How does diffusion work?
Diffusion reduces concentration differences through random particle motion. Net transport generally proceeds from higher to lower concentration. Individual molecules do not aim toward a destination; the statistical outcome of many random movements produces macroscopic spreading.
Effects of distance and temperature
Diffusion rate depends on particle size, temperature, and medium properties. In simple models, characteristic diffusion time grows with distance squared. It is effective over short cellular distances but insufficient alone across a large organism. Bulk fluid movement is advection; a smell spreading across a room may involve airflow as well as diffusion.
Movement across cell membranes
Some small molecules cross the lipid bilayer, while others require channels or carriers. Facilitated diffusion uses these proteins but does not actively pump material uphill. Electric forces also act on ions, so their electrochemical gradient matters. Equilibrium does not mean molecular motion stops: opposing flows continue while their net effect approaches zero.