The diffusion of molecules occurs when molecules move along a concentration gradient. Diffusion is an important process in living systems. It is diffusion that moves substances within and outside of cells. A higher concentration of molecules is transferred to a lower concentration region and vice versa until all of the molecules are in the same quantity.

In other words, diffusion is the passive movement of molecules or particles from an area of high concentration to an area of low concentration. They diffuse due to the gradient in concentration. The movement of the object is downward, so no chemical energy is expended (in contrast to active transportation). A process by which molecules travel through a biological membrane without the use of membrane proteins is called simple diffusion.

What is Diffusion?

Diffusion refers to the passive movement of molecules or particles from regions of higher concentration to regions of lower concentration. There must be a gradient of concentration for diffusion to happen. As a result of the differences in solute, particle, or molecule amounts between the two regions, they are likely to move between them.


In the biological context, diffusion can be defined as the random movement between crowded and less crowded zones (fluids), often across semipermeable membranes. A gradient will develop between two fluids when the concentration of particles is unequal, which will compel them to move and equalize the disparity in concentration.

However, the movement of solutes during the diffusion process is not a one-way street. Movements can occur in both directions. The movement, however, is inclined towards the area with the lowest concentration. The particle is therefore said to diffuse by moving down a gradient of concentration – that is, moving from a high concentration area to a low concentration area.

The net movement of oxygen and carbon dioxide across the alveolar-capillary membrane of mammalian lungs and the net movement of glucose down the gradient of concentration are examples of diffusion in biological systems.

Diffusion in Physics, Chemistry, and Biology

Particles move from a region of higher concentration to a region of lower concentration as a result of thermal energy, or diffusion in physics.  This concept also applies to chemistry. As a result of the collision of particles suspended in liquids or gases, they are always moving randomly. Particle collisions cause a movement known as pedesis or the Brownian motion.

As the number of particles increases, they become compacted and the Brownian movement is lost as what can be observed in concentrated solutions. The law of Fick observed that they will begin to move in a rather organized manner from a region of high concentration to a region of low concentration when provided with an augmented space.

Temperature, concentration, distance, and material are all factors that affect diffusion rates. Hot temperatures, in particular, cause particles to accumulate more kinetic energy (the energy that particles possess when moving), which moves and pushes other particles more. As a result, the diffusion of particles becomes faster at higher temperatures. In contrast, at lower temperatures, it occurs more slowly. With regard to particle concentration, the greater the number of particles in a solution, the faster the rate of diffusion will be.

In terms of distance, the shorter the distance the diffusing particles have to travel, the faster the rate. Likewise, the type of substance in a solution also influences how fast it diffuses. Particles that are smaller and lighter can spread more easily than those that are larger and heavier. In liquids or solids, particles in gaseous media diffuse much more quickly than they do in liquids. Meanwhile, liquids diffuse much more quickly than solids.

Dropping food dye in an aqueous solution demonstrates how diffusion occurs in chemistry. As the dye particles spread throughout the solution, they can be seen moving. For instance, when spraying perfume, the scent is picked up throughout the room as it diffuses.

The diffusion of objects from an initial area of high concentration is defined fundamentally in physics and chemistry. A semipermeable membrane in biology functions in a similar way to diffusion in chemistry, but with a biological system as an intermediary. In addition, it is a means for particles or molecules to move through space. For biological systems, it is a passive transport method.

The passive transport method uses a concentration gradient to move substances across cells. Compared to passive transport, active transport is when substances have to be moved to a previously saturated or high concentration area.

Passive transport does not require chemical energy (e.g. ATP) to move substances because they do not move against the concentration gradient. They move using kinetic energy rather than chemical energy (e.g. ATP). Other passive transport methods include filtration and osmosis.

Diffusion in Plants and Animals

Experiments on plants and animals have demonstrated diffusion through the diffusion of gasses. Plants receive and emit carbon dioxide through the openings formed by the guard cells called stomata. Plants take in carbon dioxide since it is a key component of photosynthesis. As a result of photosynthesis, oxygen is produced and diffused into the environment by stomata.

Similar to humans, most respiratory gases are transported by simple diffusion in animals. As blood is separated from tissue fluid by capillary beds, respiratory gases diffuse through them. In the lungs, carbon dioxide is released by diffusing from the blood into the alveoli and then exhaled. Oxygen is breathed into the lungs and then diffuses into the blood through the alveoli. Through the circulating blood, oxygen diffuses to various tissues within the body.

