The basic difference between simple diffusion and facilitated diffusion across a cell membrane lies in whether or not membrane proteins assist the movement of molecules:
- Simple diffusion is the passive movement of small, nonpolar molecules (like oxygen and carbon dioxide) directly through the lipid bilayer of the cell membrane without any assistance. This movement occurs down the concentration gradient (from higher to lower concentration) and does not require energy or transport proteins
- Facilitated diffusion is also a passive process that moves molecules down their concentration gradient, but it requires specific membrane proteins-either channel proteins or carrier proteins-to help large, polar, or charged molecules (such as glucose, amino acids, and ions) cross the hydrophobic lipid bilayer. No energy (ATP) is needed, but the transport is protein-mediated and can be saturable and selective
In summary:
Feature| Simple Diffusion| Facilitated Diffusion
---|---|---
Molecule type| Small, nonpolar molecules| Large, polar, or charged molecules
Protein involvement| None| Requires channel or carrier proteins
Energy requirement| None (passive)| None (passive)
Movement direction| Down concentration gradient| Down concentration gradient
Specificity| Non-specific| Specific to certain molecules
Rate of transport| Generally slower| Faster due to protein assistance
Thus, simple diffusion is unassisted passive transport through the membrane, while facilitated diffusion is passive transport aided by specific membrane proteins to allow molecules that cannot freely cross the membrane to enter or exit the cell
