What is diffusion in a cell in biology? Diffusion examples

Table of Contents

What is Diffusion in Biology?

Diffusion is the transportation or movement of molecules in the presence of a concentration gradient. It is an important process that occurs in all biological forms. Diffusion is a process that aids in the movement of substances into and out of cells. The molecules move from a high-concentration region to a low-concentration region until the concentration is uniform throughout. It is also known to be a process of passive transport.

The diffused material could be a solid, liquid, or gas. In the same way, the medium in which diffusion takes place could be in one of the three physical states (solid, liquid, or gas). One of the significant features of diffusion is that the movements of molecules must take place along a concentration gradient.

Diffusion is a concept used in many fields, including physics, biology, chemistry, sociology economics, and finance. But for the sole purpose of specialization, a closer look will be taken on what is diffusion in biology as it states in the topic. It is required in the human body for the absorption of digested nutrients, gas exchange, nerve impulse propagation, hormone, and other metabolite movements to their target organ, and nearly every event during embryonic development.  Therefore, let’s dive deep into the matter at hand.

diffusion in cells
Diagram of diffusion in cells; Starting from (a), it illustrates how the concentration is greater outside the cell and (b) shows how the concentration is even on both sides of the cell after diffusion.

Definition of Terms

Before understanding what is diffusion in biology? it is good to know some terminologies used in the explanation of diffusion in biology.

Concentration gradient

When the concentration of particles in one area is higher than in another, this is known as a concentration gradient. Particles diffuse down a concentration gradient, from higher to lower concentrations, until they are evenly spaced in passive transport.

Cell transport

The movement of substances across the cell membrane is referred to as cell transport. The ability to selectively allow some particles across the phospholipid membrane is probably the most important feature of a cell’s phospholipid membranes.

Passive transport

The movement of a solute from a region of high concentration on one side of the cell membrane to a region of lower concentration on the opposite side is referred to as passive transport.

Cell membrane

The cell membrane (also referred to as the plasma membrane or the cytoplasmic membrane) is a biological membrane that separates the interior of all cells from the extracellular space, protecting the cell from its surroundings.

Types of diffusion

The two major types of diffusion are; facilitated and simple diffusion.

Facilitated diffusion

It is the passive movement of molecules along a concentration gradient. It is a selective process, meaning that only certain molecules and ions are allowed to pass through the membrane. However, other molecules are unable to pass through the membrane. Such movement across the membrane is aided by the electric charge and pH.

For example, in the living system, the molecules that are allowed to pass through the membrane are; ions, small molecules proteins, and other solutes (water-soluble substances). The movement is aided by the lipid-based membrane that creates compartments that allow for the movements of molecules. On the other hand, molecules that are hydrophilic, polar, or charged are unable to pass through the membrane.

In several ways, facilitated diffusion differs from simple diffusion.

  1. The cargo and the membrane-embedded channel or carrier protein are bound together by molecular binding in facilitated diffusion.
  2. In contrast to free diffusion, which is linear in the concentration difference, the rate of facilitated diffusion is saturable with respect to the concentration difference between the two phases.
  3. Due to the presence of an activated binding event, the temperature dependence of facilitated transport is significantly different from that of free diffusion, which has a mild temperature dependence.

Factors that affect facilitated diffusion

Brownian motion is the driving force behind fluid diffusion. The following are the main factors that influence the facilitated diffusion process:

  • Temperature– As the temperature rises, the energy in the molecules rises, causing the molecules to move faster.
  • Concentration– Molecules move from a region of higher concentration to a region of lower concentration.
  • Diffusion Distance– Diffusion is faster over shorter distances than it is over longer distances. The gas diffuses much faster through a thin wall than it does through a thick wall, for example.
  • Size of the molecules– Smaller molecules are lighter than larger molecules and thus diffuse faster.

Importance of facilitated diffusion

Facilitated diffusion is important because some of the molecules needed for biochemical processes require help. Therefore, here are some biological importances;

  • It is required for the movement of large or polar molecules across the hydrophobic lipid bilayer
  • Facilitated diffusion is required for every cell’s biochemical processes, like the communication between the various subcellular organelles.

