Which statement best describes diffusion in the context of passive transport across biological systems?

• A. The movement of substances across a membrane via vesicle formation and fusion (endocytosis or exocytosis).
• B. The net movement of molecules from a region of higher concentration to a region of lower concentration due to random thermal motion, without requiring cellular energy.
• C. The movement of water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration.
• D. The transport of molecules against their concentration gradient using ATP-driven membrane pumps.

Answer: (B)

Verified step by step guidance

1. Identify the key concept being asked about: diffusion as a form of passive transport across biological membranes.
2. Recall that passive transport means movement of substances without the input of cellular energy (ATP).
3. Understand that diffusion specifically refers to the movement of molecules from an area of higher concentration to an area of lower concentration, driven by random thermal motion.
4. Compare the given options to this definition: exclude processes involving energy use (like ATP pumps) or vesicle formation (endocytosis/exocytosis), and also exclude osmosis, which specifically involves water movement.
5. Conclude that the best description of diffusion is the net movement of molecules from higher to lower concentration due to random motion, without requiring cellular energy.