The concentration gradient is the process of particles, which is referred as solutes which moves through a gas or solution from an higher number of particles area to lower number of particles area and this area is typically separated by a membrane.  This membrane can be permeable, semi-permeable, or non-permeable in nature. Permeable is referred to the term as a membrane that can be crossed by particles, ions, or water. Semi-permeable is defined as the membrane that some particles, ions, or water can cross the membrane. Finally, non-permeable membrane means that no particles, ions, or water can cross the membrane.

This theory can be illustrated by giving an example as an wooden log would allow all the particles to pass through, this would be an example of permeable membrane and a chain link fence which would allow only less particles to pass through can be referred as an example for semi-permeable membrane.  A solid plastic fence would not allow items to pass through it at all - this would represent a non-permeable membrane.

Concentration Gradient Process

Molecules diffuse from an area of high concentration to the area of low concentration down through a concentration gradient. The concentration gradient works under the process of diffusion. The solutes moving through the solution may be gas or liquid happen by the random motion of the solutes until they are equal number of solute particles in both the areas.  The Random motion is referred to the movement that occurs by default because there is no order or regular system through which the movements of the particles shift in the solution (liquid or gas). The actual movement is called as diffusion.

There are three types of diffusion: passive diffusion, facilitative diffusion, and active transport. Since, the active transport does not happen along the concentration gradient, it will not be covered in this lesson. Instead, let's first discuss passive diffusion.

The concentration gradient therefore represents the concept that, just as a low weight thing rolls down a slope, during diffusion molecules move down the concentration gradient. Higher concentration gradients will result in higher rates of diffusion. As the molecules move the gradient evens out until equilibrium is reached. The solute does not really “want” anything, of course. But the laws of thermodynamics state that due to the constant movements of atoms and molecules, substances will move from areas of higher concentration to lower concentration, in order to produce a random solution. This can be easily demonstrated at home by adding a drop of food coloring to a glass of water. At first, the food coloring will only occupy the small spot in the water glass where it was added. But over time, the colored particles will spread, creating an equal distribution of colored particles throughout the bottom of the glass.

In order to power life process some life forms use this theory like in neurons, they are able to send signals so rapidly because they use a concentration gradient of charge particles to create an electrochemical impulse when they need to fire. 20-25% of all calories consumed by the human body are used to maintain this vital concentration gradient.

There are two types of diffusion:

Passive Diffusion

The passive diffusion does not require any energy, it happens by the random motion. In a solution or gas that has an area of high numbers of particles and an area of low numbers of particles, the particles tend to diffuse or move from the area of higher concentration to the area of lower concentration. A most common example of this is a cup of water and when a drop of food coloring is put. The food coloring is concentrated when dropped into the cup of water, however after a few seconds pass, the particles become lighter in color as they move to the lower concentration level. Again, this happens without energy hence referred as passive diffusion.

Soak a raisin in water, and you will get a grape. More than “re-juicing,” soaking raisins constitutes another instance of passive transport – this time, osmosis. Different from other types of passive transport, it seeks equilibrium rather than simple movement along a concentration gradient. Water passes through the raisin’s membrane not only to reach a less-concentrated interior, but also to make the grape “equal” to its outside environment. This process can happen with other fruits and vegetables, as well, as long as the produce has undergone some form of dehydration

Facilitated Diffusion

It is also known as the process of spontaneous passive transport, as opposed to active transport of molecules or ions across a biological membrane or specific transmembrane integral protein. Facilitated diffusion is the passive movement of molecules across the cell membrane via the aid of a membrane protein

It is utilized by molecules that are unable to freely cross the phospholipid bilayer (e.g. large, polar molecules and ions) 

This process is mediated by two distinct types of transport proteins – channel proteins and carrier proteins. The driving force behind diffusion of fluids is simply the probability behind Brownian motion. All molecules have some degree of erratic, random movement, largely dependent on temperature. As temperature increases, the energy of these molecules increases.

When a substance is highly concentrated in a certain region, molecular movement, especially at the periphery, will lead to the gradual spread of the substance. When all the molecules within the region are moving randomly, some are bound to move outwards, into a region where its concentration is low. On the other hand, it is less likely that random molecular movement will result in the directional movement from a region of low concentration specifically towards regions of high concentration.