Active transport moves substances across the cell membrane using the cell's energy.

• A. It always happens spontaneously without proteins
• B. It is a form of diffusion down the gradient
• C. It occurs only in plants
• D. It requires energy

Answer: (D)

Active transport uses cellular energy to move substances across the membrane against their concentration gradient, usually through specialized transport proteins called pumps. Because moving uphill requires work, energy from ATP (or another energy source) powers the protein to push or shuttle molecules into or out of the cell. This is what sets active transport apart from diffusion, which is a passive process that moves substances from higher to lower concentration without using energy or proteins. Active transport isn’t limited to plants; animals, bacteria, and fungi all use energy-driven pumps to balance ions and nutrients. That energy requirement is the key reason this statement is true.