In passive transport, what is the primary driving force of diffusion across a membrane?

• A. A concentration gradient (difference in solute concentration) that produces net movement from high to low concentration
• B. Active pumping by membrane transporters that move solutes from low to high concentration
• C. Vesicular transport that packages molecules into vesicles for movement across the membrane
• D. ATP hydrolysis providing energy to move molecules against their concentration gradient

Answer: (A)