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

What Is Simple Diffusion?

In a solution or gas that has an area of high numbers of particles and an area of lower numbers of particles, the particles will diffuse, or move, from the area of higher to the area of lower concentration. This is known as simple diffusion, or passive diffusion.

Simple diffusion does not require energy. Instead, it happens by random motion. 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 a solution or gas.

These areas of higher or lower numbers of particles in a solution or gas are separated by a membrane. A membrane is a thin substance that may allow substances to pass through it or not. Therefore, a membrane is classified as permeable (which means things can pass through it), impermeable (which means things can’t pass through it), or semi permeable (which means things can pass through it by diffusion). In the case of simple diffusion, the membrane is semi permeable, which means the particles or solutes can pass through it by random motion.

To help you understand this process, think of a small brook with flowing water. Imagine that the brook has small leaves and other debris in the water. Then imagine 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.

So, let’s put it all together. Concentration gradient is the process by which particles, which are sometimes called solutes, move through a solution or gas from an area of higher number of particles to an area of lower number of particles. The actual movement is called diffusion. Solutes moving through a solution or a gas happen by random motion along the concentration gradient until there are equal numbers of particles in the two areas.

Simple Diffusion Process

There are actually three types of diffusion. We’ve been talking about simple diffusion (also known as passive diffusion), but there’s also facilitative diffusion and active transport. We will only go into detail about the focus of this lesson: simple diffusion.

An easy way to see simple diffusion in action is to drop food coloring into a cup of water. If you have ever done this, you have likely noticed that the food coloring is concentrated when dropped into the cup of water. However, as a few seconds pass, the particles become lighter in color as they move to the lower concentration level. Again, this happens without energy, which is why it is known as passive diffusion.

Example of Simple Diffusion

In the cell, examples of molecules that can use simple diffusion to travel in and out of the cell membrane are water, oxygen, carbon dioxide, ethanol and urea. They pass directly through the cell membrane without energy along the concentration gradient. With what you have learned so far, what would be the reason for these molecules to move either into or out of the cell? The answer is that the molecules either in the cell or outside the cell have higher concentration in numbers.

In the case of the lungs, as a person breathes in room air (a.k.a., oxygen), there is more carbon dioxide in the lungs and less oxygen. So, with simple diffusion, oxygen moves into the lungs through the cell membrane and carbon dioxide moves out the same way. Remember, this happens along the concentration gradient and does not require energy.

Lesson Summary

Simple diffusion does not require energy and occurs along the concentration gradient. The concentration gradient is the process of particles moving, or diffusing, from an area of higher concentration to an area of lower concentration.

In the human body, molecules, such as water, oxygen, carbon dioxide, ethanol and urea, pass directly through the cell membrane into and out of a cell using simple diffusion. Remember, this does not require energy.

Learning Outcomes

Upon completing this lesson, you should be able to:

• Define simple diffusion and concentration gradient
• Identify the types of membranes
• Explain how simple diffusion occurs
• List examples of molecules in the human body that move through simple diffusion