Such importance cannot be achieved without the help of transmembrane proteins.

Transmembrane Proteins

Transmembrane proteins are proteins found in the cell membrane that allow specific molecules to move across it more easily. Certain channel proteins and carrier proteins help to speed up the transportation process.

Channel proteins aid in the entry and exit of substances into and out of cells; open channel proteins and gated channel proteins are the two types of channel proteins. The charged molecules can pass through the pore created by open channel proteins in the cell membrane whereas the gated channel proteins regulate the entry and exit of substances by being either closed or open.

Carrier proteins are proteins that are found on the cell membrane and they function by transporting the molecules, changing their confirmation, and then releasing them on the other side. The carrier proteins are affected by temperature and saturation.

Certain transmembrane proteins can also aid in the transportation of solutes and ions.

Simple diffusion

Simple diffusion is defined as the movement of molecules along a concentration gradient without the involvement of other molecules. It is vital in chemical reactions, and some physical events or situations, and even global weather patterns and geological events. It occurs in most biological systems across a lipid bilayer membrane, which is semi-permeable. The membrane has pores and openings that allow specific molecules to pass through.

Process of simple diffusion

It is powered by kinetic energy and concentration gradient, just like other mechanisms. As long as there is a concentration gradient, molecules will continue to move between the two areas. The molecules collide during simple diffusion and as a result, molecules are constantly moving at random. In biological systems, ATP (a chemical form of energy) has been observed not to directly drive simple diffusion.

Factors that affect simple diffusion

Because several parameters influence the rate of diffusion, these parameters/factors have an impact on the diffusion mechanism and they are as follow;

Concentration gradient
  • Simple diffusion of a nonelectrolyte is propelled by a concentration gradient across a biological membrane.
  • The higher the concentration difference across the membrane, the faster the diffusion rate.
  • It has been observed that the rate of diffusion slows as the distribution of molecules across the membrane becomes more uniform.
  • The process of diffusion comes to an end once equilibrium is achieved across the membrane.
Size and mass of the solute molecules
  • The rate of diffusion across a biological membrane is also affected by the size of the molecules.
  • If the molecules are large, they will have a harder time moving across the membrane, slowing the rate of diffusion.
  • The rate of diffusion for smaller molecules is faster, while the rate of diffusion for larger molecules is slower.
  • The temperature of the system has an impact on the simple diffusion process.
  • As the temperature rises, the energy of the molecules rises as well.
  • Molecules with more energy can cross the membrane faster, while particles with less energy take longer.
  • The rate of particle diffusion is also affected by the solubility of molecules in a medium.
  • Lipid-soluble molecules can move quickly across a lipid layer, such as the plasma membrane.
  • Similarly, depending on the nature of the biological membrane, polar and non-polar molecules move at different rates.
Solvent Density
  • The rate of diffusion slows as the density of the solvent increases.
  • The solute will have a harder time moving around in a dense solvent.
  • The movement of solute in the cytoplasm of the cell is influenced by the density of the solvent.
  • The movement of molecules and gases is slowed as the density of the cytoplasm increases, and the opposite is true for less dense cytoplasm.
Surface area and thickness of the biological membrane
  • The rate of diffusion increases as the membrane’s surface area increases.
  • Because mobility is one of the factors that cause flux, increasing the surface area increases the permeability or mobility of the molecules.
  • Similarly, as the membrane thickness increases, the rate of diffusion decreases.
  • The free surface membrane of cells involved in rapid absorption, such as those in the kidney or intestine, is frequently formed into hundreds of tiny projections called microvilli, which increase the absorbing surface.
The Concentration Gradient’s Steepness

Diffusion is essentially driven by the probability of molecules moving away from a region of higher saturation, it follows that when the medium (or solvent) contains a very low concentration of the solute, the likelihood of a molecule diffusing away from the central area is higher.

