The form of passive transport that allows small molecules such as oxygen to cross the cell membrane is known as simple diffusion.
Simple diffusion involves the movement of small, nonpolar molecules directly through the lipid bilayer of the cell membrane. Because oxygen is a small molecule and nonpolar, it can easily pass through the membrane without the need for any energy input or assistance from transport proteins. This process occurs along the concentration gradient, meaning that oxygen will move from an area of higher concentration to an area of lower concentration until equilibrium is reached.
Unlike facilitated diffusion, which requires specific transport proteins to help larger or polar molecules cross the membrane, simple diffusion relies solely on the intrinsic properties of the molecules and the cell membrane itself. This is an essential process for cells, as it allows them to take in oxygen necessary for respiration while maintaining proper cellular function.