Diffusion takes place constantly and in many areas of chemistry and biology. For example, diffusion plays an important role in vital functions such as water uptake by plants and gas exchange in our lungs. But diffusion can also be observed regularly in everyday life, for example when the smell of coffee spreads throughout the apartment in the morning or when sugar dissolves in hot tea.
Definition of the term diffusion
Diffusion describes the passive movement of material particles or the passive transport of material along a concentration gradient. Ultimately, the diffusion leads to a concentration equalization between different substances. These substances are mostly liquid or gaseous, but in rare cases can also be solid.
If a balance between the concentrations is established, one speaks of thermodynamic equilibrium.
Literally, the word diffusion derives from Latin diffusion for «spreading» or «diverging». Diffusion is irreversible. It can only be undone by using energy.
In biology, a distinction can be made between active transport and passive transport. Compared to passive transport, active transport requires energy (e.g. through ATP).
Active transport is required when substances have to be transported against a concentration gradient – e.g. B. by the sodium-potassium pump.
You can find more information about mass transport in the article of the same name on .
Diffusion simply explained
Diffusion compensates for a concentration gradient between two substances through the undirected movement of particles. This leads to passive mass transport along the concentration gradient. The particles therefore diffuse from the place of higher concentration to the place of lower concentration. Ultimately, this brings about a concentration equalization, i.e. a mixing of the substances. This achieves the uniform distribution of particles in space that nature strives for.
Theory of Diffusion
In nature, a concentration balance is always sought. Diffusion is one of several possibilities.
Diffusion is caused by the accidental and informed movement of the material particles. These particles include, for example, atoms, molecules or free neurons. Brownian motion is the physical law governing the cause of diffusion.
Brownian motion describes the random and undirected thermal motion of particles. These particles move alone in space, but they collide with each other, changing their direction of movement.
Only at -273.15* Celsius (absolute zero) do the particles stop moving.
Diffusion of gases
The diffusion of gases differs little from the diffusion of solutes in liquids. However, the movement of particles in gases is much faster than in solutions, so that there is a higher speed of diffusion and faster gas exchange.
Diffusion of Solids
Diffusion is usually the mixing of gaseous or liquid substances. However, diffusion is also possible with solids such as crystals. This requires errors in the otherwise «perfect» crystal lattice. If these lattice defects exist, the vacancies are occupied by ions or atoms changing places and thus move through the lattice. A flow of particles is created.
This mechanism was demonstrated by the Kirkendall Effect.
In addition, it happens Swapping rings when individual particles simply change places. It is also possible for small particles to move through the interstices of the gratings.
Examples of Diffusion
Diffusion takes place in various everyday situations. For example, when the smell of coffee or scented candles spreads throughout the room, when sugar dissolves in hot tea and when wind is created by balancing out temperature differences.
To make the diffusion visible, the example with ink and water is often used.
If you put a few drops of ink or food coloring in a glass of water, diffusion can be observed. It happens through the random and instructed movement of the particles of ink and water. It can be seen that the color does not remain rigid in one place, but that the entire water is colored evenly after a short time.
Incidentally, the particles continue to move even after the concentration has been completely equalized – it’s just that no change can be perceived with the human eye.
speed of diffusion
The rate at which diffusion occurs is called diffusion rate designated. The higher it is, the faster the concentration equalization occurs. Several factors affect this speed. These include the temperature, the difference in concentration, the size of the molecules, the viscosity and the distance between the molecules.
in the Fick’s law of diffusion summarizes the quantitative relationship of these factors.
temperature
The higher the temperature, the faster the movement of the particles. This means that the speed of the particles’ own movements increases with the temperature and thus the collisions and the number of changes in direction also increase. Therefore, at a high temperature, the concentration equalization and thus a thorough mixing occurs more quickly than at a low temperature.
The influence of the temperature factor on the diffusion speed, for example when preparing tea, can be seen. The hotter the water, the faster the concentration equalization occurs.
Figure 2: Diffusion speed and the temperature factor: Preparation of tea. Source: sofatutor.com
concentration difference
The driving force of diffusion is the difference in concentration. The greater this difference between the two concentrations, the faster the rate of diffusion.
The more ink that is dropped into the glass of water, the sooner the concentration equalizes -and the faster the entire water is uniformly blue.
size of the molecules
The particle size of the molecules affects the resistance they experience as a result of their movement in the solvent. The larger the diameter of the molecule, the more resistance it experiences from the solvent and the slower the rate of diffusion. The smaller the molecules are, the faster they can diffuse.
Especially in the case of diffusion through biomembranes, the size of the molecules plays a crucial role.
viscosity
The nature of the solvent through which the molecules move during diffusion also affects the rate of diffusion. If the solvent (medium) has a high viscosity, i.e. it is very viscous, the molecules can only move slowly. However, if the medium has only a low viscosity, the molecules can move faster and the rate of diffusion increases.
Water is a medium with a very low viscosity – this means that the ink can diffuse in water quite quickly. Oil, on the other hand, is much more viscous than water – i.e. has a higher viscosity. The diffusion speed of ink in oil is therefore lower.
distance
The distance to be covered by the molecules also influences the rate of diffusion. The longer the distance, the lower the diffusion speed and the slower the concentration equalization.
Accordingly, the diffusion of molecules within a cell usually takes place very quickly, within milliseconds. However, if molecules have to diffuse over a longer distance, for example several centimetres, this can take hours.
Diffusion in Biology
Diffusion is on paactive transport processin which the substances along the concentration gradients move, i.e. from the place of high concentration to the place of low concentration. This concentration difference is the driving force for passive transport, which is why no externally supplied energy (ATP) is required.
In particular, diffusion through the cell membrane, which all prokaryotes and eukaryotes possess, plays an important role in biology. This membrane is called semi-permeable, meaning it is semi-permeable. This means that only some substances can diffuse through the membrane and others cannot.
Diffusion also plays an important role in plants, especially for the uptake of water. Water gets through the cell wall and the cell membrane into the interior of the cell and is stored there in the vacuoles.
A distinction is made in the diffusion through biomembranes in easy diffusion and facilitated diffusion.
You can find more detailed information about the properties of the biomembrane in the article semipermeable membrane.
Easy diffusion
Simple diffusion is automatic and does not rely on added energy from ATP or other aids. Small and fat-soluble particles and molecules such as water and oxygen can easily diffuse through the biomembrane. The speed of diffusion depends, among other things, on the size of the molecules.
Simple diffusion is driven by the difference in concentration between the intercellular and extracellular spaces, i.e. the inside and outside of the cell.
Facilitated Diffusion
Facilitated diffusion (also: directed diffusion, channel-mediated diffusion or permeability) is used when channel proteins (tunnel proteins) are required for passive transport through the biomembrane. The channel proteins act like tunnels through the biomembrane in facilitating diffusion. Molecules that otherwise cannot easily penetrate the biomembrane are thus facilitated or enabled to diffuse.
This is mostly necessary for quite large or charged particles (ions).
These particles can Not only with channel proteins, but also with the help of von transport proteins (carriers) are transported through the biomembrane. However, this is slower than using the channel proteins.
See the article on facilitated diffusion for more information.
osmosis
Osmosis is a special case of diffusion.
osmosis describes the concentration equalization of two dissolved different substances that are separated from each other by a semi-permeable biomembrane.
In the case of osmosis, usually only the solvent diffuses through the semi-permeable biomembrane to compensate for the difference in concentration.
Due to the diffusion of the solvent, there is a dilution on the side with the more concentrated solution and thus an equalization. The osmosis lasts until the concentration of the substances is the same on both sides.
Mostly it is the…