Diffusion in Cells

Through its plasma membrane, the cell regulates the entry and exit of substances. This membrane has been designed to allow certain molecules to pass while others cannot. It is the lipid bilayer that blocks the passage of polar molecules through the plasma membrane. It is nevertheless possible for nonpolar molecules and ions to pass through the lipid bilayer. Bypassing the plasma membrane is the only way polar molecules can pass through. Membrane proteins that transport substances into and out of the cell are transmembrane integral proteins.

Types of Diffusion

There are two major types of diffusion. In the first type, the concentration gradient does not require assistance. The process is called simple diffusion. Facilitated diffusion, on the other hand, is one that requires assistance. Biological membranes, for instance, contain proteins that assist in the process.

Basically, these types of diffusion differ in the mechanisms by which substances move – one happens without assistance and the other with the assistance of transport proteins. A molecule can only be transported if it has been able to bind to the membrane protein transporter in facilitated diffusion. The net result of both of them is a movement of substances downhill without requiring chemicals to move.

Simple diffusion

Substances that move through semipermeable membranes and in solutions without the aid of transport proteins. Bacteria, for example, deliver small nutrients, water, and oxygen into the cytoplasm through diffusion.

Facilitated diffusion

A carrier molecule facilitates the passive movement of molecules across the cell membrane from the region of higher concentration to the region of lower concentration.


The diffusion of solutes through selectively permeable membranes. A selectively permeable membrane is one that allows only specific ions and molecules to pass through while blocking the movement of others.


It occurs when solvent molecules move from the region of lower concentration to the region of higher concentration through a semipermeable membrane. Biologists define osmosis as the diffusion of water across a selectively permeable membrane since water is the solvent in every living organism. With the help of osmosis, plants extract water and minerals from roots.

Diffusion and Osmosis

Osmosis and diffusion are both passive transport methods. Both of these events happen in a downhill manner without requiring any energy expenditure. They differ in that diffusing particles or molecules are involved. It is the solutes of a solution that are diffusing in diffusion.

Diffusion VS Osmosis
Diffusion VS Osmosis

During osmosis, the diffusing particles are the solution’s solvent, i.e. the water molecules. Osmosis occurs when water molecules diffuse across a biological membrane from an area of high water concentration to an area of low water concentration. Osmosis is not the process of drawing water towards a concentrated solution without the use of a biological membrane.

Factors Affecting Diffusion

The process of diffusion is affected by several factors, which individually and collectively determine the rate and extent of diffusion. These factors are:

  • The temperature.
  • The area of interaction.
  • The particle’s size.
  • The steepness of the gradient in concentration.

Examples of Diffusion

  • When immersed in hot water, a tea bag will diffuse into the water and change its color.
  • We can smell the odor in the air when a perfume or room freshener spray is sprayed on it.
  • The sugar dissolves evenly and sweetens the water without stirring.
  • When we light an incense stick, its smoke diffuses into the air and spreads throughout the room.
  • Adding boiling water to dried noodles causes the water to diffuse, rehydrating and making the noodles plumper and more saturated.

Causes of Diffusion

Diffusion is a natural and physical process that happens on its own, without stirring or shaking the solutions. Diffusion occurs in liquids and gases because molecules can move randomly. When molecules collide, the directions of the molecules are changed.

Significance of Diffusion

The process of diffusion is important to all life processes. A particle, ion, molecule, solution, etc. is involved in the movement of information. During the metabolic process of all living organisms, diffusion plays an important role in the movement of molecules.

Importance Of Diffusion

  • During the process of respiration, this process diffuses carbon dioxide through the cell membrane into the bloodstream.
  • Plant cells also undergo diffusion. Through the root hair cells of all green plants, water from the soil diffuses into the plants.
  • It is diffusion that causes ions to move across neurons and generate an electrical charge.


What is diffusion in biology?

Diffusion is the net movement of molecules from an area where they are at a higher concentration to areas where they are at a lower concentration. The difference in the concentration of a substance between two areas is called the concentration gradient.

What is diffusion in simple words?

Diffusion is the movement of a substance from an area of high concentration to an area of lower concentration. Diffusion occurs in liquids and gases when their particles collide randomly and spread out. Diffusion is an important process for living things – it is how substances move in and out of cells.

What is diffusion vs osmosis?

In diffusion, particles move from an area of higher concentration to one of lower concentration until equilibrium is reached. In osmosis, a semipermeable membrane is present, so only the solvent molecules are free to move to equalize concentration.

What are examples of diffusion in daily life?

  • The smell of perfumes/Incense Sticks.
  • Opening the Soda/Cold Drinks bottle and the CO2 diffuses into the air.
  • Dipping the tea bags in hot water will diffuse the tea in hot water.
  • Small dust particles or smoke diffuse into the air and cause air pollution.

What is the cause of diffusion?

The primary cause of diffusion is the random motion of atoms and molecules in a substance.