What Is Facilitated Diffusion: Types, Importance & Examples

What is Facilitated Diffusion?

Facilitated diffusion is a type of transport where molecules move across a cell membrane with the help of specific carriers or channel proteins. Unlike simple diffusion, it allows larger or polar molecules, such as glucose and ions, to pass through the lipid bilayer.

Definition and Basic Concept

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.

The process does not use energy supplied by the cell and is driven by the concentration gradient i.e, from high to low concentration. It 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 the plasma membrane of a cell. 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.

Mechanism of Facilitated Diffusion

The mechanism of facilitated diffusion includes:

Steps Involved in Facilitated Diffusion

• The molecule binds to transport protein.

• The transport protein changes shape.

• The molecule is transported across the membrane.

• The molecule is released on the other side.

Energetics and Direction of Movement

• There is no energy (ATP) required.

• The movement of molecules is from high to low concentration (down the concentration gradient).

Comparison: Simple Diffusion vs Facilitated 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.

Although both the processes do not require energy, there are differences between the two:

Types of Facilitated Diffusion

Facilitated diffusion can be classified according to transport protein type.

Carrier-Mediated Diffusion

• A carrier protein is involved.

• It is specific to certain molecules.

• Protein changes shape to move the molecule across the membrane.

Channel-Mediated Diffusion

• An example is a channel protein.

• It creates pores for ions or water molecules.

• Rapid transport of small molecules or ions is allowed.

Role of Transport Proteins

The role of transport proteins is given below:

Carrier Proteins

• It binds to the specific molecule.

• It undergoes conformational change to transport the molecule.

• It releases the molecule on the other side of the membrane.

Channel Proteins

• It forms hydrophilic pores in the membrane.

• It allows specific ions or molecules to pass through.

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 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 cause high kinetic energy and speed up the rate of diffusion.

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

Saturation Effect

• Occurs because there are limited number of carrier proteins that transport specific molecules

• When molecules are high, carrier proteins become occupied and the rate of transport cannot be increased further.

Examples of Facilitated Diffusion

Following are the important examples of facilitated diffusion:

Glucose Transporters (GLUT, SGLT)

Glucose transporters 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 (Na⁺, K⁺)

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

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.

Biological Significance

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

Role in Nutrient Uptake

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

Waste Removal

• Enables removing waste from cells.

Ion Regulation and Homeostasis

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

Facilitated Diffusion NEET MCQs (With Answers & Explanations)

Important topics for NEET exam are:

• Simple diffusion vs Facilitated diffusion

• Examples of Facilitated diffusion

• Factors affecting Facilitated diffusion

Practice Questions for NEET

Q1. Porins are present in

1. Membrane of plastids

2. Membrane of mitochondria

3. Membrane of bacteria

4. All of these

Correct answer: 4) All of these

Explanation:

Some specialized proteins called porins and channels are used to facilitate the movement of hydrophilic substances across the plasma membrane down their concentration gradient. Facilitated diffusion never causes a net movement of particles from the region of their lower concentration to the region of their higher concentration. The transport rate reaches a maximum when all the protein transporters are saturated. Some channel proteins are always open, such as aquaporins for water transport.

Porins are pore-forming proteins present in the outer membranes of plastids, mitochondria, and some bacteria.

Hence, the correct answer is option 4) All of these.

Q2. Which of the following criteria does not pertain to facilitated transport?

1. Requirement of special membrane protein

2. High selectivity

3. Transport saturation

4. Uphill transport

Correct answer: 4) Uphill transport

Explanation:

Facilitated transport is the process of spontaneous passive transport or downhill transport of molecules or ions across a biological membrane via specific transmembrane integral proteins. The movement of molecules or ions from a region of lower concentration to a region of higher concentration is uphill transport.

Hence, the correct answer is option 4) Uphill transport.

Q3. Voltage-gated ion channels are regulated by means of

1. Water movement across membrane

2. Regular voltage is needed to open channels

3. Change in membrane potential leads to opening and closing of voltage gated ion channels.

4. Simple diffusion of ions

Correct answer: 3) Change in membrane potential leads to opening and closing of voltage gated ion channels.

Explanation:

Tunnel shaped that acts as a gate which can open and close.

Two types:

1. Ion channels - voltage-gated, mechanical and ligand-gated.

2. Porins - Large protein pores

Hence, the correct answer is option 3) Voltage-gated ion channels are regulated using a change in membrane potential that leads to the opening and closing of voltage-gated ion channels.

Also Read:

• MCQs on Facilitated Diffusion

• Transport in Plants

• Osmosis

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