Final answer:
Molecules cross the outer membrane of Gram-negative bacteria via diffusion, porins, or facilitated transport for passive movement, and through active transport mechanisms such as efflux pumps for substances against a concentration gradient.
Step-by-step explanation:
Molecules get across the outer membrane of Gram-negative bacteria through various methods, depending on the nature of the molecules. Small hydrophobic molecules can diffuse directly through the lipid bilayer. Water and gases may pass through porins, which are protein channels that permit the passage of certain substances. Larger or charged molecules, such as glucose, amino acids, and ions, typically utilize facilitated transport through specific protein channels that do not require energy and follow a concentration gradient.
Furthermore, active transport mechanisms are in place for molecules that cannot passively diffuse across membranes. This process involves the use of cellular energy, usually in the form of ATP, to transport substances against their concentration gradient. In Gram-negative bacteria, efflux pumps play a crucial role in actively expelling antibiotics and other toxins from the cell, contributing to their multi-drug resistance capabilities.