Diffusion Definition

The passive movement of molecules or particles from higher concentration to lower concentration region is called diffusion. A concentration gradient is needed for the diffusion. The molecules can only move when there is dissimilar amount of solutes or molecules between two regions that form a concentration gradient.

In biology, diffusion is the passive movement of ions or other particles such as glucose, respiratory gases that move from higher concentration to lower concentration of these solutes. The concentration gradient generated by unequal amount of the particles incite them to equalize the concentrations among both the regions.

During diffusion, the solutes move in bidirectional route. It is defined as net movement of particles along their concentration gradient. For example- carbon dioxide and oxygen move across the alveolar-capillary membrane of mammalian lungs, similarly the movement of glucose is also an example of diffusion.

Diffusion Etymology

The solutes or particles move from their higher concentration to lower concentration region by the process of diffusion. It is a type of passive transport. In diffusion, the particles move along their concentration gradient therefore it is an energy independent process.

The word diffusion is derived from the Latin word “diffusionem” that means “a pouring forth”.

What is Diffusion?

In physics, diffusion is defined as movement of particles from their higher concentration to their lower concentration that is driven by thermal energy.

Diffusion is defined in chemistry as motion of the particles suspended in any liquid or gas due to the collision of particles. The collision is called pedesis or the Brownian movement. The Brownian movement of particles decreases when the particles increase in number.

It can be easily observed in a concentrated solution. The movement again starts with decrease in concentration and follows fick’s Law. The rate of diffusion is affected by some factors like temperature, concentration, distance, and material. The rate of diffusion increases when the temperature is higher and decreases at a lower temperature.

The rate of diffusion get faster with increasing concentration of particles in a solution. Distance also affect the rate of diffusion, shorter distance increases the rate of diffusion. Similarly, small and lighter particles diffuse faster than large molecules.

The example of diffusion in chemistry includes food dye dropped in an aqueous solution. Diffusion is also defined as spreading out of the particles.

In biology, the diffusion occur within a biological system. For example cell membrane that is a semipermeable membrane. The diffusion is defined in biology as a type of passive transport used for the movement of particles or molecules.

Transport is of two types- Active transport and passive transport. When the particles move down to the concentration gradient it is called passive transport that is energy-independent process. In active transport the particles move from lower concentration region to higher concentration region or against the concentration gradient therefore it requires chemical energy.

Types of Diffusion

Diffusion is classified into two types- simple diffusion, and facilitated diffusion. In simple diffusion, the movement does not require assistance whereas the facilitated diffusion requires any assistance to complete the process.

The assistance is provided by transport proteins found in the cell membrane. Thus, the facilitated diffusion can only occur in presence of any transport proteins when the molecule binds to the protein transporter.

Diffusion vs Active Transport

Diffusion is a type of passive transport where the particles move from their higher concentration region to the lower concentration region, whereas active transport facilitates the movement of particles from lower concentration to higher concentration region.

The passive transport does not require chemical energy but active transport requires energy. The energy used in the form of ATP.

Plant Cell: Definition, Labeled Diagram, and Examples

Diffusion and Osmosis

The passive transport can occur by different types. For example- diffusion and osmosis. Both facilitates the movement of particles in a downhill manner and do not require energy. The main difference between both the methods is the diffusing molecules.

Solutes of a solution are diffused by the process of diffusion, whereas, osmosis diffuse the particles of solvent of the solution. In osmosis, the solvent molecules diffuse through a biological membrane.

Importance of Diffusion

Diffusion plays a significant role in a cell. The cell has an outer covering that regulates the transport of substance. It is called cell membrane or plasma membrane that is selectively permeable membrane. Plasma membrane is composed of lipid bilayer, transport proteins, and carbohydrates.

Due to its structure, only small nonpolar molecules and ions can pass directly through the lipid bilayer. The cell membrane does not allow large polar molecules to diffuse easily therefore they cross the membrane via transport proteins. Based on the polarity and size, different molecules cross the membrane by different mechanisms.

For example- Small non-polar molecules diffuse by simple diffusion, Polar molecules such as glucose and larger ions use facilitated diffusion, and large non-polar ions such as retinol are diffused by facilitated diffusion and require membrane proteins (e.g. retinol-binding protein).

Diffusion in Plants and Animals

The common example of diffusion in plants is diffusion of gases. For example – The transport of carbon dioxide and oxygen in leaves of the plant. Plant take carbon dioxide to perform photosynthesis and release oxygen into the environment through stomata.

Similarly in animals the transport of respiratory gases also occur by diffusion. In humans, the respiratory gases diffuse through the capillary beds that separate blood from the tissue fluid.

The alveoli of the lungs diffuse carbon dioxide from the blood and oxygen into the blood respectively. The oxygen then diffuses from the blood to the cells and tissues of the body.

Diffusion Citations

• Enhanced Diffusion and Chemotaxis at the Nanoscale. Acc Chem Res . 2018 Oct 16;51(10):2365-2372.
• Complex Diffusion in Bacteria. Adv Exp Med Biol . 2020;1267:15-43.