Final answer:
Molecules primarily cross the outer membrane of Gram-negative bacteria through porin channels via facilitated diffusion, a passive process that utilizes specific protein channels and does not require energy.
Step-by-step explanation:
Molecules get across the outer membrane of Gram-negative bacteria primarily through porin channels. While some small molecules might diffuse directly across the membrane by simple diffusion, most of them, including polar molecules and nutrients, require facilitated diffusion to pass through. This is a passive process that does not require energy, where molecules move along their concentration gradient through specific protein channels known as porins in the outer membrane of Gram-negative bacteria.
Other methods of transport like active transport involve the movement of molecules against their concentration gradient and require energy usually in the form of ATP.