Application of simple diffusion

  • In the action of medicines in the body, the simple diffusion process is used. After ingesting a medicine, the molecules are released into their respective sites of action through a simple diffusion process.
  • Another example of a diffusion-related phenomenon is air pollution. Air pollution is caused by the diffusion of various gases released by agricultural, industrial, and mechanical processes.

On a whole, the comparison of the two types of diffusion is illustrated in the table below;

Comparison of facilitated and simple diffusion
Name of the type of diffusion
Type of transport
Conditions for occurrence
Direction of movements
Facilitated  diffusion
Concentration gradient, in addition to channel or carrier proteins
Towards lower concentration
Glucose, amino acids, and water
Simple diffusion
Concentration gradient
Towards lower concentration
Hormones, steroids, water, and gases that includes CO2 and O2

Examples of diffusion

Diffusion is used in all most every field and below are some of the examples that are pertaining to biology. These examples are based on the types.

Examples of simple diffusion

Diffusion of gases (CO2 and O2)

  • The movement of gases across a membrane in animals is a classic example of simple diffusion.
  • Simple diffusion is used to exchange oxygen and carbon dioxide dissolved in the blood.
  • The direction of gas movement in the cells is determined by the concentration gradient of these gases in the cells.
  • The oxygen concentration in the alveoli is higher than in the blood vessels during inhalation. As a result, oxygen moves from the alveoli to the bloodstream.
  • Similarly, during exhalation, the concentration of carbon dioxide in the blood is higher than in the alveoli, causing carbon dioxide to move towards the lungs.
  • The exchange of gases between blood and cells follows a similar pattern.
  • When compared to the oxygen levels in actively respiring cells, oxygen concentrations inside arteries and arterioles are higher. Only a single layer of cells separates oxygen from hepatocytes or skeletal muscle fibers by the time blood flows into capillaries in the muscle or liver.

Transportation of waste materials

  • It is used to remove waste materials in animals.
  • The waste material urea is excreted onto the blood by a simple diffusion process in the liver.
  • Similarly, simple diffusion is used in the kidneys to remove waste chemicals and toxins and to absorb water. In some parts of the kidneys, there is a separate active transport system.

Bacterial nutrition

  • Bacteria and other prokaryotes lack a specialized mechanism for transporting nutrients, water, gases, and other solutes throughout their bodies.
  • As a result, they rely on simple diffusion to transport these molecules throughout the cytoplasm.
  • Furthermore, simple diffusion, which occurs through the general body surface, aids the excretion of waste materials in bacteria.

Examples of facilitated diffusion

The following are some of the most well-known examples of facilitated diffusion:

Glucose Transporter

These make it easier for glucose to cross the plasma membrane. These are found in the plasma membrane and are responsible for binding and transporting glucose molecules across the lipid bilayer.

Sodium-glucose cotransporters, which are found in the small intestine and renal tubules and are responsible for glucose transport against a concentration gradient, and facilitative glucose transporters, which are responsible for bidirectional glucose molecules movement across the plasma membrane, are the two types of glucose transporters.


Water transport across the lipid bilayer is aided by these proteins. They play a crucial role in plant cells, red blood cells, and certain kidney structures (where they minimize the amount of water lost as urine).

Channels of Ionization

These are transmembrane proteins that allow ions and solutes to be transported across the plasma membrane selectively. These ionic pumps keep the extracellular fluid’s concentration different from the cytosol’s.

Resting potential is obtained when an excess of sodium ions is present in the extracellular region and an excess of potassium ions is present inside the cell. The sodium ion channels open with a small voltage change, allowing sodium ions to enter the cell quickly. The potassium ion channels open as well, allowing ions to exit the cell.

Proteins on chromatin

Facilitated diffusion occurs in the nucleoplasm on chromatin filaments in eukaryotes, just as it does in prokaryotes, and is explained by the switching dynamics of a protein when it is either bound to a chromatin thread or freely diffusing in the nucleoplasm.

