Discussion
Active transport of a substance across a membrane requires :
Answer: C
Active transport requires ATP.
Active transport moves a substance against its concentration or electrochemical gradient, which needs an input of energy. In primary active transport, such as the Na-K pump, a carrier protein uses ATP hydrolysis directly. Secondary active transport uses an ion gradient instead, but that gradient is itself built by ATP-driven pumps, so ATP is the ultimate requirement.
A gradient is what active transport works against, not what defines it; passive diffusion also involves one. Water and phospholipids are present at any membrane but are not what makes transport active.
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