How are smaller molecules like o2 and co2 able to pass through the openings of a cell membrane

The cell membrane is the outermost covering of any cell. It serves to protect, maintain and regulate the internal processes of the cell. It has a lipid bilayer which helps monitor the passage of any chemical or molecule however, nonpolar and small polar molecules are able to pass through because of simple diffusion.

Generally travels west to east but often shifts to north and south