All the molecules that are present in the body which includes dissolved substances and water are in constant motion on different pathways. These motion never ceases except at absolute zero temperature.
As the moving molecule gains kinetic energy and when it reaches to the stationary molecule the electrostatic and nuclear forces of moving molecule repel stationary molecule and transfer some energy to it.
The process continues and the stationary molecule which is now a moving molecule collides with another stationary molecule and the process repeats. These collisions occur thousands of times each second.
This continual movement of molecules among one another in liquid or in gases is called diffusion.
Ions, molecules, and colloids diffuse in the same manner except that the colloids diffuse far less rapidly because of their large size.

Diffusion through the cell membrane

Diffusion through the cell membrane can be divided into

Simple Diffusion

it is the kinetic movement of molecules or ions through the membrane without the interaction of carrier proteins present in the membrane.
The rate  depends on
the number of substances available
velocity of molecules
size of  molecules
shape of molecules

it occurs by two pathways which are
1. Through lipid bilayer
2. through watery channels

Diffusion through a lipid bilayer

Diffusion through a lipid bilayer depends on the lipid solubility of a substance. Substances which lipids solubility are high can dissolve directly in the lipid bilayer and diffuse through the cell membrane.
The diffusion of highly lipid soluble molecules occurs in the same way as the diffusion of water solutes occur in the watery solution.
For example,
Oxygen, carbon dioxide, nitrogen, and alcohol can diffuse easily through the lipid bilayer because of their solubility in fats.
The rate of diffusion through a lipid bilayer is directly proportional to lipid solubility

through protein channels

Substances which are insoluble in lipids but are needed to enter the cell diffuse through a different pathway which is known is protein channels.

These proteins channels penetrate all the way through the membrane.

Through these channels lipid, insoluble substances enter the cell. e.g water

These channels allow water soluble and small molecules. when the molecules become larger their penetration falls off rapidly.

Diffusions through these protein channels can be controlled by two ways

1. Selective permeability
2. Gating of channels

Selective permeability

Protein channels are highly selective for the transport of one or more specific ion or molecules.
The selectivity of these protein channels occurs due to their characters which are
nature of electrical charges
chemical bonds
To understand permeability lets see the example
Potassium channels allow passage of potassium ions across the cell membrane about 1000 times more readily than sodium ions.

Gating of channels

Gating of protein channels means switching on or off due to a threshold stimulus.
This gating of channels controls the ions permeability of the channels.
The opening or closing of gates are controlled in two ways
1. Voltage gating
2.  Chemical gating
In voltage gating the molecular conformation of the gate or if its chemical bonds respond to the electrical potential across the cell membrane.
While chemical gating occurs due to the binding of chemical substances to the protein channels.

Facilitated diffusion

it is also known is carrier-mediated because substances transported in this way diffuses through the cell membrane with the help of a specific carrier protein.

it differs from simple diffusion due to the following reason,
The rate of simple diffusion increases with the concentration of diffusing substances while in facilitated diffusion the rate reaches a maximum Called Vmax as the concentration of the diffusing substances increases.
An example of molecules which diffuses through facilitated diffusion is glucose and most of the amino acids.

Factors affecting the rate of diffusion

Substances diffuse across the cell membrane but the study reveals that the net rate of diffusion depends on several factors which are given below.

1. The net diffusion rate is proportional to the concentration difference across the membrane which means that the rate at which the substances diffuses inward is proportional to the concentration of molecules on the outside and vice versa.
2.  If an electric potential is applied across the membrane it causes the movement of molecules through the membrane even though no concentration gradient exists to cause the movements.
3. At times considerable pressure difference developed between the two sides of a diffusable membrane. Having high pressure on one side of the membrane means that the sum of all the forces of the molecules striking the channels on that side of the membrane is greater than the other side due to which molecules from high-pressure area diffuses into a low-pressure area.


it is the activity of a chemical from an area of higher concentration to a region of lower concentration. Diffusion is the movement of gas or solute molecules from a region of higher attentiveness to a region of lower concentration until they are really equal.

it is important to organisms since it is the procedure by which useful molecules enter your body skin cells and waste products are removed.


The Intestines

Food molecules which are digested and present in the intestine move down from the intestine to the blood because of the concentration difference present between blood and intestine.


The Lungs. Oxygen techniques from the high amount (in the air sac) to lesser attention (in the bloodstream). Skin tightening and steps from high awareness (in the blood) to less focus (in the air sac).
Carbon dioxide present in the tissues also diffuses into the blood through the process of diffusion.


it is the continual movement of molecules through the cell membrane and this occurs due to the concentration gradient, electric potential, and pressure difference.

It is an important process in the body because of a different molecule, ions, and colloids enter or leave cells, organs, and tissues through the process of diffusion. diffusion can be controlled by two process that is the selective permeability and gating of the channel. Diffusion is a very important process and without it, life may be impossible. Disturbance in diffusion can cause a huge disturbance in the body.