How can charged molecules cross the lipid membrane according to ion-pair concepts?

How can charged molecules cross the lipid membrane according to ion-pair concepts?

• A. They pair or attach to protein to become neutral and diffuse
• B. They always cross as charged species
• C. They can only cross via active transport
• D. They never cross

Answer: (A)

Charged molecules can cross a lipid membrane by forming a neutral ion pair with an oppositely charged partner, creating a complex that is less polar and can partition into the hydrophobic bilayer and diffuse across. Once it traverses, the pair can dissociate and release the drug on the other side. This shows why becoming neutral through pairing enables passage through the membrane, whereas remaining charged would keep the molecule on the aqueous side. This mechanism is a passive diffusion route and does not require active transport. It isn’t correct to say they never cross, and crossing strictly as a charged species would be energetically unfavorable in the lipid core.

Charged molecules can cross a lipid membrane by forming a neutral ion pair with an oppositely charged partner, creating a complex that is less polar and can partition into the hydrophobic bilayer and diffuse across. Once it traverses, the pair can dissociate and release the drug on the other side. This shows why becoming neutral through pairing enables passage through the membrane, whereas remaining charged would keep the molecule on the aqueous side. This mechanism is a passive diffusion route and does not require active transport. It isn’t correct to say they never cross, and crossing strictly as a charged species would be energetically unfavorable in the lipid core.