Define Diffusion, Examples of Diffusion, Factors that Affect Diffusion

Define Diffusion is a physical process that refers to the net movement of molecules from an area of ​​high concentration to an area of ​​lower concentration. The diffusing material can be solid, liquid or gaseous. Likewise, the medium in which the diffusion takes place could be in one of three physical states.

One of the main characteristics of diffusion is the movement of molecules along the concentration gradient. While this could be facilitated by other molecules, they are not directly high-energy molecules such as adenosine triphosphate (ATP) or guanosine triphosphate (GTP).

The rate of diffusion depends on the type of interaction between the medium and the material. For example, one gas diffuses very quickly into another gas.

An example of this is the way the noxious smell of ammonia gas spreads in the air. When a canister of liquid nitrogen loses something, the escaping nitrogen gas quickly diffuses into the atmosphere. The same gas would diffuse a little more slowly in a liquid like water and more slowly in a solid.

Similarly, two miscible liquids also diffuse into one another to form a uniform solution. For example, when water is mixed with glycerin, the two liquids will diffuse radially from one another over time. This can even be observed visually by adding different colored dyes to each of the liquids. However, the same phenomenon is not observed when mixing immiscible liquids such as gasoline and water. The diffusion takes place slowly and only through the small interaction surface between the two liquids.

Examples of Diffusion

Diffusion is an important part of many biological and chemical processes. In biological systems, diffusion occurs at all times through the membranes of each cell and throughout the body.

For example, oxygen is present in arteries and arterioles in a higher concentration than the oxygen content in actively breathing cells. For example, when blood flows into the capillaries of the muscle or the liver, there is only one layer of cells that separates this oxygen from the hepatocytes or skeletal muscle fibers. Oxygen crosses the capillary membrane and penetrates the cells through a passive diffusion process without the active participation of another molecule.

Cells use oxygen in the mitochondria for aerobic respiration, which creates carbon dioxide as a by-product. As the concentration of this gas increases within the cell, it in turn diffuses into the capillaries, where the force of blood flow removes excess gas from the tissue area. In this way, the capillaries stay at a low concentration of carbon dioxide, which allows the molecule to move away from the cells continuously.

This example also shows that the diffusion of any material is independent of the diffusion of any other substance. When oxygen enters tissues from capillaries, carbon dioxide enters the bloodstream.

In chemical processes, diffusion is often the central principle that drives many reactions. As a simple example, some sugar crystals slowly dissolve in a glass of water over time. This happens because there is a net movement of sugar molecules in the water medium. Even in large industrial reactions, when two liquids are mixed, diffusion binds the reactants and allows the reaction to proceed smoothly. For example, polyester is synthesized by mixing the appropriate organic acid and alcohol in liquid form. The reaction occurs when the two reactants diffuse into each other and undergo a chemical reaction to form esters.

Factors that Affect Diffusion

The diffusion is influenced by the temperature, the interaction area, the steepness of the concentration gradient and the particle size. Each of these factors can independently and collectively change the rate and degree of diffusion.


In every system the molecules move with a certain kinetic energy. Usually this is not targeted in any particular way and can appear random. When these molecules collide with each other, the direction of movement as well as the momentum and speed change. For example, if a block of dry ice (carbon dioxide in solid form) is placed in a box, the carbon dioxide molecules collide with one another in the middle of the block and become trapped in the solid mass. In the case of molecules on the periphery, however, rapidly moving molecules in the air also influence their movement so that they can diffuse through the air. This creates a concentration gradient in which the carbon dioxide concentration gradually decreases with the distance from the dry ice lump.

As the temperature rises, the kinetic energy of all particles in the system increases. This increases the speed at which solute and solvent molecules move and increases collisions. This means that dry ice (or even normal ice) evaporates faster on a warmer day, simply because each molecule moves with greater energy and is more likely to leave the confines of a solid.

Area of interaction

To expand on the previous example, when the block of dry ice breaks into several pieces, the area that interacts with the atmosphere increases instantly. The number of molecules that only collide with other carbon dioxide particles in dry ice is decreasing. Therefore, the diffusion speed of the gas in the air also increases.

This property is even more visible when the gas has a smell or a color. For example, when iodine is sublimed on a hot stove, purple vapors begin to appear and mix with the air. If the sublimation takes place in a narrow crucible, the vapors slowly diffuse into the mouth of the container and then quickly disappear. Although they are limited to the smallest surface area within the crucible, the rate of diffusion remains slow.

This is also seen when two liquid reactants are mixed with one another. Stirring increases the area of interaction between the two chemicals and allows these molecules to diffuse towards each other more quickly. The reaction proceeds towards completion at a faster rate. On a similar note, any solute that is broken into small pieces and stirred into the solvent dissolves rapidly – another indicator of molecules diffusing better when the area of interaction increases.

Steepness of the Concentration Gradient

Since the diffusion is mainly determined by the probability that the molecules move away from an area of ​​higher saturation, it immediately follows that with a very low concentration of the solute in the medium (or solvent) the probability that a molecule is outside the central area diffuses it is higher. If, for example, in the example of iodine gas diffusion, the crucible is placed in another closed container and the iodine crystals are heated for a longer period of time, the speed at which the violet gas in the mouth of the glass seems to “disappear” is lowered. This apparent slowdown is due to the fact that over time the larger vessel begins to have enough iodine gas for some of it to move “back” towards the crucible. Although this is undirected random movement, with a large volume you can create a scenario in which there is no net movement of gas out of the container.

Particle size

At any given temperature, the diffusion of a smaller particle is faster than that of a larger molecule. This is related to both the mass of the molecule and its surface area. A heavier molecule with a larger surface area will diffuse slowly, while smaller, lighter particles will diffuse faster. For example, while oxygen gas diffuses slightly faster than carbon dioxide, both move faster than iodine gas.

Broadcast functions

Diffusion in the human body is necessary for the absorption of digested nutrients, gas exchange, the propagation of nerve impulses, the transport of hormones and other metabolites to their target organs and for almost all embryonic development processes.

Types of Diffusion

Diffusion can be simple diffusion and can be facilitated by another molecule.

Simple Diffusion

Simple diffusion is simply the movement of molecules along their concentration gradient without the direct involvement of other molecules. This can be the propagation of a material through a medium or the transport of a particle through a membrane. All of the examples above were simple diffusion cases.

Facilitated Diffusion

On the other hand, facilitated diffusion, as the term suggests, requires the presence of another molecule (the facilitator) for diffusion to take place. Facilitated diffusion is required for the movement of large or polar molecules through the hydrophobic lipid bilayer. Facilitated diffusion is necessary for the biochemical processes of every cell, since communication takes place between different subcellular organelles. For example, while gases and small molecules such as methane or water can freely diffuse through a plasma membrane, larger charged molecules such as carbohydrates or nucleic acids require the help of transmembrane proteins that form pores or channels.

The picture shows the movement of an insoluble molecule from the extracellular space into the cytoplasm.

Since the openings in the plasma membrane are relatively large, these integral membrane proteins also have a high specificity. For example, the channel protein that carries potassium ions has a much higher affinity for that ion than a very similar sodium ion, with almost the same size and charge.

Define Diffusion, Examples of Diffusion, Factors that Affect Diffusion

Define Diffusion, Examples of Diffusion, Factors that Affect Diffusion

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