Which statement about lipid bilayers is correct?

• A. Polar molecules cannot cross at all
• B. Protein channels allow polar molecules to cross the hydrophobic interior
• C. Lipids form a rigid barrier with no proteins
• D. Protein channels are not involved

Answer: (B)

The main idea here is that the lipid bilayer has a hydrophobic interior that strongly resists the passage of polar or charged molecules, so movement across the membrane typically requires membrane proteins that form pores or transporters. The statement that channels allow polar molecules to cross the hydrophobic interior captures how membranes enable polar species to pass: channels create aqueous, hydrophilic pathways that bypass the energy barrier of the lipid core, letting polar or charged molecules diffuse through down their concentration gradient (water uses aquaporins as a classic example). This is why channels are essential for such transport.

The other options misstate the situation: polar molecules can cross in some cases, especially through channels or after being transported by specific proteins; membranes are not simply rigid barriers with no proteins—proteins are integral to many forms of transport; and channels are indeed involved in this process, so saying they are not involved would be incorrect.