Which statement correctly describes passive and active transport across cell membranes?

• A. Passive transport requires energy input to move solutes against their gradient.
• B. Passive transport moves substances down their gradient without energy.
• C. Active transport relies on carrier proteins but does not require energy in some cases.
• D. Endocytosis transports solutes down gradient.

Answer: (B)

Understanding transport across the cell membrane hinges on energy use and direction relative to the concentration gradient. Passive transport carries substances down their concentration gradient without using cellular energy, examples being diffusion and facilitated diffusion via a channel or carrier. Active transport, by contrast, moves substances against their gradient and requires energy, often from ATP or an existing ion gradient, with carrier proteins acting as pumps. Endocytosis is a form of active transport that also consumes energy to engulf material into the cell. So the statement that correctly describes passive transport is that it moves substances down their gradient without energy.