Facilitated diffusion is a type of passive transport that moves molecules across cell membranes using specific carrier and channel proteins. It enables larger or polar molecules like glucose and ions to cross the lipid bilayer without energy expenditure. A key Class 11 Biology & NEET concept under Transport in Plants.
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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.
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.
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.
The mechanism of facilitated diffusion includes:
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.
There is no energy (ATP) required.
The movement of molecules is from high to low concentration (down the concentration gradient).
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:
Feature | Simple Diffusion | Facilitated Diffusion |
Transport Medium | Directly through lipid bilayer | Via carrier/channel proteins |
Molecule Size | Small, nonpolar | Large, polar or charged |
Speed of Transport | Slow | Fast (protein-assisted) |
Specificity | Non-specific | Highly specific |
Example | O₂, CO₂ | Glucose, ions, amino acids |
Facilitated diffusion can be classified according to transport protein type.
A carrier protein is involved.
It is specific to certain molecules.
Protein changes shape to move the molecule across the membrane.
An example is a channel protein.
It creates pores for ions or water molecules.
Rapid transport of small molecules or ions is allowed.
The role of transport proteins is given below:
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.
It forms hydrophilic pores in the membrane.
It allows specific ions or molecules to pass through.
The factors affecting facilitated diffusion are:
The greater the gradient, the higher the diffusion rate.
The transport of molecules continues till the attainment of equilibrium.
A higher number of transport proteins elevates the diffusion rate.
The specificity of the transport proteins towards certain molecules.
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.
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.
Following are the important examples of facilitated diffusion:
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.
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.
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.
Facilitated diffusion is vital for numerous biological activities a cell needs to survive and perform its functions.
Transfers essential nutrients, including glucose and amino acid, into the cell.
Enables removing waste from cells.
Regulates the balance of ions and the flow of water in the cell.
Important topics for NEET exam are:
Simple diffusion vs Facilitated diffusion
Examples of Facilitated diffusion
Factors affecting Facilitated diffusion
Q1. Porins are present in
Membrane of plastids
Membrane of mitochondria
Membrane of bacteria
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?
Requirement of special membrane protein
High selectivity
Transport saturation
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
Water movement across membrane
Regular voltage is needed to open channels
Change in membrane potential leads to opening and closing of voltage gated ion channels.
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.
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Frequently Asked Questions (FAQs)
Factors include the gradient of concentration, the number, and kind of transport proteins and environmental conditions, like temperature and pH.
Facilitated diffusion is a type of passive transport in which transporter proteins help the transmission of molecules across the cell membrane in concentration gradients, without energy consumption.
Examples include the transport of glucose in red blood cells via the GLUT1 transporter, ion channels in neurons and aquaporins in the kidney cells.
Facilitated diffusion requires transport proteins to move larger or polar molecules, while in simple diffusion, movement occurs directly through the lipid bilayer without the aid of proteins.
Facilitated diffusion enables the uptake of nutrients and removal of waste and metabolic products, besides also being concerned with ion balance and water content, essential for a cell's survival and functions.