Diffusion

Diffusion is the passive movement of substances from an area of higher to lower concentration.

These molecules are in constant random motion. They collide and bounce off each other, getting dispersed.

High molecular concentration leads to more frequent collisions, increasing the diffusion rate, and eventually, the molecules are evenly distributed.

In cells, molecules diffuse across the lipid membrane through either simple or facilitated diffusion.

In simple diffusion, gases and lipid-soluble molecules like alcohol, steroids, and fatty acids move directly through the lipid bilayer.

In contrast, water-soluble molecules such as glucose, and charged ions, cannot cross the membrane and require facilitated diffusion via transmembrane carriers or channels. 

In humans, gaseous exchange during respiration occurs through diffusion. The inhaled oxygen-rich air with low carbon dioxide reaches the alveoli, surrounded by carbon dioxide-rich blood capillaries. This concentration difference allows the gases to exchange between the alveoli and the blood via diffusion.