What is Facilitated Diffusion?
Definition: “Facilitated diffusion is a type of diffusion in which the molecules move from the region of higher concentration to the region of lower concentration assisted by a carrier.”
Facilitated diffusion is the passive movement of molecules along the concentration gradient. It is a selective process, i.e., the membrane allows only selective molecules and ions to pass through it.
It, however, prevents other molecules from passing through the membrane. The electric charge and pH helps in the diffusion across the membrane.
In living systems, the lipid based membrane creates compartments which allow the transport of a selective concentration of water-soluble substances.
The ions, small molecules, proteins, and other solutes have different concentration across the membranes. Hydrophilic, polar or charged molecules cannot cross the membrane.
Factors Affecting Facilitated Diffusion
Brownian motion is the force behind the diffusion of fluids. The main factors affecting the process of facilitated diffusion are:
- Temperature– As the temperature increases, the movement of the molecules increases due to an increase in energy.
- Concentration– The movement of the molecules takes place from the region of higher concentration to lower concentration.
- Diffusion Distance– The diffusion rate is faster through smaller distance than through the larger distance. For eg., gas diffuses much faster through a thin wall than through a thick wall.
- Size of the molecules– The smaller molecules are lighter and hence diffuse faster than the larger molecules.
Characteristics Of Facilitated Diffusion
Facilitated diffusion is one of the many types of passive transport. This means that it is a type of cellular transport where substances move along their concentration gradient.
The difference in concentrations between areas creates a gradient that incites substances to inherently move to be distributed between the two areas to achieve equilibrium.
Because the movement is downhill (i.e. from higher to lower concentrations), chemical energy is not directly required. What drives facilitated diffusion, just like the other types of passive transport, is kinetic energy.
Nevertheless, what characterizes facilitated diffusion from the other types of passive transport is the need for assistance from a transport protein lodged in the plasma membrane.
Examples of Facilitated Diffusion
Glucose and amino acid transport
Glucose transport is a facilitated diffusion example. Since glucose is a large polar molecule, it cannot pass through the lipid bilayer of the membrane.
Thus, it needs carriers called glucose transporters to pass through. The epithelial cells of the small intestine, for instance, take in glucose molecules by active transport right after the digestion of dietary carbohydrates.
These molecules will then be released into the bloodstream via facilitated diffusion. The rest of the body takes in glucose by means of facilitated diffusion as well. 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.
Gas transport
The hemoglobin is the carrier protein in the red blood cells whereas the myoglobin is the carrier in the red skeletal muscle cells. Both of these membrane proteins 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. A similar mechanism occurs with carbon monoxide and carbon dioxide.
In adult humans, the red blood cells lack a nucleus and other organelles in order to maximize space for hemoglobin that can bind with oxygen or carbon dioxide.
Ion transport
Ions, although small molecules, cannot diffuse through the lipid bilayer of biological membranes because of the charge they carry. Thus, they are transported in their concentration gradient by facilitated diffusion.
Potassium ions, sodium ions, and calcium ions need membrane proteins that can provide a passageway. These proteins are referred to as ion channels (or gated channel proteins).
These channels can allow the passage of ions down their concentration gradient at a very fast rate, often about 106 ions per second or more, without using chemical energy.
Importance Of Facilitated Diffusion
The unequal distribution of substances between the intracellular fluid and the extracellular fluid drives cellular transport, including facilitated diffusion. The movement between these two regions is an attempt to establish equilibrium.
In living organisms, this form of transport is essential to regulate what goes in and what goes out of the cell. The plasma membrane surrounding the cell is responsible for this crucial biological function.
Facilitated diffusion in biology systems is, therefore, crucial to maintaining homeostatic optimal levels of molecules and ions inside the cell.