What is the primary difference between simple diffusion and facilitated diffusion?

The primary difference between simple diffusion and facilitated diffusion lies in the involvement of transport proteins. In facilitated diffusion, molecules move across a cell membrane with the help of specific transport proteins. This process allows larger or polar molecules, which cannot readily pass through the lipid bilayer, to enter or exit the cell. These proteins selectively assist in the transport, ensuring that substances can move down their concentration gradient without the use of energy.

In contrast, simple diffusion allows small, nonpolar molecules, such as oxygen and carbon dioxide, to move directly through the lipid bilayer of the cell membrane without any assistance from proteins. This process also occurs without the expenditure of energy, as it relies on the natural movement of molecules from areas of higher concentration to areas of lower concentration.

Understanding the role of transport proteins in facilitated diffusion highlights its importance in cellular processes, especially for the transport of essential biomolecules and ions that cannot freely traverse the lipid bilayer. This distinction is fundamental in cell biology and helps clarify the mechanisms cells use to regulate their internal environment.