Why can some molecules diffuse directly across a phospholipid bilayer, while other molecules can only cross the membrane with the assistance of a transport protein?

The phospholipid bilayer is a semi-permeable membrane which selects against polar and charged molecules. This is due to the hydrophilic head and the hydrophobic tail in the bilayer. Non-polar, uncharged or small molecule are able to diffuse through the membrane easily. However, polar molecules such as ions are repelled by the phospholipid bilayer. Transport proteins are required for the transport of such molecules into the cell.