Which molecules can easily diffuse across a plasma membrane?

The plasma membrane is made up of lipid bilayer and hence small nonpolar molecules, such as oxygen (O2) and carbon dioxide (CO2), are soluble in the bilayer and therefore they readily cross cell membranes. Small uncharged polar molecules, such as water (H2O), can also diffuse through membranes, but larger uncharged polar molecules like glucose cannot and therefore need transport systems.