Explain why some substances are able to pass through the cell membrane

The most important property of the cell membrane is its selective permeability: some substances can pass through it freely, but others cannot. Small and nonpolar (hydrophobic) molecules can freely pass through the membrane, but charged ions and large molecules such as proteins and sugars are barred passage.

Selective permanently, is the reason why the cell membrane only let's certain things pass through. small noon polar molecules (oxygen & carbon dioxide) pass through by simply diffusing. But other molecules use other types of passive transport or active transport.