What is Simple Diffusion?

Simple diffusion is the process by which molecules, atoms, or ions diffuse through a semipermeable membrane down their concentration gradient without the assistance of transporter proteins.

A semipermeable membrane is a membrane that allows certain molecules, atoms, or ions through while blocking others.

Diffusion is essential in the anatomy and physiology of a living thing, especially in homeostasis.

It is one of the ways by which the body employs to regulate the concentration of substances. Without it, biological molecules, ions, and other compounds will not move down their concentration gradients readily and with ease.

Thus, the ideal or optimal concentrations of such molecules may not be promptly achieved in the body. Thus, this type of diffusion is pivotal in maintaining equilibrium.

The process entails the transport of molecules and substances relatively with ease and without the need for ATP expenditure

What is Simple Diffusion

Simple Diffusion Definition

It is the process in which solutes are passed through the concentration gradient in a solution across a semipermeable membrane. The assistance of membrane proteins is not required in this process of diffusion wherein substances move from higher concentration to lower.

The process is conducted by the actions of hydrogen bonds which form between solutes and water molecules. Molecules of water move in to surround the solute molecules which maximizes hydrogen bonding.

The hydrogen bonds are temporary and the solution is stirred constantly which aids even the distribution of solutes through the solution. If molecules are tiny enough, simple diffusion can occur across the cell membranes between the phospholipids that make up the membrane.

Water passes along their concentration gradient through the cell membrane in this state, a type of simple diffusion referred to as osmosis.


Simple diffusion is one of the major types of passive transport. The others are facilitated diffusion (also called facilitated transport), filtration, and osmosis. All of them are characterized by a downhill movement that is, a movement from an area of high concentration to an area of low concentration.

By contrast, active transport entails an uphill movement of substances, i.e. from an area of low concentration to an area of high concentration. And since the movement is downhill or “passive”, chemical energy (ATP) is not needed for the process to proceed.

This is distinct from the other types of membrane transport in being independent. This means substances do not use any membrane proteins to move from one area to another.

By contrast, facilitated diffusion requires carrier protein and channel proteins whereas osmosis requires aquaporins (also called water channels) for molecules to move into and out of the cells.

Solutes are the entities that move as opposed to osmosis, which is based upon the movement of the solvent (e.g. water) across the plasma membrane.

Process of Simple Diffusion

In biological systems, ATP (a chemical form of energy) does not directly drive simple diffusion. Similar to other mechanisms, the energy that fuels simple diffusion is kinetic energy and concentration gradient. In simple diffusion, the molecules struck each other. As a result, molecules are in random constant motion.

The collision of particles is called pedesis. When an area is concentrated, the molecules tend to be compacted. The motion is reduced as well. Thus, when a larger space becomes available, the molecules tend to move toward an area with a larger space.

Another requirement is a concentration difference, also referred to as the concentration gradient, which pertains to the difference in concentration between two areas. The movement of molecules will continue between the two areas as long as there is a concentration gradient.

Simple Diffusion Example

Simple diffusion can be better understood with the following example –

Bacteria are simple entities that have no way to intake nutrients other than diffusion across the membrane of the cell. It may use facilitated diffusion for the transportation of most of the nutrients, it depends on simple diffusion to pass water, oxygen, and small nutrients to the cytoplasm.

In its cells, there are no special organelles to transport or hold substances, hence bacteria depend on simple diffusion of substances in the cell to make sure that matter is found in it for reaction to regulate its life processes.

Facilitated diffusion

While some molecules can diffuse across the plasma membrane such as oxygen and carbon dioxide, some other molecules require assistance in crossing the hydrophobic core.

Molecules in facilitated diffusion diffuse across the plasma membrane with aid from the membrane proteins such as the carrier and channels. A concentration gradient is seen in these molecules for it has the potential for diffusing into cells by moving down them.

However, as they are polar or charged, they cannot cross the phospholipid part of the membrane with no assistance. The facilitated transport proteins protect such molecules from the hydrophobic core of the membrane rendering a path through which it can cross. Carrier proteins and channels are the two broad classes of facilitated transport proteins.


It is the spontaneous gross diffusion or movement of solvent molecules via a selectively permeable membrane from an area of high water potential to a region of lower water potential in the direction that tends to equalizing concentrations of solute on both sides.

It can be described as a physical phenomena wherein any solvent moves through a selectively permeable membrane that separates two solutions of varying concentrations.

Osmosis renders a chief mode by which water is moved in and out of the cells. The turgor pressure of cells is majorly maintained by osmosis across the cell membrane between the interior of the cell and its comparative hypotonic ambiance.

The osmotic pressure is the external pressure needed to be applied such that there is no gross solvent movement across the membrane. This pressure is a colligative property, the pressure relies on the molar concentration.

Active transport

Active transport is a kind of cellular transportation that is involved in the movement of molecules across membranes of cells from an area of lower concentration to that of an area having higher concentration. This happens against the concentration gradient.

The process of active transportation necessitates cellular energy to move. Active transportation commonly occurs in root hair cells, walls of the small intestine (villi), etc.

Active transport is of two types

  • Primary active transport – utilizes adenosine triphosphate
  • Secondary active transport – utilizes an electrochemical gradient

Difference between Simple Diffusion and Facilitated Diffusion

The downhill movements of solutes particularly define passive diffusion. Both simple and facilitated diffusion mechanisms are types of passive transport where solute molecules move from a region of higher to a region of lower concentration.

The difference between them is that in simple diffusion the molecules move without the aid of membrane proteins whereas in facilitated diffusion it helps the molecules move downhill. The table below is shown to describe the similarities and differences between the two.]

Simple vs Facilitated Diffusion

Simple DiffusionFacilitated Diffusion
Simple diffusion (definition in biology): a type of passive transport where molecules diffuse unassistedFacilitated diffusion (definition in biology): a type of passive transport where molecules diffuse with assistance from membrane proteins
Concentration gradient (a form of potential energy) results in the movement of molecules from an area of higher concentration to an area of lower concentration
Apart from a concentration gradient, kinetic energy or natural entropy of molecules further fuels the process.
Example of simple diffusion: passive transport of small nonpolar molecules across the plasma membraneExample of facilitated diffusion: passive transport of glucose and ions into and out of the cell