Amanda has taught high school science for over 10 years. They have a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. They also are certified in secondary special education, biology, and physics in Massachusetts.
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.
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.
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.
This process happens with every breath, allowing diffusion across the cell membrane to keep you alive.
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.
Inside cells, estrogen can access DNA, or the cells master instructions. Estrogen, in combination with other proteins, can control processes like cell growth and the health of our cardiovascular system. Specifically, estrogen is known to decrease blood pressure and help prevent heart disease. Again, all these wonderful benefits are thanks to diffusion.
Drugs, like alcohol, antidepressants, cocaine, anxiety medications, and others easily slip through the cell membrane. These molecules target our brain, which has an extensive system to keep things out. The blood brain barrier is made of several layers of cells that are packed tightly together, keeping your brain safe. Only very small, hydrophobic molecules can diffuse directly through the cell membrane.
Sometimes the ability to slip through the cell membrane is beneficial, like for patients who need antidepressants. Other substances like alcohol and cocaine can be damaging to our brain. Either way, from ingesting the medication there is more of it in the blood than in the brain. The drugs diffuse through the cell membrane from a high concentration to low concentration.
In summary, the cell membrane is a thin, flexible barrier that protects the cell. The cell membrane only lets certain things in and out due to the hydrophobic tails of phospholipids that make up the membrane. Only very small, hydrophobic molecules can diffuse through the membrane. One example is how carbon dioxide and oxygen diffuse between the blood and lungs. Another is the steroid hormones that regulate many organs in the body, which diffuse from the blood to the cells. Lastly, drugs can diffuse from high concentration in the blood to low concentration in the brain across the blood brain barrier.
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