Simple Diffusion

A substance tends to move from a region of high concentration in diffusion to a region of lower concentration until equilibrium is reached. The total movement of molecules in and out of the more concentrated region and into the lesser concentrated one is till the concentration is equal. Such a process does not require any input of energy. A concentration gradient itself is a form of stored energy, which is utilized as concentrations tend to equalize.

The directional movement as a result of diffusion along a gradient is passive and continues this way till the molecules are dispersed evenly. Non-polar and small molecules can diffuse freely across the cell membranes. The rate of diffusion could be affected by many factors such as –

  • Size of molecules – large particles have a greater resistance in a fluid medium
  • Temperature
  • Rate of diffusion is higher if the concentration gradient is higher – steepness of gradient

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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 that maximizes hydrogen bonding.

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

Mechanism of Simple Diffusion

ATP in biological systems directly does not drive simple diffusion. As seen in other mechanisms, the kinetic energy and concentration gradient is the energy which fuels the process of simple diffusion. In this type of diffusion, molecules hit one another because of which molecules are constantly in a random motion. Pedesis is the collision of particles.

The molecules have the tendency to be compact and motion is reduced when a given region is concentrated. As a result when larger spaces are available, molecules have the tendency to move towards a region with a larger space. One more necessity is a concentration gradient or concentration difference which is the difference of concentration between two regions. Molecular movement continues between both areas until there is a concentration gradient that is established.

Simple Diffusion Example

Simple diffusion can be better understood with the following example –

Bacteria are simple entities who 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 there are some molecules which 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. But, as they are polar or charged, it 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 which has a tendency for equalizing concentrations of solute on both sides. It can be described as a physical phenomena wherein any solvent moves through a selectively permeable membrane which 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 ambience.

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 which 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 in order 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

This was an overview on simple diffusion. Explore related topics on NEET at BYJU’S.

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