The movement of molecules of a substance in a mass unit from a higher concentration zone to a lower concentration zone allied with an active force due to concentration (the ratio of a substance in a compound mixture) difference is the meaning of diffusion. The migration of molecules persists until the state of equilibrium (the sum of all forces is zero when the nature of the body is at rest or is in uniform motion) is achieved between two different concentration zones. The factors that influence the rate of diffusion are temperature, viscosity and mass (particle size) of the substance.

Types of diffusion

There are three kinds of diffusion as listed below:-

• Simple diffusion  
• Facilitated diffusion  
• Channel diffusion

Simple diffusion

• Simple diffusion is an example of passive transport that occurs when electrochemical potentials vary on both sides of a permeable barrier. 
• In this type of diffusion, very small molecules can move through gaps.
• Location – between the various phospholipid molecules in cell membranes. 
• Small inhibitor molecules do not inhibit the simple diffusion process.
• This type of diffusion does not require any energy. The speed of simple diffusion is very low. 
• In this type of diffusion, the movement of particles takes place in the direction of the concentration gradient.
•  Example:- water, oxygen, carbon dioxide, etc.
•  More examples of simple diffusion are:-
  • Steroid hormones can move freely across different membranes down their concentration gradient. So, this is an excellent example of simple diffusion.
  • At the time of respiration, oxygen is transferred into the blood from the lungs. Also, oxygen is transferred into the muscles from the blood cells. 
  • Another simple diffusion example- In the body of pregnant women, food and oxygen travels from the mother’s body into the foetus.
  • Bacteria are single-celled micro organisms. Bacteria deliver water, oxygen and nutrients to the cytoplasm through simple diffusion. No specialised organelles in bacteria perform this type of activity.

Facilitated diffusion

• This type of diffusion is the transport of substances or molecules across a permeable membrane through a concentration gradient. Only large polar molecules pass through in this type of diffusion. Examples:- Amino acids, glucose, etc.
• Facilitated diffusion occurs through transmembrane proteins.
• These transport large or polar particles.
• It happens through specific facilitator molecules.
• In this type of diffusion, the kinetics of carrier-mediated transport affects the diffusion rate.
• Examples of this type of diffusion are:-
  • Glucose (sugar molecule) is the main energy source for cells. Outside a cell, the blood stream is constantly supplying glucose, but glucose is continuously consumed inside the cell. 
  • Molecules flow from high to low concentration- Some molecules cannot enter/exit a cell during diffusion. So, this type of diffusion can check some of these molecules while passing through the plasma membrane. They bind the molecules to special carrier proteins by opening channels located between the cells.

Channel diffusion

• When the materials pass through one end, the ion channels open in channel diffusion. This diffusion is carried out by a protein channel. This is called “channel-mediated diffusion.” 
• Proteins that are embedded in the membrane act as a doorway for diffusion. 
• The molecules will move down a concentration gradient via channel diffusion.
• Some examples of channel diffusion are:-
  • Perfume diffusing into the surrounding (air).
  • When watering plants, water molecules get diffused into the plant. It keeps the leaves from wilting. It means carbon dioxide is diffused from the air (located between the mesophyll cells) and transfers to the chloroplast of leaves.

Graham’s Law of diffusion

Graham’s Law which is conspicuously known as Graham’s Law of Effusion was itemised by Thomas Graham in 1848. Thomas Graham attempted various things with the spread cycle and tracked down a huge part of gas particles that are lighter and will travel faster than the heavier gas atoms.

Fick’s Law of diffusion

The process of diffusion explained by Adolf Fick states that diffusion is the movement of molecules from a higher concentration to a lower concentration region. Fick’s Law of Diffusion was proposed in 1855. He categorised the process under two conditions:

1. Normal diffusion: The process is often known as Fickian diffusion; the process of diffusion that follows Fick’s rules is called normal diffusion.
2. Irregular diffusion: The respective term is also known as non-Fickian diffusion, categorising the process of diffusion that does not follow Fick’s rules.

Examples of diffusion

• Diffusion of waste products, salts and water in the kidneys.
• Gaseous exchange in lungs
• Dialysis: It is an artificial process that partially replaces renal function.

Applications of diffusion

• Absorption of drugs released in the body (tablets, powder, ointment, etc.)
• Excretion process through kidneys
• Changes of gases in alveoli (lungs)

Factors affecting diffusion

• Temperature
• Concentration difference
• Electrical potential
• Osmosis pressure

Conclusion 

Thus, diffusion is very useful in the separation of a variety of gases. Diffusion is also involved in a detection technique called the methane detecting process. Osmosis is also part of diffusion, which involves the infiltration of water by a technique called reverse osmosis. In our kitchens, the gas in LPG comes out by diffusion.

There are various types of diffusion like simple diffusion, facilitated diffusion, dialysis, osmosis.