Why can oxygen diffuse across a cell membrane but a protein cannot?
Cell membrane allows small molecules to pass through it. Oxygen molecule has smaller size as compared to the protein molecule, so an oxygen molecule an diffuse easily as compared to protein molecule. For more info visit: http://www.sparknotes.com/testprep/books/sat2/biology/chapter4section4.rhtml
Oxygen can diffuse across a cell membrane because it is smaller than protein. In fact, most gases can diffuse due to being smaller in size compared to proteins.
Oxygen is small enough to pass through the membrane of a cell. The protein is bigger therefore cannot pass through the membrane.
Cell membranes have tiny holes that allow only small particles to pass through. Oxygen is micro molecular compared to proteins which are macromolecular and that are why oxygen passes the cell membrane easily than protein.