Solute transport across membrane with external concentration gradient

Steps:

• Note higher solute X concentration outside cell creates inward down-gradient for passive entry.
• Identify passive solute transport: simple diffusion (lipid-soluble, no proteins) and facilitated diffusion (channel/carrier proteins).
• Consider active transport: uses energy to move solutes against gradient, enabling outward movement here.
• Exclude non-solute processes: osmosis (water only) and exocytosis (vesicle bulk export).

Why A is correct:

• Active transport moves solutes against concentration gradient via ATP hydrolysis (e.g., sodium-potassium pump), while diffusion and facilitated diffusion follow gradient per Fick's law.

Why the others are wrong:

• B and C include osmosis, which drives water movement via osmosis, not solutes.
• D includes exocytosis, a bulk secretory process unrelated to individual solute diffusion.

Final answer: A