TRA - Diffusion [LESSON]

Diffusion

Atoms and molecules are always moving. Atoms are in constant violent motion. Just look around you. Doesn't look that way? If you could see the molecules that make up the air around you, you'd see molecules speeding past at 1000 miles per hour—that is, you'd see them if they weren't such a blur.

In liquids and solids, the atoms and molecules move just as fast, but they collide much more frequently. In this activity, you're going to explore some different factors that affect how quickly molecules move.

Use the interactive below to watch how a drop of food coloring diffuses (moves) through a glass of water.  Click anywhere in the model container to add a drop of dye to the water. Use the "Trace Random Molecule" button to follow a single molecule. 

What did you notice about the movement of particles? Why?

The particles seem to spread out. This is because they move from high to low concentration.

Diffusion is the movement of particles from high to low concentration. In biological systems, there is often a membrane through which these particles have to move. Living organisms don't just let any molecules across their cell membranes. The membranes are selectively permeable, which means that they let some things in while keeping other things out. One way that they do this is through different pore sizes. You learned about the structure of the cell membrane in the last module. Start thinking about what types of particles can move through using diffusion.

Diffusion is a passive process of transport which means that no energy is required. A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across a space. You are familiar with the diffusion of substances through the air. For example, think about someone opening a bottle of ammonia in a room filled with people. The ammonia gas is at its highest concentration in the bottle; its lowest concentration is at the edges of the room. The ammonia vapor will diffuse, or spread away, from the bottle; gradually, more and more people will smell the ammonia as it spreads. Materials move within the cell's cytosol by diffusion, and certain materials move through the plasma membrane by diffusion. Diffusion expends no energy.

Biological membranes are selectively permeable; some molecules can cross while others cannot. One way to affect this is through pore size. Change the pore size with the slider to change the permeability of the membrane to the different types of molecules. Trace an individual molecule to see the path it takes.

How does pore size affect the diffusion of different molecules?

The larger the pore, the more molecules can pass through per unit of time.

Remember (especially for the upcoming activity): Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient independent of the concentration gradients of other materials. In addition, each substance will diffuse according to that gradient. Within a system, there will be different rates of diffusion of the different substances in the medium.

Take a moment to learn more about the Factors below.

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