What Substances Pass Through the Cell Membrane by Diffusion?

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we'll discuss the structure of the cell membrane and how this influences diffusion. Then, we'll give three examples of substances that can move by diffusion alone through the membrane.

What Is the Cell Membrane?

Think of the walls of a classroom. They are designed to keep students who are enrolled in a particular class in and separate from other students, shielded from distractions in the hallway, and protected from wildlife that might venture in. However, we don't want to imprison the students forever! Those students need to leave, and others need to come in, and sometimes we need some fresh air from the outdoors. The doors and windows in a classroom serve this purpose. Although millions of times smaller than a classroom, the cells inside your body have a similar system.

Like the walls of the classroom, the cell has a cell membrane, which is a thin, flexible barrier outside the cell that only lets certain things in and out. The structure of the plasma membrane controls what is allowed in and out at any particular time. Let's look more at the structure of the cell membrane before we learn how things move across it.

Cell Membrane Structure

The cell membrane is two layers thick and made of tiny fat molecules called phospholipids. The phospholipids are unique because the outside of them, or the heads, are hydrophilic, meaning they love water. These interact with the liquid solution inside and outside the cell. The tails are hydrophobic or water-fearing. They are sandwiched in between the heads, shielding them from the water.

Structure of the phospholipid bilayer

Hydrophobic things don't get along with hydrophilic things, so if something wants to go through the cell membrane, it has to be hydrophobic too! Since the phospholipids are packed tightly together, only really small, hydrophobic things can sneak through.

What Is Diffusion?

Molecules that fit the profile, small and hydrophobic, can move through the membrane by diffusion. Diffusion is the process of a substance moving from high concentration to low concentration. This process requires no energy. Think of it like rolling a ball down a hill. The ball rolls from a higher elevation to a lower elevation. Gravity does the work, and you expend no energy getting your ball to the bottom. There are a few key examples in the body of molecules that diffuse through the cell membrane, gasses, hormones, and certain drugs. Next, we'll take a look at each of these in detail.

Diffusion across the cell membrane

Gas Exchange

Take a big breath in. Now, slowly release it through your nose. That calming breath is achieved through diffusion. When you breathe in, oxygen flows into your lungs and stops at tiny sacs called alveoli. The alveoli are where oxygen goes from your lungs to your blood, and then to your body. There is more oxygen inside the alveoli than in the blood coming from your body. Oxygen is very tiny and hydrophobic, so it diffuses from the alveoli through the cell membrane and into your blood. Then your blood carries it away to do its job in the body.

The inverse process happens to get rid of carbon dioxide, a waste product from your cells. The blood coming from your body brings carbon dioxide to the lungs. There is more carbon dioxide in the blood than your lungs, so it diffuses into the alveoli. Then you exhale and carbon dioxide is released.

Gas exchange in the lungs
gas exchange

This process happens with every breath, allowing diffusion across the cell membrane to keep you alive.

Steroid Hormones

Another example of a small, hydrophobic molecule is steroid hormones. Steroid hormones are chemical messages sent to your body through the blood. They are made from cholesterol and are made of mostly carbon and hydrogen atoms. Estrogen is an example of a steroid hormone. We commonly think of estrogen as a female hormone, causing the development of breasts and menstruation during teenage years. However, estrogen actually has many functions in the body and men produce some estrogen as well. Estrogen diffuses from the blood, where there is a higher concentration, to cells, where there is lower concentration.

Diffusion of estrogen across the plasma membrane

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