Which statement describes active transport?

• A. The net movement of substances into or out of cells from a low concentration to a high concentration (against gradient) through carrier proteins, requiring energy
• B. Movement down the concentration gradient by simple diffusion
• C. Movement of water across membranes via osmosis
• D. Movement of gases through the lipid bilayer

Answer: (A)

The key idea being tested is that active transport moves substances against their concentration gradient using energy and carrier proteins. The statement describes net movement of substances into or out of cells from a low concentration to a high concentration, which is against the natural gradient, and it specifies that this movement occurs through carrier proteins with the input of energy. This combination—going against the gradient and requiring energy provided to a carrier protein (such as a pump using ATP)—is the defining feature of active transport. An example is the sodium‑potassium pump, which uses ATP to move Na+ and K+ against their gradients.

In contrast, movement down a concentration gradient by simple diffusion happens without energy and does not require carrier proteins. Osmosis refers specifically to water moving across membranes, usually through aquaporins, and is a form of diffusion rather than active transport. Movement of gases through the lipid bilayer occurs by simple diffusion as well and does not require energy or carrier proteins.