Simple diffusion is a passive transport process that allows molecules to move across a membrane without the help of membrane proteins. Molecules that can move across a membrane using simple diffusion are typically small, uncharged, and hydrophobic. Two examples of molecules that move across a membrane using simple diffusion are oxygen and carbon dioxide.

Oxygen is a small, uncharged, and hydrophobic molecule that is essential for cellular respiration. Oxygen molecules can diffuse across the plasma membrane and into the cytoplasm of cells. Once inside the cytoplasm, oxygen molecules can be used to generate ATP, the energy currency of the cell.

Carbon dioxide is a small, uncharged, and hydrophobic molecule that is produced as a waste product of cellular respiration. Carbon dioxide molecules can diffuse across the plasma membrane and out of the cytoplasm of cells. Once outside the cytoplasm, carbon dioxide molecules can be transported to the lungs and exhaled.