Diffusion

Diffusion is a spontaneous process of penetration of particles of one substance into another with an effort to uniform penetration into the entire volume.

Basic description

Diffusion occurs because of the disordered thermal movement of particles. The movement of particles is essentially random, but substances tend to move from an environment of higher concentration to an environment of lower concentration. A natural property of substances is that if their particles can move, they disperse throughout the space and gradually equalize the concentration in all its parts. We say that substances diffuse. During diffusion, no energy is consumed.

Diffusion

The speed of particle propagation is influenced by particle size, temperature and environmental properties. Mathematically, they describe diffusion Fick's Laws.

Diffusion is a ``spontaneous, ``irreversible and thermally activated process. This fact is clarified by the Einstein-Stokes equation, which describes the dependence of the diffusion coefficient on the temperature T, the dynamic viscosity of the liquid η and the size of the diffusing particles R.

Diffusion enables the movement of substances inside cells and thereby metabolism. In living organisms, other factors play an important role, for example, the ``electric charge of particles or the ``[Biological membrane and transport of substances through biological membrane|transport through semipermeable membranes]].

Division

1. Simple Diffusion
2. Facilitated Diffusion
3. Diffusion through ion channels
   • Aquaporins

A specific case of diffusion is osmosis

Diffusion in different states

Gas environment

In a gaseous environment there is a much faster diffusion than in any other environment. Gas particles have the highest kinetic energy'. An example of this process is the very rapid spread of the scent throughout the room.

Liquid environment

Particles of a liquid substance have lower kinetic energy' than particles of a gas. For that reason, slower diffusion occurs in it than in gas. An example is the release of substances from a tea bag after it is poured with hot water.

Solid

Diffusion in the solid state is more difficult, time-consuming, and is the only possible method of substance transfer. However, it also depends on the type of material. An example is the connection of two copper wires with tin.

Diffusion in biological systems

Transmission of substances through a semipermeable membrane

Diffusion is one of the most fundamental phenomena occurring in living organisms. For the transport of substances through the cell membrane, gaseous and liquid diffusion can be used in the human body.

A constant composition of body fluids is absolutely necessary for organisms. One of the most important factors for maintaining homeostasis is transport across the membrane'. The cell membrane is a selectively permeable (semipermeable) barrier. Its task is to maintain the osmotic and ionic balance between the intracellular and extracellular environments. The cell membrane functions as a barrier that regulates the passage of particles between the intra- and extracellular environment. The easiest to penetrate the membrane are small non-polar molecules such as oxygen and carbon dioxide, which dissolve very well and therefore diffuse very quickly. This rapid diffusion is important for gas exchange in alveoli and tissues. Lipophilic substances penetrate directly through the phospholipid bilayer of the membrane. The rate of diffusion is directly proportional to their fat solubility.

Electroneutral polar molecules, if they are small enough (eg water) diffuse relatively easily (not always by the process of simple diffusion). Hydrophilic substances (eg water and ions) are insoluble in the lipid environment of the biological membrane, so they can only diffuse through transmembrane transporters built into the membrane. This is of course important in the possibility of regulation of the entry of these substances.

Examples of diffusion in an organism

Diffusion is one of the most important physical events that enable the movement of substances inside the organism.

One of the very important events is the creation of action potential. This is caused by the transfer of ions across the cell membrane.

 For more information see Resting Membrane Potential.

 For more information see Action Potential (Physiology).

Another example is the need to give the patient an isotonic solution (a solution with the same concentration as blood plasma). In the case of administration of pure water, the water would diffuse into human cells, which would increase in size and could 'burst by prolonging the exposure too much. On the contrary, when a too concentrated solution is delivered to the patient, water would be sucked out of the cells - this would lead to the cells shrinking. If the exposure time were long, the cells would be so concentrated that cell death would also occur.

 For more information see Osmosis.

Links

Related Articles

• Transmembrane transport
• Ion Channels
• Active transport
  • Symport
  • Antiport
• Passive transport
  • Diffusion
    • Simple Diffusion
    • Facilitated Diffusion
  • Filtering
  • Osmosis
  • Drug penetration through membranes

References

1. LEOŠ NAVRÁTIL, ROSINA JOZEF AND COLLECTIVE, Medical biophysics [online]. [feeling. 2014-16-11]. <https://www.grada.cz/medicinska-biofyzyzika-3633/>