Molecules are able to go through diffusion because they naturally move from areas of higher concentration to areas of lower concentration, driven by their kinetic energy and random thermal motion. This passive process does not require energy. Small, nonpolar, or hydrophobic molecules can simply dissolve in and pass through the phospholipid bilayer of a cell membrane by passive diffusion. Larger or charged molecules usually require facilitated diffusion, which involves transport proteins that help them cross the membrane. Diffusion continues until equilibrium is reached, when molecule concentration is the same on both sides.
In more detail:
- Diffusion is the net movement of molecules down their concentration gradient, from a higher to a lower concentration.
- Molecules move randomly due to thermal motion, and the directionality results from the concentration difference.
- Small gases (like oxygen and carbon dioxide), hydrophobic molecules, and small uncharged polar molecules (like water and ethanol) can diffuse directly through the membrane.
- Larger polar or charged molecules need assistance from specific proteins (channel or carrier proteins) to cross the membrane by facilitated diffusion.
- The membrane structure and molecule properties, such as size and polarity, affect diffusion capability and rate.
Thus, diffusion takes advantage of the natural random movement and concentration differences of molecules, allowing them to spread through a medium or cross cell membranes passively.
