Which of the following cell structures is significantly different between Gram-positive and Gram-negative bacteria

Outer membrane is absent in Gram-positive bacteria but present in Gram-negative bacteria. The wall of Gram-positive bacterium is smooth, with higher peptidoglycan but low in lipid content while Gram-negative bacterium's wall is wavy and comes in contact with plasma membrane, low in peptidoglycan but high in lipid content. Porins are absent in Gram-positive bacteria but present in Gram-negative bacteria. Gram-positive bacteria remain colored with Gram staining even after washing with alcohol or acetone while Gram-negative bacteria do not retain stain after wasing of alcohol or acetone.