Question

which solute is most likely to cross a membrane by active transport?
tetracycline
dissolved sodium ions
penicillin
dissolved oxygen

Explanation:

Active transport moves substances against concentration gradients, requiring energy. Dissolved sodium ions (charged, polar) cannot cross membranes passively easily and rely on active transport (e.g., Na⁺-K⁺ pump). Tetracycline/penicillin often use facilitated diffusion; oxygen uses simple diffusion.

Answer:

B. dissolved sodium ions