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Diffusion Across Membranes

Diffusion Across Membranes Background

The basic structure of almost all biological membranes is a lipid bilayer. As the name implies, the bilayer is made up of two layers of lipid molecules. The fatty acid tails of the lipids face each other, forming the middle of the bilayer, and the hydrophilic heads line both surfaces. Most membranes also have proteins attached or embedded in the lipid bilayer. The actual composition of proteins varies depending on the cell type and the subcellular location.

Biological membranes are selectively permeable barriers; in other words, they will let some molecules through but not others. This animation discusses two types of transport across cell membranes: active and passive transport.

In the kidneys, water and salt move in and out of the nephron through the membranes of the cells that make up the tube. The cell membrane must prevent important chemicals from leaking out and undesirable chemicals from leaking in, as well as allow movement of material such as oxygen and waste products in and out of the cell. How is this accomplished? We'll describe two different ways by which chemicals move through the membrane: passive diffusion and active transport.

Active Transport

Active transport across membranes can move molecules regardless of the concentration gradient (see passive diffusion below). It requires energy (hence the term "active"), which is provided by ATP molecules. One well-studied active transport mechanism, shown in this animation, is the sodium-potassium pump, which pumps potassium ions into the cell while pumping sodium ions out. In the animation, the purple tube that spans through the lipid bilayer is the pump.

Sodium ion Phosphate ADP Potassium ion
Sodium ion Phosphate ADP Potassium ion

To start the pumping process, a sodium ion attaches to a protein. Next, an ATP molecule splits (yellow explosion), yielding a phosphate group and an ADP molecule. The sodium is released outside the cell, and a potassium ion enters the pump to be released inside the cell after the phosphate group is released.

Passive Diffusion

Passive diffusion is the process by which molecules space themselves apart into the surrounding area. For example, if you were in a gym with a large group of people running in place, you wouldn't huddle in a corner; instead, you would all spread out. Molecules thus move from a more concentrated area to a less concentrated one down the concentration gradient.


A cell membrane is selectively permeable; some molecules such as oxygen and carbon dioxide, can pass through unimpeded. Water, represented by a green molecule in the animation, is also one such molecule. The animation shows water passing through the lipid bilayer from the inside (top) to the outside (bottom) of the cell.

Diffusion Across Membranes Animation Tips

Want to download this animation for classroom use? The files are in QuickTime format and are available for nonprofit educational use. Howard Hughes Medical Institute holds the copyright for all of these materials and should be credited.

The animations in this section have a wide variety of classroom applications. Use the tips below to get started but look for more specific teaching tips in the near future. Please tell us how you are using the animations in your classroom by sending e-mail to biointeractive@hhmi.org.

  1. Use the animations to make abstract scientific ideas visible and concrete.
  2. Explain important scientific principles through the animations. For example, the biological clocks animations can be used to demonstrate the fundamentals of transcription and translation.
  3. Make sure that students learn the material by repeating sections of the animations as often as you think necessary to reinforce underlying scientific principles. You can start, restart, and play back sections of the animations.
  4. Urge students to use the animations in accordance with their own learning styles. Students who are more visually oriented can watch the animations first and read the text later, while others might prefer to read the explanations first and then view the graphics.
  5. Incorporate the animations into Web-based learning modules that you create to supplement your classroom curricula.
  6. Encourage students to incorporate the animations into their own Web-based projects.

Diffusion Across Membranes Credits

Director: Dennis Liu, Ph.D.

Scientific Content: Satoshi Amagai, Ph.D.

Animator: Mike Chelen

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