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Chapter 6: Cell Membrane Structure and Functions Back To Chapter

6.7: Diffusion

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Diffusion

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Diffusion is the passive movement of substances from an area of higher to lower concentration.

These molecules are in constant random motion. They collide and bounce off each other, getting dispersed.

High molecular concentration leads to more frequent collisions, increasing the diffusion rate, and eventually, the molecules are evenly distributed.

In cells, molecules diffuse across the lipid membrane through either simple or facilitated diffusion.

In simple diffusion, gases and lipid-soluble molecules like alcohol, steroids, and fatty acids move directly through the lipid bilayer.

In contrast, water-soluble molecules such as glucose, and charged ions, cannot cross the membrane and require facilitated diffusion via transmembrane carriers or channels.

In humans, gaseous exchange during respiration occurs through diffusion. The inhaled oxygen-rich air with low carbon dioxide reaches the alveoli, surrounded by carbon dioxide-rich blood capillaries. This concentration difference allows the gases to exchange between the alveoli and the blood via diffusion.

6.7: Diffusion

Diffusion is a type of passive transport. In passive transport, a substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space. For example, take the diffusion of substances through the air. When someone opens a perfume bottle in a room filled with people, the perfume is at its highest concentration in the bottle and is at its lowest at the edges of the room. The perfume vapor will diffuse, or spread away, from the bottle, and gradually, more and more people will smell the perfume 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. Rather the different concentrations of materials in different areas are a form of potential energy, and diffusion is the dissipation of that potential energy as materials move down their concentration gradients from high to low.

Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient, independent of the concentration gradients of other materials. Additionally, each substance will diffuse according to that gradient.

Several factors affect the rate of diffusion.

• The extent of the concentration gradient: The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes.
• Mass of the molecules diffusing: Larger molecules move more slowly because it is more difficult for them to move between the molecules of the substance they are moving through; therefore, they diffuse more slowly.
• Temperature: Higher temperatures increase the energy and, therefore, the movement of the molecules, increasing the rate of diffusion.
• Solvent density: As the density of the solvent increases, the rate of diffusion decreases. The molecules slow down because they have a more difficult time getting through the denser medium.

This text is adapted from Openstax, Concepts of Biology, Section 3.5, Passive Transport.

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Diffusion Passive Transport Concentration Gradient High Concentration Low Concentration Equal Concentration Perfume Bottle Room Perfume Vapor Cytosol Plasma Membrane Potential Energy Materials Extracellular Fluid Rate Of Diffusion

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