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Diffusion: Definition with Types and Examples

What is Diffusion

It can be defined as the movement of atoms, ions, or molecules from a region of high concentration to a region of low concentration. The word ‘diffusion’ is derived from the Latin word, ‘diffundere’, meaning ‘to spread out’.

What Causes Diffusion and What Happens During the Process

The random movement of molecules existing in any of the three states of matter (solid, liquid, or gas) increases the kinetic energy of the system. Since molecules always tend to attain the state of equilibrium or minimum randomness, through the process of diffusion they move from an area of higher to the lower concentration to equalize the concentration in both the regions, also called ‘moving down the concentration gradient’.

If Fick’s laws can describe a diffusion process, it is called a normal or Fickian diffusion); otherwise, it is named as anomalous or non-Fickian diffusion).

Basic Characteristics of Diffusion

  • It is a fast and spontaneous process
  • When occurring across a biological membrane, it is a type of passive transport
  • Requires no energy expenditure
  • It depends upon the nature of interaction between the diffusing material and the medium in which diffusion occurs
  • It will continue until the concentration of the material becomes even on both the regions or equilibrium is reached
  • It can only take place when the diffusing material on both sides of the concentration gradient is fully or partially miscible
  • Diffusion of any one material is independent of the diffusion of any other substance

Examples of Diffusion

Some of the common examples of diffusion taking place in everyday life are:

  • The spreading of the odor of a scent or a perfume from the region applied to the whole nearby region
  • Dissolving ice, sugar, salt crystals in water to form a uniform solution

What is the Importance of Diffusion in Cellular Processes

a) Gas exchange – oxygen passes through the capillary membrane and enters cells through the process of diffusion to make the concentration even on both the regions

b) Respiration – diffusion restores the balance between oxygen and carbon dioxide within the cell by removing the excess carbon dioxide from the blood

c) Excretion – eliminating waste products from the body

d) Cellular Transport – Uptake of essential ions, small molecules, food, water, and minerals inside the cell

What Factors Affect Diffusion

The rate of diffusion is affected by many factors, each of which independently and collectively alter the rate and extent of the process

i) Temperature

Diffusion is directly proportional to temperature. Thus an increase in temperature also increases its rate.

In any system, molecules are moving randomly with a certain amount of kinetic energy. After the collision, the molecules change their direction of movement along with momentum and velocity. An increase in temperature also increases the kinetic energy of all particles in the system. This phenomenon increases the rate at which solute and solvent molecules move and collide, thus increasing diffusion.

ii) Area of Interaction

Diffusion is directly proportional to the area of interaction. Thus increasing the area of interaction of the molecules undergoing diffusion increases its rate.

When the surface area of the molecules undergoing diffusion increases, the area of interaction between the colliding molecules also increases, thus helping the reaction towards faster completion.

iii) The Extent of the Concentration Gradient

Diffusion is directly proportional to the concentration gradient. Thus greater the concentration gradient of the molecules, faster is its rate.

Higher the difference in concentration of the molecules between the regions undergoing diffusion more is the random collision of the molecules, thereby increasing the rate of diffusion.

iv) Diffusion Distance

The rate of diffusion is inversely proportional to the distance through which the material is diffusing. That is, smaller distance results in faster diffusion rates and larger distance result in slower diffusion rates.

v) Types of Diffusing Materials

The rate of also diffusion depends on the particle and the diffusing medium. At a specific temperature, all particles have the same average energy, which means lighter atoms, travel faster, and are more mobile than larger atoms. Thus materials made of lighter atoms diffuse faster than heavier materials.

vi) Particle Size

At any given temperature, the diffusion of a smaller particle will be more rapid than that of a larger-sized molecule. This phenomenon 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 more quickly.

What are the Different Types of Diffusion

Since the distribution of molecules occurs in a variety of conditions, diffusion can be classified into two major types:

1) Simple Diffusion

Substances moves across a biologically active semi-permeable membrane along the concentration gradient without the involvement of any other molecules.


  • Breathing oxygen into the body and sending carbon dioxide out of the body during respiration

2) Facilitated Diffusion

It is defined as a process where the diffusing material requires the presence of another molecule or a facilitator to perform diffusion.


Glucose, sodium ions, and potassium ions are transported in and out of the cell with the help of specific carrier proteins and protein channels

Article was last reviewed on Tuesday, January 14, 2020

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