Which statement best describes a passive transport process across a cell membrane?

• A. Movement of a substance against its concentration gradient using ATP-powered pumps
• B. Movement of a substance down its concentration gradient without direct input of cellular energy (ATP)
• C. Movement of large polar molecules through the lipid bilayer without transport proteins
• D. Bulk movement of extracellular fluid and dissolved solutes into the cell via membrane vesicles

Answer: (B)

Verified step by step guidance

1. Understand the definition of passive transport: it is the movement of molecules across a cell membrane without the use of cellular energy (ATP).
2. Recognize that passive transport moves substances down their concentration gradient, meaning from an area of higher concentration to an area of lower concentration.
3. Identify common types of passive transport, such as simple diffusion, facilitated diffusion through transport proteins, and osmosis (water movement).
4. Contrast passive transport with active transport, which requires energy to move substances against their concentration gradient.
5. Evaluate each statement in the problem by checking if it involves movement down the concentration gradient and no direct energy use, which characterizes passive transport.