# Concentration Gradient: Definition & Example

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• 0:00 Concentration Gradient Defined
• 1:18 Concentration Gradient…
• 1:59 Passive Diffusion
• 2:40 Facilitative Diffusion
• 4:17 Lesson Summary

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Lesson Transcript

Nadine has taught nursing for 12 years and has a PhD in Nursing research

In this lesson, you'll learn about the concentration gradient and its processes, as well as about the two types of diffusion, passive and facilitative, that happen along the concentration gradient.

## Concentration Gradient Defined

The formal definition of a concentration gradient is the process of particles, which are sometimes called solutes, moving through a solution or gas from an area with a higher number of particles to an area with a lower number of particles. The areas are typically separated by a membrane. This membrane can be permeable, semi-permeable, or non-permeable. Permeable is defined as a membrane that can be crossed by particles, ions, or water. Semi-permeable means 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.

An example that might help you understand the different types of membranes would be different types of fences. A wooden log fence would allow many things to pass through - this would be an example of a permeable membrane. A chain link fence would allow some small items to pass through it - this would be like a 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 and Diffusion Types

Solutes moving through solution (liquid or gas) happen by random motion until there are equal numbers of particles in the two areas. Random motion is defined as movement that occurs by chance because there is no order or regular system by which the movements of the particles shift in the solution or gas.

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

## Passive Diffusion

Passive diffusion does not require energy; it happens by 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 will diffuse or move from the area of higher to the area of lower concentration. A common example of this is a cup of water that you drop food coloring into. 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 so it is known as passive diffusion.

## Facilitative Diffusion

Another type of diffusion that is passive because it does not require energy is facilitative diffusion. This is how particles move across a membrane. This usually refers to a cell membrane, so that is the description we will use. A cell membrane is a barrier that usually cannot be crossed by all particles or ions. Ions are just charged particles, which means they have either a positive charge or a negative change. The particles cannot move freely across the membrane into and out of the cell; they must be assisted by a carrier. This is called facilitation.

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