What Is Facilitated Diffusion: Types, Importance & Examples

Facilitated Diffusion Definition

Facilitated diffusion is the type of passive transport that makes use of specific transport proteins to move molecules across cell membranes. It is, therefore, a method of passive transport requiring the facilitation of proteins in the transport of substances incapable of diffusing directly through the lipid bilayer because of their size, polarity, or charge.

This process does not use energy supplied by the cell and therefore is driven by the concentration gradient, moving substances from high to low concentration areas. It, therefore, plays a major role in the uptake of major molecules and ions that the cell requires to run its activities properly.

Importance in Cellular Processes

Facilitated diffusion is important in the maintenance of cellular homeostasis and the selective permeability of a cell's plasma membrane. In this way, nutrients are quickly taken up by the cells waste products are removed and ion concentrations are regulated, which are important for the maintenance of cellular health and function.

Comparison with Simple Diffusion

Simple diffusion is a method of transport in which the movement of molecules occurs directly through the lipid bilayer without the help of transport proteins.

Facilitated diffusion requires specific transport proteins, generally for large or polar molecules which cannot easily diffuse through the cell membrane.

Role of Transmembrane Proteins

The role of transport proteins is given below:

Carrier Proteins

• Bind to the specific molecule.

• Undergo a conformational change to transport the molecule.

• Release the molecule on the other side of the membrane.

Channel Proteins

• Form hydrophilic pores in the membrane.

• Allow specific ions or molecules to pass through.

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Steps Involved in Facilitated Diffusion

• The molecule binds to transport protein.

• Transport protein changes shape.

• The molecule is transported across the membrane.

• The molecule is released on the other side.

Energetics and Direction of Movement

• No energy (ATP) is required.

• Movement is from high to low concentration (down the concentration gradient).

Types of Facilitated Diffusion

Facilitated diffusion can be classified according to transport protein type.

Carrier-Mediated Diffusion

• A carrier protein is involved.

• Specific to certain molecules.

• Changes shape to move the molecule across the membrane.

Channel-Mediated Diffusion

• An example is a channel protein.

• Creates pores for ions or water molecules.

• Rapid transport of small molecules or ions is allowed.

Factors Affecting Facilitated Diffusion

The factors affecting facilitated diffusion are:

Concentration Gradient

• The greater the gradient, the higher the diffusion rate.

• The transport of molecules continues till the attainment of equilibrium.

Number and Type of Transport Proteins

• A higher number of transport proteins elevates the diffusion rate.

• The specificity of the transport proteins towards certain molecules.

Temperature And pH

• Higher temperatures speed up the rate of diffusion.

• Extremely high or low pH can denature the transport proteins, hence effectiveness.

Biological Significance

Facilitated diffusion is vital for numerous biological activities a cell needs to survive and perform its functions.

Intake Of Nutrients

• Transfers essential nutrients, including glucose and amino acid, into the cell.

Waste Disposal

• Enables removing waste from cells.

Homeostasis

• Regulates the balance of ions and the flow of water in the cell.

Examples of Facilitated Diffusion

Following are the important examples of facilitated diffusion:

Glucose Transporter

These make it easier for glucose to get through the plasma membrane. They are found in the plasma membrane, which binds and moves the glucose molecules across the lipid bilayer.

There are two types of glucose transporters: the facilitative glucose transporters, which move glucose molecules across the plasma membrane in both directions and the sodium-glucose cotransporters, which are found in the renal tubules and small intestine and are in charge of transporting glucose against concentration gradients.

Aquaporins

These proteins make it easier for water to pass through the lipid bilayer. Diseases like diabetes insipidus may arise from mutations in the proteins that create aquaporins.

Ion Channels

The selective movement of ions and other solutes across the plasma membrane is made possible by these transmembrane proteins. These ionic pumps keep the extracellular fluid's concentration distinct from the cytosol's.

A resting potential is achieved when there is an excess of potassium ions within the cell and sodium ions in the extracellular space. The sodium ion channels open and the sodium ions quickly enter the cell when there is a slight voltage shift. Additionally, the potassium ion channels open, allowing the ions to exit the cell.

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