrier protein known when all the active binding site are occupied with the target molecule. There is not single site available for binding of selected molecules.

But when the concentration gradient attains the equilibrium across the membrane, there is no transport of molecules. Thus, facilitated diffusion attained the saturation stage.

What happens to facilitated diffusion when the protein carrier become saturated?

Carrier proteins have limited number of binding sites for the targeted molecules. At the low solute concentration, a greater number of binding sites are available, maximum rate of diffusion. At the high solute concentration of solute, available binding site become less, so the level of transport falls off.

When all the binding site occupies, the point of saturation achieved called as transport maximum (Tm). Then the transport level became constant.

How does facilitated diffusion by carrier protein is differ from simple diffusion?

Simple is the passive movement of molecule as per concentration gradient but in facilitated diffusion, carrier protein is required for transport across the membrane and movement according to downhill of concentration gradient.

The speed of simple diffusion is very low as compare to the rate of facilitated diffusion. The simple diffusion process in not solute specific but facilitated diffusion transport selected solutes.

Simple diffusion in not affected by the presence of inhibitor. The facilitated diffusion is completes inhibited in the presence of inhibitors.

What is the difference between diffusion and facilitated diffusion?

In the simple diffusion, the movement of solute only in one direction i.e. high concentration to low. In the facilitated diffusion, it can be bidirectional downhill as well uphill direction of concentration.

Simple diffusion involved the movement of only small molecules. Large and polar molecules are transported by the facilitated diffusion. In simple diffusion, molecule movement occurs through the surface of the membrane, in the facilitated diffusion, certain kind of transport protein like: carrier, channel and gated protein require for transport of molecules.


To wrap the article, we state that facilitated diffusion by the carrier protein required for transport of large and polar molecule across the membrane which usually difficult to pass through the membrane to less porosity of the biological membrane.

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