What is passive transport?

• A. The movement of molecules into higher concentration that does require energy
• B. The movement of molecules into higher concentration that does not require energy
• C. The movement of molecules out of the cell using energy
• D. The excretion of waste from cells

Answer: (B)

Passive transport is the movement of substances across a cell membrane without using the cell’s energy. It relies on the natural spread of particles from areas of higher concentration to lower concentration, so substances move down their gradient rather than up. Since no energy is expended, processes like diffusion, osmosis, and facilitated diffusion (through channels or carrier proteins) carry substances across the membrane down the gradient. Moving into a higher concentration without energy would go against the gradient, which is why that description isn’t how passive transport works. In contrast, transporting substances out of the cell or moving against the gradient requires energy (active transport), and simply excreting waste isn’t a transport mechanism by itself. So passive transport is diffusion and related processes that move substances down the concentration gradient without energy input.