Diffusion in cells

Through its plasma membrane, the cell regulates the entry and exit of substances. Because of its structure, not all molecules can easily pass through this selective membrane. The plasma membrane’s lipid bilayer prevents polar molecules from passing through. Small nonpolar molecules and ions do, however, pass through the lipid bilayer. Polar molecules can pass through the plasma membrane thanks to the proteins that make it up.

Causes of diffusion

Diffusion is a natural and physical process that occurs without the need to stir or shake the solutions. It occurs in liquids and gases because molecules can move at random and the molecules collide and change the direction of the flow.


Diffusion is an important process that occurs in a variety of life processes. It is an important factor in the movement of molecules during the metabolic process in all living organisms. For instance,

  1. The waste carbon dioxide produced by respiration, the amount of carbon dioxide in the cell rises and the carbon dioxide concentration in the cell eventually exceeds that of the surrounding blood. The carbon dioxide then diffuses into the bloodstream through the cell membrane.
  2. Plants absorb water through their root hair cells and the water moves from a high-concentration area (in the soil) to a lower-concentration area (in the root hair cell).
  3. Diffusion is responsible for the movement of ions across neurons, which results in electrical charges in living things.

Diffusion in plants and animals

Diffusion is exemplified in plants and animal cells by gas diffusion. Carbon dioxide enters the plant through the openings formed by the guard cells called stomata, and oxygen leaves through the openings formed by the guard cells called stomata. Carbon dioxide is one of the major reactants in photosynthesis, so the plant takes it in. Photosynthesis also produces oxygen as a byproduct, which is then released into the environment through the stomata.

In animals, respiratory gases are typically moved or transported by simple diffusion. Diffusion of respiratory gases occurs in humans at the capillary beds that separate blood from tissue fluid. Carbon dioxide is released from the lungs by diffusing from the blood into the alveoli and then being exhaled. In turn, oxygen is inhaled and diffuses into the bloodstream via the alveoli of the lungs. The oxygen then diffuses through the body’s tissues from the circulating blood.

Diffusion that is controlled

Although the rate of diffusion through a cell membrane varies depending on the substance, when there is a concentration gradient, the rate is often faster or slower. Slower than anticipated, water diffuses at a slower rate, and amino acids pass through a membrane at a faster rate than one might anticipate.

This is thought to happen in some cases because ions or molecules can only pass through the membrane through special pores. These pores can be few in number or open or closed depending on the situation. When a molecule enters a cell, it is subjected to a variety of structures and processes that may transport it from where it enters to where it is required. Diffusion alone is unlikely to play a significant role in this movement.


In conclusion, the importance of diffusion cannot be overstated, as it can be seen in every part of life. It can be stated without doubt that it is a critical aspect of biology as its functions can be seen in both flora and fauna cellular activities.

Frequently Asked Questions

Does facilitated diffusion require energy?

Facilitated diffusion occurs when there is a concentration difference on both sides of the membrane, and it takes place in the direction of the lowest concentration. Therefore it does not require energy.

How long does diffusion take?

Until it reaches equilibrium, at which point an equal amount of the substance is distributed throughout the entire system.

Does facilitated diffusion require ATP?

Facilitated diffusion necessitates the use of an ATP source and can move materials in both directions.

What exactly is diffusion?

It is the transportation of molecules down a concentration gradient from a region of higher concentration to a region of lower concentration.

What is the list of the different types of diffusion?

It is classified into two categories: simple diffusion and facilitated diffusion.

What is the definition of simple diffusion?

It is the movement of a substance through a semipermeable membrane or in a solution without the assistance of transport proteins.

What is the definition of facilitated diffusion?

It is the passive movement of molecules across the cell membrane by means of a carrier molecule from a region of higher concentration to a region of lower concentration.

What is the process of dialysis?

Dialysis works by allowing solutes to diffuse across a selectively permeable membrane. A selectively permeable membrane allows only certain ions and molecules to pass through while preventing other molecules from moving.

What factors have an impact on diffusion?

Temperature, interaction area, particle size, and the steepness of the concentration gradient are all factors that influence the diffusion process.