Diffusion and facilitated diffusion

Diffusion is the movement of substances from an area of higher concentration to an area of lower concentration. Simple diffusion can occur directly across a phospholipid bilayer if the particles are small, nonpolar, uncharged, and therefore lipid soluble (like oxygen and carbon dioxide). This is because they are small enough to move between the phospholipid molecules and are neither repelled nor attracted to the fatty acid tails of the phospholipid bilayer. Although water is polar, it is very small and can also pass freely into the cell.

Diffusion continues until the concentration of the substance is equal in all areas (reaching equilibrium). Several factors influence the speed of diffusion:

• Temperature: Higher temperatures increase the kinetic energy of particles, causing them to move faster and thus speeding up diffusion.

• Surface area: A larger surface area of the membrane allows more particles to diffuse at once, increasing the rate of diffusion.

• Distance: Shorter distances for particles to travel result in faster diffusion.

• Concentration gradient: A greater difference in concentration between two areas results in faster diffusion.

Facilitated diffusion

Large or charged particles (such as glucose and ions) cannot pass into the cell by simple diffusion. These molecules enter the cell with the help of channel or carrier proteins, which allow specific substances to pass through. This movement does not require energy and is therefore passive. However, since it involves additional proteins, it is known as facilitated diffusion as the proteins facilitate the diffusion of substances into the cell. 

Channel and carrier proteins are highly specific, permitting only certain substances to pass through. Small, charged, or polar, and therefore water-soluble particles (like sodium and potassium ions) use channel proteins to enter cells by diffusion. Larger water-soluble molecules (such as glucose) use carrier proteins to enter cells by diffusion. While channel proteins generally do not change shape to allow facilitated diffusion (they simply act as a channel), the tertiary structure of carrier proteins does change causing the particle to be moved across the membrane.

Simple diffusion and facilitated diffusion.

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