Transport Across the Cell Membrane

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.

This lesson is on transport across the cell membrane. In this lesson, we'll review what a cell membrane is and what it's made of, and learn in detail about the three types of transport across the membrane.

What Is the Cell Membrane?

At a high security building, humans use doors, guards, and locks to let some people into the building and keep others out. But a cell can't use these things to let in or keep out unwanted material, so how does it do this? We'll talk about that, but first let's review what a cell membrane is.

Picture a piece of fruit you enjoy, maybe an apple, a banana, or an orange. What do all these have in common besides being a healthy snack? They all have a peel, or skin on the outside. That skin is designed to protect the fruit. It's pretty thin, and is generally flexible (think how the orange peel bends as you remove it).

Now, if we imagine the fruit as a cell with the important stuff like the fruit flesh inside, the skin of the fruit becomes the cell membrane. The cell membrane is a thin, flexible barrier outside the cell. It's designed to let only certain things in and out, so we call it selectively permeable. Fruit skin is designed to keep out pests and toxins, like the plasma membrane keeps out unwanted substances. However, for fruit to ripen, it needs to let some chemicals in. The same thing happens with the plasma membrane, since it lets in important things the cell needs.

So how does the cell membrane do this? The answer is through three different types of transport: diffusion, facilitated diffusion, and active transport.


Diffusion is when things naturally drift from a high concentration to a low concentration. Think of a cup of tea. When you put your tea bag in a mug of water, all the tea is concentrated near the bag. However, as time goes on, the tea steeps is slowly mixed in to all parts of the water, creating a uniform color. That's diffusion! It happens completely on its own and uses no energy. Diffusion is the most basic type of transport and happens everywhere in the world, all the time. The plasma membrane is no different.

In the cell, the fats, or phospholipids, keep large or charged substances from diffusing straight through the membrane. However, some gases can diffuse directly in and out of the cell. This process is extremely important for breathing.

Blood cells carry carbon dioxide, a waste product, from the body to the lungs. The high concentration of carbon dioxide in the blood cell diffuses to the low concentration in the lungs where it can be exhaled. Meanwhile, the high concentration of oxygen in the lungs diffuses to the blood cell. Without diffusion we would be unable to oxygenate our blood or remove toxic carbon dioxide from our body.

Carbon dioxide diffuses out of the blood into the lungs; oxygen diffuses from the lungs into the blood
gas exchange

Another type of diffusion is osmosis, which is the movement of water. Water moves from where there is more water to where there is less without using energy. Cells need to keep a balance of water to stay alive. If there is too much water in the cell they can burst, but if there is too little water they will become dehydrated, shrink and die.

Osmosis causes water to move into or out of the cell at different rates depending on the concentration of water

Facilitated Diffusion

Facilitated diffusion is a bit more complex a process. When you facilitate a meeting, you help organize and move it along. The same is true for facilitated diffusion, which is diffusion, but through a channel protein that provides a highway for compounds in and out of the cell. This method still doesn't use any energy, it just provides a route.

Diffusion versus facilitated diffusion
diffusion versus facilitated diffusion

An important example of this type of transport occurs in your brain. Your brain cells send signals between each other to tell your body and brain what to do. These signals require the movement of sodium and potassium ions into or out of the cell. To start the signal, sodium rushes into the cell via a channel protein. Although it uses a protein, sodium moves from a high concentration outside the cell, to a low concentration inside the cell, by diffusion. Without facilitated diffusion, you wouldn't be able to read this very article.

Channels in the membrane of brain cells let sodium in and potassium out
facilitated diffusion

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