This lesson will provide you with a definition of facilitated diffusion. Also, an example and illustration will be provided to support your understanding of the process.

Definition of Facilitated Diffusion

Because facilitated diffusion is a complex topic, you will be learning the definitions of several terms in this lesson. Be careful not to get facilitated diffusion confused with other types of diffusion. It is not a thermal motion process like other types of diffusion (such as simple diffusion), but it is a type of transport.

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Facilitated diffusion occurs in the cell body. It is the process of transporting particles into and out of a cell membrane. Energy is not required because the particles move along the concentration gradient. The concentration gradient is the process of particles, which are sometimes called solutes, moving through a solution or gas from an area of higher number of particles to an area of lower number of particles.

Let’s define particles and membranes now. A particle is a substance that may be visible or invisible to the naked eye. A membrane is a thin layer that may or may not allow particles to pass through. Permeable means that particles, ions, or water can cross the membrane. 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.

To help you understand membranes, think of a small brook with flowing water that is filled with small leaves and other debris. There is a window screen in the brook. The screen represents the semi-permeable membrane and the leaves and other debris in the water represent particles. In this case, small leaves and debris can pass through the screen, while larger leaves and debris cannot pass through.

Facilitated Diffusion Process

Solutes moving through solution or a gas move randomly along a concentration gradient until there are equal numbers of particles in the two areas. The particles in the cell body are usually charged ions that are dissolved in water. The cell membrane is made up of fatty acids and lipid layers that repel water.

To assist you with this concept, think of a measuring cup in your kitchen in which you pour 3/4 of a cup of oil. Then you add 1/2 cup of water. What happens? The water and oil do not mix together, do they? No – in fact, the water is on the bottom and the oil is on the top. This is an example of the repelling that occurs at the fatty lipid cell wall when particles dissolved in water approach.

The particles that need to go into or come out of the cell cannot do so by themselves. This type of transport requires the use of a carrier that facilitates this process – thus the name facilitated diffusion.

The carrier is usually a protein that is able to cross the cell membrane. So why can the protein cross the membrane and not the other particles? The answer is that the protein has a special shape and usually can carry a small particle or ion across the cell membrane through a channel. The carrier protein only accepts a particular particle or ion, not many different ones. Therefore, there is a carrier protein for each particle or ion needing to cross the cell membrane. This process can happen all the time or the channel may be open and closed at different times, depending on the cell’s needs.

As the particle or ion is accepted into the carrier protein, the protein changes shape and the particle or ion flows down the concentration gradient either into or out of the cell. After the process occurs, the carrier protein changes back to the original shape to allow for the next transport process.

Facilitated diffusion

Example of Facilitated Diffusion

In the cell, examples of molecules that must use facilitated diffusion to move in and out of the cell membrane are glucose, sodium ions, and potassium ions. They pass using carrier proteins through the cell membrane without energy along the concentration gradient. With what you have learned so far, what would be the reason for the molecules’ movement either into or out of the cell? The answer is that the number of molecules either in the cell or outside the cell has higher concentration in numbers.

Lesson Summary

Facilitated diffusion is the process of transporting particles into and out of a cell membrane. Energy is not required, because the particles move along the concentration gradient. The concentration gradient is the process of particles, which are sometimes called solutes, moving through a solution or gas from an area of higher number of particles to an area of lower number of particles.

In the human body, particles and ions that cannot cross the cell membrane use carrier proteins to get into and out of the cell. Each carrier protein accepts only one type of particle or ion to transport either into or out of the cell.

Things to Remember

facilitateddiffusion

Terms Definitions
Facilitated Diffusion the process of transporting particles into and out of a cell membrane
Concentration gradient the process of particles (solutes) moving through a solution or gas from an area with a higher number of particles into a section with a lower number of particles

Learning Outcomes

Attention should be given to the main points of this lesson so that you can later do the following with ease:

  • Write the definition of facilitated diffusion from memory
  • Outline the process of this type of diffusion
  • Recite examples of molecules that use facilitated diffusion